Unix(3)

📦
version 5.5.0 · 19 versions
, in arch rolling

💖 Unix(3) — OCaml library 💖

okay so bestie, this is the OCaml Unix module, and it's literally your backstage pass to the Unix system. This is part 1 of 7, so buckle up. 💅

🌸 NAME 🌸

Unix — Interface to the Unix system. ✨

🧚 Module 🧚

Module Unix
 : sig end

📖 Documentation 📖

Interface to the Unix system.

To use the labeled version of this module, add module Unix = UnixLabels in your implementation.

⚠️ Note: all the functions of this module (except Unix.error_message and Unix.handle_unix_error) are liable to raise the Unix.Unix_error exception whenever the underlying system call signals an error. no cap, that's the vibe check.

💥 Error report 💥

type error =
 | E2BIG  (* Argument list too long
 *)
 | EACCES  (* Permission denied
 *)
 | EAGAIN  (* Resource temporarily unavailable; try again
 *)
 | EBADF  (* Bad file descriptor
 *)
 | EBUSY  (* Resource unavailable
 *)
 | ECHILD  (* No child process
 *)
 | EDEADLK  (* Resource deadlock would occur
 *)
 | EDOM  (* Domain error for math functions, etc.
 *)
 | EEXIST  (* File exists
 *)
 | EFAULT  (* Bad address
 *)
 | EFBIG  (* File too large
 *)
 | EINTR  (* Function interrupted by signal
 *)
 | EINVAL  (* Invalid argument
 *)
 | EIO  (* Hardware I/O error
 *)
 | EISDIR  (* Is a directory
 *)
 | EMFILE  (* Too many open files by the process
 *)
 | EMLINK  (* Too many links
 *)
 | ENAMETOOLONG  (* Filename too long
 *)
 | ENFILE  (* Too many open files in the system
 *)
 | ENODEV  (* No such device
 *)
 | ENOENT  (* No such file or directory
 *)
 | ENOEXEC  (* Not an executable file
 *)
 | ENOLCK  (* No locks available
 *)
 | ENOMEM  (* Not enough memory
 *)
 | ENOSPC  (* No space left on device
 *)
 | ENOSYS  (* Function not supported
 *)
 | ENOTDIR  (* Not a directory
 *)
 | ENOTEMPTY  (* Directory not empty
 *)
 | ENOTTY  (* Inappropriate I/O control operation
 *)
 | ENXIO  (* No such device or address
 *)
 | EPERM  (* Operation not permitted
 *)
 | EPIPE  (* Broken pipe
 *)
 | ERANGE  (* Result too large
 *)
 | EROFS  (* Read-only file system
 *)
 | ESPIPE  (* Invalid seek e.g. on a pipe
 *)
 | ESRCH  (* No such process
 *)
 | EXDEV  (* Invalid link
 *)
 | EWOULDBLOCK  (* Operation would block
 *)
 | EINPROGRESS  (* Operation now in progress
 *)
 | EALREADY  (* Operation already in progress
 *)
 | ENOTSOCK  (* Socket operation on non-socket
 *)
 | EDESTADDRREQ  (* Destination address required
 *)
 | EMSGSIZE  (* Message too long
 *)
 | EPROTOTYPE  (* Protocol wrong type for socket
 *)
 | ENOPROTOOPT  (* Protocol not available
 *)
 | EPROTONOSUPPORT  (* Protocol not supported
 *)
 | ESOCKTNOSUPPORT  (* Socket type not supported
 *)
 | EOPNOTSUPP  (* Operation not supported on socket
 *)
 | EPFNOSUPPORT  (* Protocol family not supported
 *)
 | EAFNOSUPPORT  (* Address family not supported by protocol family
 *)
 | EADDRINUSE  (* Address already in use
 *)
 | EADDRNOTAVAIL  (* Can't assign requested address
 *)
 | ENETDOWN  (* Network is down
 *)
 | ENETUNREACH  (* Network is unreachable
 *)
 | ENETRESET  (* Network dropped connection on reset
 *)
 | ECONNABORTED  (* Software caused connection abort
 *)
 | ECONNRESET  (* Connection reset by peer
 *)
 | ENOBUFS  (* No buffer space available
 *)
 | EISCONN  (* Socket is already connected
 *)
 | ENOTCONN  (* Socket is not connected
 *)
 | ESHUTDOWN  (* Can't send after socket shutdown
 *)
 | ETOOMANYREFS  (* Too many references: can't splice
 *)
 | ETIMEDOUT  (* Connection timed out
 *)
 | ECONNREFUSED  (* Connection refused
 *)
 | EHOSTDOWN  (* Host is down
 *)
 | EHOSTUNREACH  (* No route to host
 *)
 | ELOOP  (* Too many levels of symbolic links
 *)
 | EOVERFLOW  (* File size or position not representable
 *)
 | EUNKNOWNERR of int
  (* Unknown error
 *)

The type of error codes. Errors defined in the POSIX standard and additional errors from UNIX98 and BSD. All other errors are mapped to EUNKNOWNERR. 🫶

  • exception Unix_error of error * string * string — Raised by the system calls below when an error is encountered. The first component is the error code; the second component is the function name; the third component is the string parameter to the function, if it has one, or the empty string otherwise.

UnixLabels.Unix_error and Unix.Unix_error are the same, and catching one will catch the other. bestie, they're literally twins. 👯‍♀️

  • val error_message : error -> string — Return a string describing the given error code.
  • val handle_unix_error : ('a -> 'b) -> 'a -> 'b — handle_unix_error f x applies f to x and returns the result. If the exception Unix.Unix_error is raised, it prints a message describing the error and exits with code 2.

🌱 Access to the process environment 🌱

  • val environment : unit -> string array — Return the process environment, as an array of strings with the format variable=value. The returned array is empty if the process has special privileges.
  • val unsafe_environment : unit -> string array — Return the process environment, as an array of strings with the format variable=value. Unlike Unix.environment, this function returns a populated array even if the process has special privileges. See the documentation for Unix.unsafe_getenv for more details.

Since 4.06 (4.12 in UnixLabels). ✨

  • val getenv : string -> string — Return the value associated to a variable in the process environment, unless the process has special privileges.

Raises Not_found if the variable is unbound or the process has special privileges.

This function is identical to Sys.getenv.

  • val unsafe_getenv : string -> string — Return the value associated to a variable in the process environment.

Unlike Unix.getenv, this function returns the value even if the process has special privileges. It is considered unsafe because the programmer of a setuid or setgid program must be careful to avoid using maliciously crafted environment variables in the search path for executables, the locations for temporary files or logs, and the like. 🚨

Since 4.06.

Raises Not_found if the variable is unbound.

  • val putenv : string -> string -> unit — putenv name value sets the value associated to a variable in the process environment. name is the name of the environment variable, and value its new associated value.
  • val unsetenv : string -> unit — unsetenv name removes the variable name from the process environment.

Since 5.5. 💅

👶 Process handling 👶

type process_status =
 | WEXITED of int
  (* The process terminated normally by exit ; the argument is the
return code.
 *)
 | WSIGNALED of Sys.signal
  (* The process was killed by a signal; the argument is the signal
number.
 *)
 | WSTOPPED of Sys.signal
  (* The process was stopped by a signal; the argument is the signal
number.
 *)

The termination status of a process. See Sys.signal for the definitions of the standard signal numbers.

🪟 On Windows: only WEXITED is used (as there are no inter-process signals) but with specific return codes to indicate special termination causes. Look for NTSTATUS values in the Windows documentation to decode such error return codes. In particular, STATUS_ACCESS_VIOLATION error code is the 32-bit 0xC0000005 : as Int32.of_int 0xC0000005 is -1073741819 , WEXITED -1073741819 is the Windows equivalent of WSIGNALED Sys.sigsegv .
type wait_flag =
 | WNOHANG  (* Do not block if no child has died yet, but immediately
return with a pid equal to 0.
 *)
 | WUNTRACED  (* Report also the children that receive stop signals.
 *)

Flags for Unix.waitpid.

  • val execv : string -> string array -> 'a — execv prog args execute the program in file prog, with the arguments args, and the current process environment. Note that the first argument, args.(0), is by convention the filename of the program being executed, just like Sys.argv.(0). These execv* functions never return: on success, the current program is replaced by the new one. 🔥

On Windows: the CRT simply spawns a new process and exits the current one. This will have unwanted consequences if e.g. another process is waiting on the current one. Using Unix.create_process or one of the open_process_* functions instead is recommended.

Raises Unix_error on failure.

  • val execve : string -> string array -> string array -> 'a — Same as Unix.execv, except that the third argument provides the environment to the program executed.
  • val execvp : string -> string array -> 'a — Same as Unix.execv, except that the program is searched in the path.
  • val execvpe : string -> string array -> string array -> 'a — Same as Unix.execve, except that the program is searched in the path.
  • val fork : unit -> int — Fork a new process. The returned integer is 0 for the child process, the pid of the child process for the parent process. It fails if the OCaml process is multi-core (any domain has been spawned). In addition, if any thread from the Thread module has been spawned, then the child process might be in a corrupted state. 😭

Raises Invalid_argument on Windows. Use Unix.create_process or threads instead.

Raises Failure if any domain has been spawned.

  • val wait : unit -> int * process_status — Wait until one of the children processes die, and return its pid and termination status.

Raises Invalid_argument on Windows. Use Unix.waitpid instead.

  • val waitpid : wait_flag list -> int -> int * process_status — Same as Unix.wait, but waits for the child process whose pid is given. A pid of -1 means wait for any child. A pid of 0 means wait for any child in the same process group as the current process. Negative pid arguments represent process groups. The list of options indicates whether waitpid should return immediately without waiting, and whether it should report stopped children.

On Windows: can only wait for a given PID, not any child process.

  • val system : string -> process_status — Execute the given command, wait until it terminates, and return its termination status. The string is interpreted by the shell /bin/sh (or the command interpreter cmd.exe on Windows) and therefore can contain redirections, quotes, variables, etc. To properly quote whitespace and shell special characters occurring in file names or command arguments, the use of Filename.quote_command is recommended. The result WEXITED 127 indicates that the shell couldn't be executed. 🐚
  • val _exit : int -> 'a

Okay so Unix._exit is the ultimate “I’m leaving, don’t follow me” move 💅 — it terminates the calling process immediately and returns the given status code to the operating system: usually 0 to indicate no errors, and a small positive integer to indicate failure. Unlike exit, Unix._exit performs no finalization whatsoever: functions registered with at_exit are not called, input/output channels are not flushed, and the C run-time system is not finalized either. Like, zero cleanup. Not even a goodbye text. 😭

The typical use of Unix._exit is after a Unix.fork operation, when the child process runs into a fatal error and must exit. In this case, it is preferable to not perform any finalization action in the child process, as these actions could interfere with similar actions performed by the parent process. For example, output channels should not be flushed by the child process, as the parent process may flush them again later, resulting in duplicate output. Periodt. 🫶

Since 4.12 ✨

  • val getpid : unit -> int — return the pid of the process. Simple. Iconic. 🎀
  • val getppid : unit -> int — return the pid of the parent process.

Raises Invalid_argument on Windows (because it is meaningless 💀).

  • val nice : int -> int — change the process priority. The integer argument is added to the nice value. (Higher values of the nice value mean lower priorities.) Return the new nice value.

Raises Invalid_argument on Windows.

Basic file input/output 💖

  • type file_descr — the abstract type of file descriptors. It’s giving “mystery box but for I/O” 🎀
  • val stdin : file_descr — file descriptor for standard input.
  • val stdout : file_descr — file descriptor for standard output.
  • val stderr : file_descr — file descriptor for standard error.

type open_flag — the flags to Unix.openfile. Here’s the whole flag squad, exactly as listed:

type open_flag =
 | O_RDONLY  (* Open for reading
 *)
 | O_WRONLY  (* Open for writing
 *)
 | O_RDWR  (* Open for reading and writing
 *)
 | O_NONBLOCK  (* Open in non-blocking mode
 *)
 | O_APPEND  (* Open for append
 *)
 | O_CREAT  (* Create if nonexistent
 *)
 | O_TRUNC  (* Truncate to 0 length if existing
 *)
 | O_EXCL  (* Fail if existing
 *)
 | O_NOCTTY  (* Don't make this dev a controlling tty
 *)
 | O_DSYNC  (* Writes complete as `Synchronised I/O data integrity
       completion'
 *)
 | O_SYNC  (* Writes complete as `Synchronised I/O file integrity
       completion'
 *)
 | O_RSYNC  (* Reads complete as writes (depending on O_SYNC/O_DSYNC)
 *)
 | O_SHARE_DELETE  (* Windows only: allow the file to be deleted while
       still open
 *)
 | O_CLOEXEC  (* Set the close-on-exec flag on the descriptor returned
       by Unix.openfile .  See Unix.set_close_on_exec for more information.
 *)
 | O_KEEPEXEC  (* Clear the close-on-exec flag.  This is currently the
       default.
 *)
  • type file_perm = int — the type of file access rights, e.g. 0o640 is read and write for user, read for group, none for others. Permission math is lowkey a whole aesthetic. 🔥
  • val openfile : string -> open_flag list -> file_perm -> file_descr — open the named file with the given flags. Third argument is the permissions to give to the file if it is created (see Unix.umask). Return a file descriptor on the named file.
  • val close : file_descr -> unit — close a file descriptor.
  • val fsync : file_descr -> unit — flush file buffers to disk.

Since 4.08 (4.12 in UnixLabels) ✨

  • val read : file_descr -> bytes -> int -> int -> int — read fd buf pos len reads len bytes from descriptor fd, storing them in byte sequence buf, starting at position pos in buf. Return the number of bytes actually read. 🦋
  • val read_bigarray — same as Unix.read, but read the data into a bigarray. Since 5.2 ✨
val read_bigarray : file_descr -> ('a, Bigarray.int8_unsigned_elt,
       Bigarray.c_layout) Bigarray.Array1.t -> int -> int -> int
  • val write : file_descr -> bytes -> int -> int -> int — write fd buf pos len writes len bytes to descriptor fd, taking them from byte sequence buf, starting at position pos in buff. Return the number of bytes actually written. write repeats the writing operation until all bytes have been written or an error occurs. No half-sends, bestie. 💅
  • val write_bigarray — same as Unix.write, but take the data from a bigarray. Since 5.2 ✨
val write_bigarray : file_descr -> ('a, Bigarray.int8_unsigned_elt,
       Bigarray.c_layout) Bigarray.Array1.t -> int -> int -> int
  • val single_write : file_descr -> bytes -> int -> int -> int — same as Unix.write, but attempts to write only once. Thus, if an error occurs, single_write guarantees that no data has been written. It’s giving “one shot, no regrets” 🎯
  • val write_substring : file_descr -> string -> int -> int -> int — same as Unix.write, but take the data from a string instead of a byte sequence. Since 4.02 ✨
  • val single_write_substring : file_descr -> string -> int -> int -> int — same as Unix.single_write, but take the data from a string instead of a byte sequence. Since 4.02 ✨
  • val single_write_bigarray — same as Unix.single_write, but take the data from a bigarray. Since 5.2 ✨
val single_write_bigarray : file_descr -> ('a,
       Bigarray.int8_unsigned_elt, Bigarray.c_layout) Bigarray.Array1.t -> int
       -> int -> int

Interfacing with the standard input/output library 🌈

  • val in_channel_of_descr : file_descr -> in_channel — create an input channel reading from the given descriptor. The channel is initially in binary mode; use set_binary_mode_in ic false if text mode is desired. Text mode is supported only if the descriptor refers to a file or pipe, but is not supported if it refers to a socket.

On Windows: set_binary_mode_in always fails on channels created with this function. 💀

Beware that input channels are buffered, so more characters may have been read from the descriptor than those accessed using channel functions. Channels also keep a copy of the current position in the file.

Closing the channel ic returned by in_channel_of_descr fd using close_in ic also closes the underlying descriptor fd. It is incorrect to close both the channel ic and the descriptor fd.

If several channels are created on the same descriptor, one of the channels must be closed, but not the others. Consider for example a descriptor s connected to a socket and two channels ic = in_channel_of_descr s and oc = out_channel_of_descr s. The recommended closing protocol is to perform close_out oc, which flushes buffered output to the socket then closes the socket. The ic channel must not be closed and will be collected by the GC eventually. 🧚

  • val out_channel_of_descr : file_descr -> out_channel — create an output channel writing on the given descriptor. The channel is initially in binary mode; use set_binary_mode_out oc false if text mode is desired. Text mode is supported only if the descriptor refers to a file or pipe, but is not supported if it refers to a socket.

On Windows: set_binary_mode_out always fails on channels created with this function. 💀

Beware that output channels are buffered, so you may have to call flush to ensure that all data has been sent to the descriptor. Channels also keep a copy of the current position in the file.

Closing the channel oc returned by out_channel_of_descr fd using close_out oc also closes the underlying descriptor fd. It is incorrect to close both the channel ic and the descriptor fd.

See Unix.in_channel_of_descr for a discussion of the closing protocol when several channels are created on the same descriptor. 🫶

  • val descr_of_in_channel : in_channel -> file_descr — return the descriptor corresponding to an input channel.
  • val descr_of_out_channel : out_channel -> file_descr — return the descriptor corresponding to an output channel.

Seeking and truncating 🦋

type seek_command — positioning modes for Unix.lseek:

type seek_command =
 | SEEK_SET  (* indicates positions relative to the beginning of the
       file
 *)
 | SEEK_CUR  (* indicates positions relative to the current position
 *)
 | SEEK_END  (* indicates positions relative to the end of the file
 *)
  • val lseek : file_descr -> int -> seek_command -> int — set the current position for a file descriptor, and return the resulting offset (from the beginning of the file). Slide to the left, slide to the right 🕺
  • val truncate : string -> int -> unit — truncates the named file to the given size.
  • val ftruncate : file_descr -> int -> unit — truncates the file corresponding to the given descriptor to the given size.

File status 👑

type file_kind — the kind of file vibe check:

type file_kind =
 | S_REG  (* Regular file
 *)
 | S_DIR  (* Directory
 *)
 | S_CHR  (* Character device
 *)
 | S_BLK  (* Block device
 *)
 | S_LNK  (* Symbolic link
 *)
 | S_FIFO  (* Named pipe
 *)
 | S_SOCK  (* Socket
 *)

type stats — the information returned by the Unix.stat calls. The record is:

type stats = {
 st_dev : int ;  (* Device number
 *)
 st_ino : int ;  (* Inode number
 *)
 st_kind : file_kind ;  (* Kind of the file
 *)
 st_perm : file_perm ;  (* Access rights
 *)
 st_nlink : int ;  (* Number of links
 *)
 st_uid : int ;  (* User id of the owner
 *)
 st_gid : int ;  (* Group ID of the file's group
 *)
 st_rdev : int ;  (* Device ID (if special file)
 *)
 st_size : int ;  (* Size in bytes
 *)
 st_atime : float ;  (* Last access time
 *)
 st_mtime : float ;  (* Last modification time
 *)
 st_ctime : float ;  (* Last status change time
 *)
 }
  • val stat : string -> stats — return the information for the named file.
  • val lstat : string -> stats — same as Unix.stat, but in case the file is a symbolic link, return the information for the link itself. Sneaky link behavior, we love to see it. 🔗
  • val fstat : file_descr -> stats — return the information for the file associated with the given descriptor.
  • val isatty : file_descr -> bool — return true if the given file descriptor refers to a terminal or console window, false otherwise. Vibe check: are you a tty? 🖥️

File operations on large files 🌸

  • module LargeFile : sig end — file operations on large files. This sub-module provides 64-bit variants of the functions Unix.LargeFile.lseek (for positioning a file descriptor), Unix.LargeFile.truncate and Unix.LargeFile.ftruncate (for changing the size of a file), and Unix.LargeFile.stat, Unix.LargeFile.lstat and Unix.LargeFile.fstat (for obtaining information on files). These alternate functions represent positions and sizes by 64-bit integers (type int64) instead of regular integers (type int), thus allowing operating on files whose sizes are greater than max_int. Obsessed with the 64-bit glow-up. ✨

Mapping files into memory 🗺️

val map_file — the page gives us this signature and then dips:

val map_file : file_descr -> ?pos:int64 -> ('a, 'b) Bigarray.kind -> 'c
       Bigarray.layout -> bool -> int array -> ('a, 'b, 'c) Bigarray.Genarray.t

Okay so picture this: your file but as a Bigarray ✨ map_file fd kind layout shared dims returns a Bigarray of kind kind, layout layout, and dimensions as specified in dims. The data inside that Bigarray? Literally the contents of the file referred to by the file descriptor fd (as opened previously with Unix.openfile, for example 🌸). The optional pos parameter is the byte offset in the file of the data being mapped; it defaults to 0 (map from the beginning of the file, obviously 💅).

If shared is true, every single modification performed on the array is reflected in the file. This requires that fd be opened with write permissions — non-negotiable, bestie. If shared is false, modifications performed on the array are done in memory only, using copy-on-write of the modified pages; the underlying file is not affected, it's giving "what happens in memory stays in memory" 🫶.

Unix.map_file is much more efficient than reading the whole file in a Bigarray, modifying that Bigarray, and writing it afterwards. Like, not even the same sport 🔥.

To adjust automatically the dimensions of the Bigarray to the actual size of the file, the major dimension (that is, the first dimension for an array with C layout, and the last dimension for an array with Fortran layout) can be given as -1. Unix.map_file then determines the major dimension from the size of the file. The file must contain an integral number of sub-arrays as determined by the non-major dimensions, otherwise Failure is raised. Read that again, it matters 💀.

If all dimensions of the Bigarray are given, the file size is matched against the size of the Bigarray. If the file is larger than the Bigarray, only the initial portion of the file is mapped to the Bigarray. If the file is smaller than the big array, the file is automatically grown to the size of the Bigarray. This requires write permissions on fd, so plan accordingly 👑

Array accesses are bounds-checked, but the bounds are determined by the initial call to map_file. Therefore, you should make sure no other process modifies the mapped file while you're accessing it, or a SIGBUS signal may be raised 😭. This happens, for instance, if the file is shrunk. Rude and disrespectful.

Invalid_argument or Failure may be raised in cases where argument validation fails.

Since 4.06 ✨

🗂️ Operations on file names 🗂️

  • val unlink : string -> unit — Removes the named file. Gone, deleted, bye 👋

If the named file is a directory, raises:

  • EPERM on POSIX compliant system
  • EISDIR on Linux >= 2.1.132
  • EACCESS on Windows
  • val rename : string -> string -> unit — rename src dst changes the name of a file from src to dst, moving it between directories if needed. If dst already exists, its contents will be replaced with those of src. Depending on the operating system, the metadata (permissions, owner, etc) of dst can either be preserved or be replaced by those of src. OS-dependent behavior, iykyk.
  • val link : ?follow:bool -> string -> string -> unit — link ?follow src dst creates a hard link named dst to the file named src.

Raises ENOSYS on Unix if ~follow:_ is requested, but linkat is unavailable.

Raises ENOSYS on Windows if ~follow:false is requested.

  • val realpath : string -> string — realpath p is an absolute pathname for p obtained by resolving all extra / characters, relative path segments and symbolic links. It's giving "no shortcuts, only truth" 💅
Since 4.13 ✨

🔐 File permissions and ownership 🔐

type access_permission =
 | R_OK  (* Read permission
 *)
 | W_OK  (* Write permission
 *)
 | X_OK  (* Execution permission
 *)
 | F_OK  (* File exists
 *)

Flags for the Unix.access call. Four little icons for four little vibes 🎀

  • val chmod : string -> file_perm -> unit — Change the permissions of the named file.
  • val fchmod : file_descr -> file_perm -> unit — Change the permissions of an opened file. Raises Invalid_argument on Windows.
  • val chown : string -> int -> int -> unit — Change the owner uid and owner gid of the named file. Raises Invalid_argument on Windows.
  • val fchown : file_descr -> int -> int -> unit — Change the owner uid and owner gid of an opened file. Raises Invalid_argument on Windows.
  • val umask : file_perm -> file_perm — Set the process's file mode creation mask, and return the previous mask. Raises Invalid_argument on Windows.
  • val access : string -> access_permission list -> unit — Check that the process has the given permissions over the named file. On Windows: execute permission X_OK cannot be tested, just tests for read permission instead. Raises Unix_error otherwise.

🎛️ Operations on file descriptors 🎛️

  • val dup : ?cloexec:bool -> file_descr -> file_descr — Return a new file descriptor referencing the same file as the given descriptor. See Unix.set_close_on_exec for documentation on the cloexec optional argument.
  • val dup2 : ?cloexec:bool -> file_descr -> file_descr -> unit — dup2 src dst duplicates src to dst, closing dst if already opened. See Unix.set_close_on_exec for documentation on the cloexec optional argument.
  • val set_nonblock : file_descr -> unit — Set the “non-blocking” flag on the given descriptor. When the non-blocking flag is set, reading on a descriptor on which there is temporarily no data available raises the EAGAIN or EWOULDBLOCK error instead of blocking; writing on a descriptor on which there is temporarily no room for writing also raises EAGAIN or EWOULDBLOCK. No waiting around, we move 🦋
  • val clear_nonblock : file_descr -> unit — Clear the “non-blocking” flag on the given descriptor. See Unix.set_nonblock.

val set_close_on_exec : file_descr -> unit

Set the “close-on-exec” flag on the given descriptor. A descriptor with the close-on-exec flag is automatically closed when the current process starts another program with one of the exec, create_process and open_process functions.

It is often a security hole to leak file descriptors opened on, say, a private file to an external program: the program, then, gets access to the private file and can do bad things with it. Hence, it is highly recommended to set all file descriptors “close-on-exec”, except in the very few cases where a file descriptor actually needs to be transmitted to another program. Please, bestie, do this 🙏

The best way to set a file descriptor “close-on-exec” is to create it in this state. To this end, the openfile function has O_CLOEXEC and O_KEEPEXEC flags to enforce “close-on-exec” mode or “keep-on-exec” mode, respectively. All other operations in the Unix module that create file descriptors have an optional argument ?cloexec:bool to indicate whether the file descriptor should be created in “close-on-exec” mode (by writing ~cloexec:true) or in “keep-on-exec” mode (by writing ~cloexec:false). For historical reasons, the default file descriptor creation mode is “keep-on-exec”, if no cloexec optional argument is given. This is not a safe default, hence it is highly recommended to pass explicit cloexec arguments to operations that create file descriptors. Iconic advice, truly 💅

The cloexec optional arguments and the O_KEEPEXEC flag were introduced in OCaml 4.05. Earlier, the common practice was to create file descriptors in the default, “keep-on-exec” mode, then call set_close_on_exec on those freshly-created file descriptors. This is not as safe as creating the file descriptor in “close-on-exec” mode because, in multithreaded programs, a window of vulnerability exists between the time when the file descriptor is created and the time set_close_on_exec completes. If another thread spawns another program during this window, the descriptor will leak, as it is still in the “keep-on-exec” mode. 😭 The more you know.

Regarding the atomicity guarantees given by ~cloexec:true or by the use of the O_CLOEXEC flag: on all platforms it is guaranteed that a concurrently-executing Caml thread cannot leak the descriptor by starting a new process. On Linux, this guarantee extends to concurrently-executing C threads. As of Feb 2017, other operating systems lack the necessary system calls and still expose a window of vulnerability during which a C thread can see the newly-created file descriptor in “keep-on-exec” mode.

  • val clear_close_on_exec : file_descr -> unit — Clear the “close-on-exec” flag on the given descriptor. See Unix.set_close_on_exec.

📁 Directories 📁

  • val mkdir : string -> file_perm -> unit — Create a directory with the given permissions (see Unix.umask).
  • val rmdir : string -> unit — Remove an empty directory. Empty, bestie. It's not doing the emotional labor of deleting the contents for you.
  • val chdir : string -> unit — Change the process working directory.
  • val getcwd : unit -> string — Return the name of the current working directory. Where are we? Now you know 💖
  • val chroot : string -> unit — Change the process root directory. Raises Invalid_argument on Windows.
  • type dir_handle — The type of descriptors over opened directories.
  • val opendir : string -> dir_handle — Open a descriptor on a directory.
  • val readdir : dir_handle -> string — Return the next entry in a directory. Raises End_of_file when the end of the directory has been reached. That's the "no more entries" alarm 😭
  • val rewinddir : dir_handle -> unit — Reposition the descriptor to the beginning of the directory.
  • val closedir : dir_handle -> unit — Close a directory descriptor.

🧚 Pipes and redirections 🧚

  • val pipe : ?cloexec:bool -> unit -> file_descr * file_descr — Create a pipe. The first component of the result is opened for reading, that's the exit to the pipe. The second component is opened for writing, that's the entrance to the pipe. See Unix.set_close_on_exec for documentation on the cloexec optional argument.
  • val mkfifo : string -> file_perm -> unit — Create a named pipe with the given permissions (see Unix.umask). Raises Invalid_argument on Windows.

🚀 High-level process and redirection management 🚀

  • val create_process : string -> string array -> file_descr -> file_descr -> file_descr -> int — create_process prog args stdin stdout stderr creates a new process that executes the program in file prog, with arguments args. Note that the first argument, args.(0), is by convention the filename of the program being executed, just like Sys.argv.(0). The pid of the new process is returned immediately; the new process executes concurrently with the current process. The standard input and outputs of the new process are connected to the descriptors stdin, stdout and stderr. Passing e.g. Unix.stdout for stdout prevents the redirection and causes the new process to have the same standard output as the current process. The executable file prog is searched in the path. The new process has the same environment as the current process. Main character process energy 👑
  • val create_process_env : string -> string array -> string array -> file_descr -> file_descr -> file_descr -> int — create_process_env prog args env stdin stdout stderr works as Unix.create_process, except that the extra argument env specifies the environment passed to the program.
  • val open_process_in : string -> in_channel

High-level pipe and process management bestie 💖 This function runs the command you give it in parallel with your program. The command's standard output gets redirected into a pipe, and you read it through the input channel you get back. The command is interpreted by the shell /bin/sh (or cmd.exe on Windows), cf. Unix.system. The Filename.quote_command function is your bestie for quoting the command and its arguments the right way for whatever shell is being used. And if your command doesn't need to go through the shell at all, Unix.open_process_args_in is a more robust and more efficient alternative to Unix.open_process_in. Slay.

val open_process_out : string -> out_channel

Same vibe as Unix.open_process_in, but it redirects the command's standard input to a pipe instead. Whatever you write to the output channel you get back gets sent to the command's standard input. ⚠️ Warning: writes on output channels are buffered, so be careful and call flush at the right times to keep everything in sync. If the command doesn't need to run through the shell, Unix.open_process_args_out can be used instead of Unix.open_process_out.

val open_process : string -> in_channel * out_channel

Same as Unix.open_process_out, but it redirects both the standard input and standard output of the command into pipes connected to the two channels you get back. The input channel is connected to the command's output, and the output channel is connected to the command's input. If the command doesn't need the shell, Unix.open_process_args can be used instead of Unix.open_process.

val open_process_full : string -> string array -> in_channel *
out_channel * in_channel

Similar to Unix.open_process, but the second argument specifies the environment passed to the command. You get back a triple of channels connected respectively to the standard output, standard input, and standard error of the command. If the command doesn't need the shell, Unix.open_process_args_full can be used instead of Unix.open_process_full.

val open_process_args : string -> string array -> in_channel *
out_channel

open_process_args prog args runs the program prog with arguments args. Heads up: the first argument, args.(0), is by convention the filename of the program being executed, just like Sys.argv.(0). The new process executes concurrently with the current process. Its standard input and output are redirected to pipes, which you can respectively read and write through the returned channels. The input channel is connected to the output of the program, and the output channel is connected to the input of the program. 🔥

⚠️ Warning: writes on output channels are buffered, hence be careful to call flush at the right times to ensure correct synchronization.

The executable file prog is searched for in the path. This behaviour changed in 4.12; previously prog was looked up only in the current directory. (iykyk)

The new process has the same environment as the current process.

Since 4.08

val open_process_args_in : string -> string array -> in_channel

Same as Unix.open_process_args, but it redirects only the standard output of the new process.

Since 4.08

val open_process_args_out : string -> string array -> out_channel

Same as Unix.open_process_args, but it redirects only the standard input of the new process.

Since 4.08

val open_process_args_full : string -> string array -> string array ->
in_channel * out_channel * in_channel

Similar to Unix.open_process_args, but the third argument specifies the environment passed to the new process. The result is a triple of channels connected respectively to the standard output, standard input, and standard error of the program.

Since 4.08

val process_in_pid : in_channel -> int

Returns the pid of a process opened via Unix.open_process_args_in, or the pid of the shell opened via Unix.open_process_in.

Since 4.08 (4.12 in UnixLabels)

val process_out_pid : out_channel -> int

Returns the pid of a process opened via Unix.open_process_args_out, or the pid of the shell opened via Unix.open_process_out.

Since 4.08 (4.12 in UnixLabels)

val process_pid : in_channel * out_channel -> int

Returns the pid of a process opened via Unix.open_process_args, or the pid of the shell opened via Unix.open_process_args.

Since 4.08 (4.12 in UnixLabels)

val process_full_pid : in_channel * out_channel * in_channel -> int

Returns the pid of a process opened via Unix.open_process_args_full, or the pid of the shell opened via Unix.open_process_full.

Since 4.08 (4.12 in UnixLabels)

val close_process_in : in_channel -> process_status

Closes channels opened by Unix.open_process_in, waits for the associated command to terminate, and returns its termination status. 🫶

val close_process_out : out_channel -> process_status

Closes channels opened by Unix.open_process_out, waits for the associated command to terminate, and returns its termination status.

val close_process : in_channel * out_channel -> process_status

Closes channels opened by Unix.open_process, waits for the associated command to terminate, and returns its termination status.

val close_process_full : in_channel * out_channel * in_channel ->
process_status

Closes channels opened by Unix.open_process_full, waits for the associated command to terminate, and returns its termination status.

val symlink : ?to_dir:bool -> string -> string -> unit

symlink ?to_dir src dst creates the file dst as a symbolic link to the file src. On Windows, ~to_dir indicates whether the symbolic link points to a directory or to a file; if it's omitted, symlink examines src using stat and picks appropriately — and if src doesn't exist, then false is assumed. (Which is exactly why it's recommended that the ~to_dir parameter be specified in new code, bestie.) On Unix, ~to_dir is ignored.

Windows symbolic links are available from Windows Vista onwards. But there are some important differences between Windows symlinks and their POSIX counterparts. 👀

Windows symbolic links come in two flavours: directory and regular, which designate whether the symbolic link points to a directory or a file. The type must be correct — a directory symlink that actually points to a file cannot be selected with chdir, and a file symlink that actually points to a directory cannot be read or written. (Note that Cygwin's emulation layer ignores this distinction.)

When symbolic links are created to existing targets, this distinction doesn't matter and symlink will automatically create the correct kind of symbolic link. The distinction only matters when a symbolic link is created to a non-existent target.

The other caveat is that, by default, symbolic links are a privileged operation. Administrators will always need to be running elevated (or with UAC disabled), and by default normal user accounts need to be granted the SeCreateSymbolicLinkPrivilege via Local Security Policy (secpol.msc) or via Active Directory.

Unix.has_symlink can be used to check that a process is able to create symbolic links.

val has_symlink : unit -> bool

Returns true if the user is able to create symbolic links. On Windows, this indicates that the user not only has the SeCreateSymbolicLinkPrivilege but is also running elevated, if necessary. On other platforms, this simply indicates that the symlink system call is available.

Since 4.03

val readlink : string -> string

Read the contents of a symbolic link. ✨

🎀 Polling 🎀

val select : file_descr list -> file_descr list -> file_descr list ->
float -> file_descr list * file_descr list * file_descr list

Waits until some input/output operations become possible on some channels. The three list arguments are, respectively, a set of descriptors to check for reading (first argument), for writing (second argument), or for exceptional conditions (third argument). The fourth argument is the maximal timeout, in seconds — a negative fourth argument means no timeout at all, i.e. an unbounded wait. The result is composed of three sets of descriptors: those ready for reading (first component), ready for writing (second component), and those over which an exceptional condition is pending (third component).

On Windows, if one of the descriptor lists exceeds FD_SETSIZE elements (64 by default), or if at least one non-socket file descriptor is used, the maximal timeout is capped to 2^32 milliseconds. 😭

💅 Locking 💅

type lock_command =
 | F_ULOCK  (* Unlock a region
 *)
 | F_LOCK  (* Lock a region for writing, and block if already locked
 *)
 | F_TLOCK  (* Lock a region for writing, or fail if already locked
 *)
 | F_TEST  (* Test a region for other process locks
 *)
 | F_RLOCK  (* Lock a region for reading, and block if already locked
 *)
 | F_TRLOCK  (* Lock a region for reading, or fail if already locked
 *)

Commands for Unix.lockf. 👑

val lockf : file_descr -> lock_command -> int -> unit

lockf fd mode len puts a lock on a region of the file opened as fd. The region starts at the current read/write position for fd (as set by Unix.lseek), and extends len bytes forward if len is positive, len bytes backwards if len is negative, or all the way to the end of the file if len is zero. A write lock prevents any other process from acquiring a read or write lock on the region. A read lock prevents any other process from acquiring a write lock on the region, but it lets other processes acquire read locks on it. Periodt.

The F_LOCK and F_TLOCK commands attempt to put a write lock on the specified region. The F_RLOCK and F_TRLOCK commands attempt to put a read lock on the specified region. If one or several locks put by another process prevent the current process from acquiring the lock, F_LOCK and F_RLOCK block until these locks are removed, while F_TLOCK and F_TRLOCK fail immediately with an exception. The F_ULOCK removes whatever locks the current process has on the specified region. Finally, the F_TEST command tests whether a write lock can be acquired on the specified region, without actually putting a lock. It returns immediately if successful, or fails otherwise.

What happens when a process tries to lock a region of a file that is already locked by the same process? Lowkey depends on the OS. 🙈 On POSIX-compliant systems, the second lock operation succeeds and may "promote" the older lock from read lock to write lock. On Windows, the second lock operation will block or fail.

⭐ Signals ⭐

Note: installation of signal handlers is performed via the functions Sys.signal and Sys.set_signal.
val kill : int -> Sys.signal -> unit

kill pid signal sends signal number signal to the process with id pid.

On Windows: only the Sys.sigkill signal is emulated, causing the receiving process to exit with code ERROR_PROCESS_ABORTED (1067). Before OCaml 5.5, the receiving process exited with code 0. 💀

Signal masks continue, bestie — this is the part where we tell the OS which signals are allowed to interrupt the vibe. 💅✨

type sigprocmask_command =
 | SIG_SETMASK
 | SIG_BLOCK
 | SIG_UNBLOCK

val sigprocmask : sigprocmask_command -> Sys.signal list -> Sys.signal list
  • val sigprocmask : sigprocmask_command -> Sys.signal list -> Sys.signal list — okay so sigprocmask mode sigs changes the set of blocked signals. If mode is SIG_SETMASK, blocked signals are set to those in the list sigs. If mode is SIG_BLOCK, the signals in sigs are added to the set of blocked signals. If mode is SIG_UNBLOCK, the signals in sigs are removed from the set of blocked signals. sigprocmask returns the set of previously blocked signals. 🫶

Each domain, and each thread when the Thread module is loaded, has its own signal mask. sigprocmask only changes the mask of the current domain or current thread. No cross-thread mask meddling, periodt.

🚨 Raises Invalid_argument on Windows (no inter-process signals on Windows). Windows said “not today” and honestly it meant it. 💀
  • val sigpending : unit -> Sys.signal list — returns the set of blocked signals that are currently pending. Like the “seen but not replied” pile, but for signals. 🙈
🚨 Raises Invalid_argument on Windows (no inter-process signals on Windows).
  • val sigsuspend : Sys.signal list -> unit — sigsuspend sigs atomically sets the blocked signals to sigs and waits for a non-ignored, non-blocked signal to be delivered. On return, the blocked signals are reset to their initial value. ⚡
🚨 Raises Invalid_argument on Windows (no inter-process signals on Windows).
  • val pause : unit -> unit — waits until a non-ignored, non-blocked signal is delivered. Literally just vibing until the OS taps you on the shoulder. 😌
🚨 Raises Invalid_argument on Windows (no inter-process signals on Windows).
  • val sigwait : Sys.signal list -> Sys.signal — sigwait sigs waits until one of the signals in the list sigs becomes pending. It then removes this signal from the set of pending signals, and returns the number of this signal. Signal handlers attached to the signals in sigs will not be invoked. The signals sigs are expected to be blocked before calling sigwait. Main character signal behavior, lowkey. 👑

Since 5.4.

🚨 Raises Invalid_argument on Windows (no inter-process signals on Windows).

🕰️ Time functions 🕰️

Clocks, timers and calendar dates — the “what time is it even” section. ⏰✨

type process_times = {
 tms_utime : float ;  (* User time for the process
 *)
 tms_stime : float ;  (* System time for the process
 *)
 tms_cutime : float ;  (* User time for the children processes
 *)
 tms_cstime : float ;  (* System time for the children processes
 *)
 }

The execution times (CPU times) of a process. 🧮

type tm = {
 tm_sec : int ;  (* Seconds 0..60
 *)
 tm_min : int ;  (* Minutes 0..59
 *)
 tm_hour : int ;  (* Hours 0..23
 *)
 tm_mday : int ;  (* Day of month 1..31
 *)
 tm_mon : int ;  (* Month of year 0..11
 *)
 tm_year : int ;  (* Year - 1900
 *)
 tm_wday : int ;  (* Day of week (Sunday is 0)
 *)
 tm_yday : int ;  (* Day of year 0..365
 *)
 tm_isdst : bool ;  (* Daylight time savings in effect
 *)
 }

The type representing wallclock time and calendar date. 🗓️

  • val time : unit -> float — return the current time since 00:00:00 GMT, Jan. 1, 1970, in seconds. The classic Unix timestamp, no cap.
  • val gettimeofday : unit -> float — same as Unix.time, but with resolution better than 1 second. More precise, more iconic. 🔥
  • val gmtime : float -> tm — convert a time in seconds, as returned by Unix.time, into a date and a time. Assumes UTC (Coordinated Universal Time), also known as GMT. To perform the inverse conversion, set the TZ environment variable to "UTC", use Unix.mktime, and then restore the original value of TZ. 🧚
  • val localtime : float -> tm — convert a time in seconds, as returned by Unix.time, into a date and a time. Assumes the local time zone. The function performing the inverse conversion is Unix.mktime. Your timezone era starts now. 🌍
  • val mktime : tm -> float * tm — convert a date and time, specified by the tm argument, into a time in seconds, as returned by Unix.time. The tm_isdst, tm_wday and tm_yday fields of tm are ignored. Also return a normalized copy of the given tm record, with the tm_wday, tm_yday, and tm_isdst fields recomputed from the other fields, and the other fields normalized (so that, e.g., 40 October is changed into 9 November). The tm argument is interpreted in the local time zone. Time math but make it fashion. 🎀
  • val alarm : int -> int — schedule a SIGALRM signal after the given number of seconds. ⏰
🚨 Raises Invalid_argument on Windows.
  • val sleep : int -> unit — stop execution for the given number of seconds. Nap mode activated. 😴
  • val sleepf : float -> unit — stop execution for the given number of seconds. Like sleep, but fractions of seconds are supported. Fancy fractional napping. 🦋

Since 4.03 (4.12 in UnixLabels).

  • val times : unit -> process_times — return the execution times of the process. 📊

On Windows: partially implemented, will not report timings for child processes.

  • val utimes : string -> float -> float -> unit — set the last access time (second arg) and last modification time (third arg) for a file. Times are expressed in seconds from 00:00:00 GMT, Jan. 1, 1970. If both times are 0.0, the access and last modification times are both set to the current time. Touch grass, but for file metadata. 🌱
type interval_timer =
 | ITIMER_REAL  (* decrements in real time, and sends the signal SIGALRM when expired.
 *)
 | ITIMER_VIRTUAL  (* decrements in process virtual time, and sends SIGVTALRM when expired.
 *)
 | ITIMER_PROF  (* (for profiling) decrements both when the process is running and when the system is running on behalf of the process; it sends SIGPROF when expired.
 *)

The three kinds of interval timers. ⏱️

type interval_timer_status = {
 it_interval : float ;  (* Period
 *)
 it_value : float ;  (* Current value of the timer
 *)
 }

The type describing the status of an interval timer. 🎚️

  • val getitimer : interval_timer -> interval_timer_status — return the current status of the given interval timer.
🚨 Raises Invalid_argument on Windows.
  • val setitimer : interval_timer -> interval_timer_status -> interval_timer_status — setitimer t s sets the interval timer t and returns its previous status. The s argument is interpreted as follows: s.it_value, if nonzero, is the time to the next timer expiration; s.it_interval, if nonzero, specifies a value to be used in reloading it_value when the timer expires. Setting s.it_value to zero disables the timer. Setting s.it_interval to zero causes the timer to be disabled after its next expiration. ⏳
🚨 Raises Invalid_argument on Windows.

👑 User id, group id 👑

Identity check, bestie — who even are you, process-wise? 🪪✨

  • val getuid : unit -> int — return the user id of the user executing the process.

On Windows: always returns 1.

  • val geteuid : unit -> int — return the effective user id under which the process runs.

On Windows: always returns 1.

  • val setuid : int -> unit — set the real user id and effective user id for the process.
🚨 Raises Invalid_argument on Windows.
  • val getgid : unit -> int — return the group id of the user executing the process.

On Windows: always returns 1.

  • val getegid : unit -> int — return the effective group id under which the process runs.

On Windows: always returns 1.

  • val setgid : int -> unit — set the real group id and effective group id for the process.
🚨 Raises Invalid_argument on Windows.
  • val getgroups : unit -> int array — return the list of groups to which the user executing the process belongs.

On Windows: always returns [|1|].

  • val setgroups : int array -> unit — setgroups groups sets the supplementary group IDs for the calling process. Appropriate privileges are required. 👮‍♀️
🚨 Raises Invalid_argument on Windows.
  • val initgroups : string -> int -> unit — initgroups user group initializes the group access list by reading the group database /etc/group and using all groups of which user is a member. The additional group group is also added to the list. 🌸
🚨 Raises Invalid_argument on Windows.
type passwd_entry = {
 pw_name : string ;
 pw_passwd : string ;
 pw_uid : int ;
 pw_gid : int ;
 pw_gecos : string ;
 pw_dir : string ;
 pw_shell : string ;
 }

Structure of entries in the passwd database. 🗂️

type group_entry = {
 gr_name : string ;
 gr_passwd : string ;
 gr_gid : int ;
 gr_mem : string array ;
 }

Structure of entries in the groups database. 🗃️

  • val getlogin : unit -> string — return the login name of the user executing the process.
  • val getpwnam : string -> passwd_entry — find an entry in passwd with the given name.
🚨 Raises Not_found if no such entry exists, or always on Windows.
  • val getgrnam : string -> group_entry — find an entry in group with the given name.
🚨 Raises Not_found if no such entry exists, or always on Windows.
  • val getpwuid : int -> passwd_entry — find an entry in passwd with the given user id.
🚨 Raises Not_found if no such entry exists, or always on Windows.
  • val getgrgid : int -> group_entry — find an entry in group with the given group id.
🚨 Raises Not_found if no such entry exists, or always on Windows.

🌐 Internet addresses 🌐

IP addresses but make it abstract, bestie. 🛰️💖

type inet_addr

The abstract type of Internet addresses. 👑

  • val inet_addr_of_string : string -> inet_addr — conversion from the printable representation of an Internet address to its internal representation. The argument string consists of 4 numbers separated by periods (XXX.YYY.ZZZ.TTT) for IPv4 addresses, and up to 8 numbers separated by colons for IPv6 addresses. 🧚
🚨 Raises Failure when given a string that does not match these formats.
  • val string_of_inet_addr : inet_addr -> string — return the printable representation of the given Internet address. See Unix.inet_addr_of_string for a description of the printable representation.
  • val inet_addr_any : inet_addr — a special IPv4 address, for use only with bind, representing all the Internet addresses that the host machine possesses. 🌈
  • val inet_addr_loopback : inet_addr — a special IPv4 address representing the host machine (127.0.0.1). Home sweet loopback. 🏠
  • val inet6_addr_any : inet_addr — a special IPv6 address, for use only with bind, representing all the Internet addresses that the host machine possesses. 🌐
  • val inet6_addr_loopback : inet_addr — a special IPv6 address representing the host machine (::1). ✨
  • val is_inet6_addr : inet_addr -> bool — whether the given inet_addr is an IPv6 address. 🕵️‍♀️

Since 4.12.

🔌 Sockets 🔌

Sockets are where the networking girlies live, and these are the domain and type menus. 🫶

type socket_domain =
 | PF_UNIX  (* Unix domain
 *)
 | PF_INET  (* Internet domain (IPv4)
 *)
 | PF_INET6  (* Internet domain (IPv6)
 *)

The type of socket domains. Not all platforms support IPv6 sockets (type PF_INET6). 😭

On Windows: PF_UNIX supported since 4.14.0 on Windows 10 1803 and later. 💀✨

type socket_type =
 | SOCK_STREAM  (* Stream socket
 *)
 | SOCK_DGRAM  (* Datagram socket
 *)
 | SOCK_RAW  (* Raw socket
 *)
 | SOCK_SEQPACKET  (* Sequenced packets socket
 *)

okay so the socket kinds? They're literally the semantics of how the communication works. SOCK_SEQPACKET is in here for completeness, but bestie, it's rarely supported by the OS, and it needs system calls that are not available in this library. 💅

type sockaddr =
 | ADDR_UNIX of string
 | ADDR_INET of inet_addr * int

This is the type of socket addresses, and it's giving "where are we even talking from" energy. ADDR_UNIX name is a socket address in the Unix domain; name is a file name in the file system. ADDR_INET(addr,port) is a socket address in the Internet domain; addr is the Internet address of the machine, and port is the port number. 🦋

val socket : ?cloexec:bool -> socket_domain -> socket_type -> int -> file_descr

Creates a new socket in the given domain, and with the given kind. The third argument is the protocol type; 0 selects the default protocol for that kind of sockets. See Unix.set_close_on_exec for documentation on the cloexec optional argument. ✨

val domain_of_sockaddr : sockaddr -> socket_domain

Returns the socket domain adequate for the given socket address. Lowkey handy. 🎀

val socketpair : ?cloexec:bool -> socket_domain -> socket_type -> int -> file_descr * file_descr

Creates a pair of unnamed sockets, connected together. See Unix.set_close_on_exec for documentation on the cloexec optional argument. Two little descriptors, holding hands. 🫶

val accept : ?cloexec:bool -> file_descr -> file_descr * sockaddr

Accepts connections on the given socket. The returned descriptor is a socket connected to the client; the returned address is the address of the connecting client. See Unix.set_close_on_exec for documentation on the cloexec optional argument. 💖

val bind : file_descr -> sockaddr -> unit

Binds a socket to an address. That's it, that's the whole vibe. 🪢

val connect : file_descr -> sockaddr -> unit

Connects a socket to an address. Slay. 🔌

val listen : file_descr -> int -> unit

Sets up a socket for receiving connection requests. The integer argument is the maximal number of pending requests. It's giving "line forms here" energy. 👑

type shutdown_command =
 | SHUTDOWN_RECEIVE  (* Close for receiving
 *)
 | SHUTDOWN_SEND  (* Close for sending
 *)
 | SHUTDOWN_ALL  (* Close both
 *)

These are the commands for shutdown. Yes bestie, the type is literally called shutdown_command. 💀

val shutdown : file_descr -> shutdown_command -> unit

Shuts down a socket connection. SHUTDOWN_SEND as second argument causes reads on the other end of the connection to return an end-of-file condition. SHUTDOWN_RECEIVE causes writes on the other end of the connection to return a closed pipe condition (SIGPIPE signal). So yeah, it's real. 😭

val getsockname : file_descr -> sockaddr

Returns the address of the given socket. 🏷️

val getpeername : file_descr -> sockaddr

Returns the address of the host connected to the given socket. iykyk. 👀

type msg_flag =
 | MSG_OOB
 | MSG_DONTROUTE
 | MSG_PEEK

These are the flags for Unix.recv, Unix.recvfrom, Unix.send and Unix.sendto. Flag city. 🚩

val recv : file_descr -> bytes -> int -> int -> msg_flag list -> int

Receives data from a connected socket. 📥

val recvfrom : file_descr -> bytes -> int -> int -> msg_flag list -> int * sockaddr

Receives data from an unconnected socket. Plus it hands you the address too, main character behavior. 📬

val send : file_descr -> bytes -> int -> int -> msg_flag list -> int

Sends data over a connected socket. 📤

val send_substring : file_descr -> string -> int -> int -> msg_flag list -> int

Same as send, but takes the data from a string instead of a byte sequence. Since 4.02 ✨

val sendto : file_descr -> bytes -> int -> int -> msg_flag list -> sockaddr -> int

Sends data over an unconnected socket. 🛫

val sendto_substring : file_descr -> string -> int -> int -> msg_flag list -> sockaddr -> int

Same as sendto, but takes the data from a string instead of a byte sequence. Since 4.02 🍓

💅 Socket options 💅

type socket_bool_option =
 | SO_DEBUG  (* Record debugging information
 *)
 | SO_BROADCAST  (* Permit sending of broadcast messages
 *)
 | SO_REUSEADDR  (* Allow reuse of local addresses for bind
 *)
 | SO_KEEPALIVE  (* Keep connection active
 *)
 | SO_DONTROUTE  (* Bypass the standard routing algorithms
 *)
 | SO_OOBINLINE  (* Leave out-of-band data in line
 *)
 | SO_ACCEPTCONN  (* Report whether socket listening is enabled
 *)
 | TCP_NODELAY  (* Control the Nagle algorithm for TCP sockets
 *)
 | IPV6_ONLY  (* Forbid binding an IPv6 socket to an IPv4 address
 *)
 | SO_REUSEPORT  (* Allow reuse of address and port bindings.

Since 4.12.
 *)

These are the socket options that can be consulted with Unix.getsockopt and modified with Unix.setsockopt. These options have a boolean (true / false) value. So it's very yes/no, very vibe check. ✅❌

type socket_int_option =
 | SO_SNDBUF  (* Size of send buffer
 *)
 | SO_RCVBUF  (* Size of received buffer
 *)
 | SO_ERROR  (* .B "Deprecated."  Use Unix.getsockopt_error instead.

Deprecated.  Use Unix.getsockopt_error instead.
 *)
 | SO_TYPE  (* Report the socket type
 *)
 | SO_RCVLOWAT  (* Minimum number of bytes to process for input
operations
 *)
 | SO_SNDLOWAT  (* Minimum number of bytes to process for output
operations
 *)

These are the socket options that can be consulted with Unix.getsockopt_int and modified with Unix.setsockopt_int. These options have an integer value. Numbers, bestie. 🔢

heads up: SO_ERROR is literally marked deprecated — use Unix.getsockopt_error instead. Not me reading the .B line, it's in the original. 💀
type socket_optint_option =
 | SO_LINGER  (* Whether to linger on closed connections that have data
present, and for how long (in seconds)
 *)

These are the socket options that can be consulted with Unix.getsockopt_optint and modified with Unix.setsockopt_optint. These options have a value of type int option, with None meaning “disabled”. So it's int, but optional, but also iconic. 🎀

type socket_float_option =
 | SO_RCVTIMEO  (* Timeout for input operations
 *)
 | SO_SNDTIMEO  (* Timeout for output operations
 *)

These are the socket options that can be consulted with Unix.getsockopt_float and modified with Unix.setsockopt_float. These options have a floating-point value representing a time in seconds. The value 0 means infinite timeout. Timeout but make it float. ⏳

val getsockopt : file_descr -> socket_bool_option -> bool

Returns the current status of a boolean-valued option in the given socket. 👀

val setsockopt : file_descr -> socket_bool_option -> bool -> unit

Sets or clears a boolean-valued option in the given socket. 💅

val getsockopt_int : file_descr -> socket_int_option -> int

Same as Unix.getsockopt for an integer-valued socket option. 🔢

val setsockopt_int : file_descr -> socket_int_option -> int -> unit

Same as Unix.setsockopt for an integer-valued socket option. ✨

val getsockopt_optint : file_descr -> socket_optint_option -> int option

Same as Unix.getsockopt for a socket option whose value is an int option. 🫶

val setsockopt_optint : file_descr -> socket_optint_option -> int option -> unit

Same as Unix.setsockopt for a socket option whose value is an int option. 💖

val getsockopt_float : file_descr -> socket_float_option -> float

Same as Unix.getsockopt for a socket option whose value is a floating-point number. 🌊

val setsockopt_float : file_descr -> socket_float_option -> float -> unit

Same as Unix.setsockopt for a socket option whose value is a floating-point number. 🧚

val getsockopt_error : file_descr -> error option

Returns the error condition associated with the given socket, and clears it. So it's like "here's the problem, and now it's gone." 😭

📞 High-level network connection functions 📞

val open_connection : sockaddr -> in_channel * out_channel

Connects to a server at the given address. Returns a pair of buffered channels connected to the server. Remember to call flush on the output channel at the right times to ensure correct synchronization. Don't forget, bestie. 🫶

The two channels returned by open_connection share a descriptor to a socket. Therefore, when the connection is over, you should call close_out on the output channel, which will also close the underlying socket. Do not call close_in on the input channel; it will be collected by the GC eventually. So yes, close_out girlboss, leave the input channel alone. 💅

val shutdown_connection : in_channel -> unit

“Shut down” a connection established with Unix.open_connection; that is, transmit an end-of-file condition to the server reading on the other side of the connection. This does not close the socket and the channels used by the connection. See Unix.open_connection for how to close them once the connection is over. 📴

val establish_server : (in_channel -> out_channel -> unit) -> sockaddr -> unit

Establishes a server on the given address. The function given as first argument is called for each connection with two buffered channels connected to the client. A new process is created for each connection. The function Unix.establish_server never returns normally. Never. Returns. Normally. 😭

The two channels given to the function share a descriptor to a socket. The function does not need to close the channels, since this occurs automatically when the function returns. If the function prefers explicit closing, it should close the output channel using close_out and leave the input channel unclosed, for reasons explained in Unix.in_channel_of_descr. So if you're an explicit-closing kind of bestie, output channel only. ✨

Raises Invalid_argument on Windows. Use threads instead. 🪟💀

🗂️ Host and protocol databases 🗂️

type host_entry = {
 h_name : string ;
 h_aliases : string array ;
 h_addrtype : socket_domain ;
 h_addr_list : inet_addr array ;
 }

Structure of entries in the hosts database. This is the shape of a hosts entry, periodt. 🏠

type protocol_entry = {
 p_name : string ;
 p_aliases : string array ;
 p_proto : int ;
 }

Structure of entries in the protocols database. Protocols but make it a record. 📜

type service_entry = {
 s_name : string ;
 s_aliases : string array ;
 s_port : int ;
 s_proto : string ;
 }

Structure of entries in the services database. Services bestie, look at you. 🛎️

val gethostname : unit -> string

Returns the name of the local host. Hi, me. 👋

val gethostbyname : string -> host_entry

Finds an entry in hosts with the given name. Raises Not_found if no such entry exists. 😭

val gethostbyaddr : inet_addr -> host_entry

Finds an entry in hosts with the given address. Raises Not_found if no such entry exists. 📍

val getprotobyname : string -> protocol_entry

Finds an entry in protocols with the given name. Raises Not_found if no such entry exists. 🔎

val getprotobynumber : int -> protocol_entry

Finds an entry in protocols with the given protocol number. Raises Not_found if no such entry exists. 🔢

val getservbyname : string -> string -> service_entry

Finds an entry in services with the given name. Raises Not_found if no such entry exists. 🧾

val getservbyport : int -> string -> service_entry

Finds an entry in services with the given service number. Raises Not_found if no such entry exists. 🚪

type addr_info = {
 ai_family : socket_domain ;  (* Socket domain
 *)
 ai_socktype : socket_type ;  (* Socket type
 *)
 ai_protocol : int ;  (* Socket protocol number
 *)
 ai_addr : sockaddr ;  (* Address
 *)
 ai_canonname : string ;  (* Canonical host name
 *)
 }

Address info returned by Unix.getaddrinfo? yeah bestie, that’s the whole little socket-tea record. 💖

type getaddrinfo_option =
 | AI_FAMILY of socket_domain
  (* Impose the given socket domain
 *)
 | AI_SOCKTYPE of socket_type
  (* Impose the given socket type
 *)
 | AI_PROTOCOL of int
  (* Impose the given protocol
 *)
 | AI_NUMERICHOST  (* Do not call name resolver, expect numeric IP
address
 *)
 | AI_CANONNAME  (* Fill the ai_canonname field of the result
 *)
 | AI_PASSIVE  (* Set address to ``any'' address for use with Unix.bind

 *)

These are the options you feed to Unix.getaddrinfo, and they are lowkey iconic. 🎀

  • AI_FAMILY of socket_domain — imposes the given socket domain.
  • AI_SOCKTYPE of socket_type — imposes the given socket type.
  • AI_PROTOCOL of int — imposes the given protocol.
  • AI_NUMERICHOST — do not call the name resolver; expect a numeric IP address. like, no DNS detours, babe.
  • AI_CANONNAME — fills the ai_canonname field of the result.
  • AI_PASSIVE — sets the address to the "any" address, for use with Unix.bind.
val getaddrinfo : string -> string -> getaddrinfo_option list ->
       addr_info list

getaddrinfo host service opts returns a list of Unix.addr_info records describing socket parameters and addresses suitable for communicating with the given host and service. The empty list is returned if the host or service names are unknown, or if the constraints expressed in opts cannot be satisfied. no cap, that’s the deal. 💅

  • host is either a host name or the string representation of an IP address. host can be given as the empty string; in this case, the "any" address or the "loopback" address is used, depending on whether opts contains AI_PASSIVE.
  • service is either a service name or the string representation of a port number. service can be given as the empty string; in this case, the port field of the returned addresses is set to 0.
  • opts is a possibly empty list of options that lets the caller force a particular socket domain (e.g. IPv6 only or IPv4 only) or a particular socket type (e.g. TCP only or UDP only).
type name_info = {
 ni_hostname : string ;  (* Name or IP address of host
 *)
 ni_service : string ;  (* Name of service or port number
 *)
 }

Host and service info returned by Unix.getnameinfo — cute little pair, honestly. 🦋

type getnameinfo_option =
 | NI_NOFQDN  (* Do not qualify local host names
 *)
 | NI_NUMERICHOST  (* Always return host as IP address
 *)
 | NI_NAMEREQD  (* Fail if host name cannot be determined
 *)
 | NI_NUMERICSERV  (* Always return service as port number
 *)
 | NI_DGRAM  (* Consider the service as UDP-based instead of the default
TCP
 *)

Options to Unix.getnameinfo, and yes they all have main-character energy: ✨

  • NI_NOFQDN — do not qualify local host names.
  • NI_NUMERICHOST — always return the host as an IP address.
  • NI_NAMEREQD — fail if the host name cannot be determined.
  • NI_NUMERICSERV — always return the service as a port number.
  • NI_DGRAM — consider the service as UDP-based instead of the default TCP.
val getnameinfo : sockaddr -> getnameinfo_option list -> name_info

getnameinfo addr opts returns the host name and service name corresponding to the socket address addr. opts is a possibly empty list of options that governs how these names are obtained.

Raises Not_found if an error occurs. 😭 facts are facts.

🖥️ Terminal interface 🖥️

The following functions implement the POSIX standard terminal interface. They provide control over asynchronous communication ports and pseudo-terminals. Refer to the termios man page for a complete description. it’s giving very “I control the terminal now” energy. 💅

type terminal_io = {

 mutable c_ignbrk : bool ;  (* Ignore the break condition.
  *)

 mutable c_brkint : bool ;  (* Signal interrupt on break condition.
  *)

 mutable c_ignpar : bool ;  (* Ignore characters with parity errors.
  *)

 mutable c_parmrk : bool ;  (* Mark parity errors.
  *)

 mutable c_inpck : bool ;  (* Enable parity check on input.
  *)

 mutable c_istrip : bool ;  (* Strip 8th bit on input characters.
  *)

 mutable c_inlcr : bool ;  (* Map NL to CR on input.
  *)

 mutable c_igncr : bool ;  (* Ignore CR on input.
  *)

 mutable c_icrnl : bool ;  (* Map CR to NL on input.
  *)

 mutable c_ixon : bool ;  (* Recognize XON/XOFF characters on input.
  *)

 mutable c_ixoff : bool ;  (* Emit XON/XOFF chars to control input flow.
  *)

 mutable c_opost : bool ;  (* Enable output processing.
  *)

 mutable c_obaud : int ;  (* Output baud rate (0 means close connection).
  *)

 mutable c_ibaud : int ;  (* Input baud rate.
  *)

 mutable c_csize : int ;  (* Number of bits per character (5-8).
  *)

 mutable c_cstopb : int ;  (* Number of stop bits (1-2).
  *)

 mutable c_cread : bool ;  (* Reception is enabled.
  *)

 mutable c_parenb : bool ;  (* Enable parity generation and detection.
  *)

 mutable c_parodd : bool ;  (* Specify odd parity instead of even.
  *)

 mutable c_hupcl : bool ;  (* Hang up on last close.
  *)

 mutable c_clocal : bool ;  (* Ignore modem status lines.
  *)

 mutable c_isig : bool ;  (* Generate signal on INTR, QUIT, SUSP.
  *)

 mutable c_icanon : bool ;  (* Enable canonical processing (line
 buffering and editing)
  *)

 mutable c_noflsh : bool ;  (* Disable flush after INTR, QUIT, SUSP.
  *)

 mutable c_echo : bool ;  (* Echo input characters.
  *)

 mutable c_echoe : bool ;  (* Echo ERASE (to erase previous character).
  *)

 mutable c_echok : bool ;  (* Echo KILL (to erase the current line).
  *)

 mutable c_echonl : bool ;  (* Echo NL even if c_echo is not set.
  *)

 mutable c_vintr : char ;  (* Interrupt character (usually ctrl-C).
  *)

 mutable c_vquit : char ;  (* Quit character (usually ctrl-\).
  *)

 mutable c_verase : char ;  (* Erase character (usually DEL or ctrl-H).
  *)

 mutable c_vkill : char ;  (* Kill line character (usually ctrl-U).
  *)

 mutable c_veof : char ;  (* End-of-file character (usually ctrl-D).
  *)

 mutable c_veol : char ;  (* Alternate end-of-line char. (usually none).
  *)

 mutable c_vmin : int ;  (* Minimum number of characters to read before
 the read request is satisfied.
  *)

 mutable c_vtime : int ;  (* Maximum read wait (in 0.1s units).
  *)

 mutable c_vstart : char ;  (* Start character (usually ctrl-Q).
  *)

 mutable c_vstop : char ;  (* Stop character (usually ctrl-S).
  *)
  }

Quick field-by-field vibe check, bestie — every single one is here: 🧚

  • c_ignbrk — ignore the break condition.
  • c_brkint — signal interrupt on break condition.
  • c_ignpar — ignore characters with parity errors.
  • c_parmrk — mark parity errors.
  • c_inpck — enable parity check on input.
  • c_istrip — strip 8th bit on input characters.
  • c_inlcr — map NL to CR on input.
  • c_igncr — ignore CR on input.
  • c_icrnl — map CR to NL on input.
  • c_ixon — recognize XON/XOFF characters on input.
  • c_ixoff — emit XON/XOFF chars to control input flow.
  • c_opost — enable output processing.
  • c_obaud — output baud rate (0 means close connection).
  • c_ibaud — input baud rate.
  • c_csize — number of bits per character (5-8).
  • c_cstopb — number of stop bits (1-2).
  • c_cread — reception is enabled.
  • c_parenb — enable parity generation and detection.
  • c_parodd — specify odd parity instead of even.
  • c_hupcl — hang up on last close.
  • c_clocal — ignore modem status lines.
  • c_isig — generate signal on INTR, QUIT, SUSP.
  • c_icanon — enable canonical processing (line buffering and editing).
  • c_noflsh — disable flush after INTR, QUIT, SUSP.
  • c_echo — echo input characters.
  • c_echoe — echo ERASE (to erase previous character).
  • c_echok — echo KILL (to erase the current line).
  • c_echonl — echo NL even if c_echo is not set.
  • c_vintr — interrupt character (usually Ctrl+C).
  • c_vquit — quit character (usually Ctrl+\).
  • c_verase — erase character (usually DEL or Ctrl+H).
  • c_vkill — kill line character (usually Ctrl+U).
  • c_veof — end-of-file character (usually Ctrl+D).
  • c_veol — alternate end-of-line char. (usually none).
  • c_vmin — minimum number of characters to read before the read request is satisfied.
  • c_vtime — maximum read wait (in 0.1s units).
  • c_vstart — start character (usually Ctrl+Q).
  • c_vstop — stop character (usually Ctrl+S).
val tcgetattr : file_descr -> terminal_io

Return the status of the terminal referred to by the given file descriptor.

Raises Invalid_argument on Windows. 🪟 not the vibe, but we move.
type setattr_when =
 | TCSANOW
 | TCSADRAIN
 | TCSAFLUSH
val tcsetattr : file_descr -> setattr_when -> terminal_io -> unit

Set the status of the terminal referred to by the given file descriptor. The second argument indicates when the status change takes place: immediately (TCSANOW), when all pending output has been transmitted (TCSADRAIN), or after flushing all input that has been received but not read (TCSAFLUSH). TCSADRAIN is recommended when changing the output parameters; TCSAFLUSH, when changing the input parameters. periodt. 💅

Raises Invalid_argument on Windows.
val tcsendbreak : file_descr -> int -> unit

Send a break condition on the given file descriptor. The second argument is the duration of the break, in 0.1s units; 0 means standard duration (0.25s). yes, it’s giving dramatic pause. 😭

Raises Invalid_argument on Windows.
val tcdrain : file_descr -> unit

Waits until all output written on the given file descriptor has been transmitted. like, hold up, let it finish. ✨

Raises Invalid_argument on Windows.
type flush_queue =
 | TCIFLUSH
 | TCOFLUSH
 | TCIOFLUSH
val tcflush : file_descr -> flush_queue -> unit

Discard data written on the given file descriptor but not yet transmitted, or data received but not yet read, depending on the second argument: TCIFLUSH flushes data received but not read, TCOFLUSH flushes data written but not transmitted, and TCIOFLUSH flushes both. choose your fighter. 🎀

Raises Invalid_argument on Windows.
type flow_action =
 | TCOOFF
 | TCOON
 | TCIOFF
 | TCION
val tcflow : file_descr -> flow_action -> unit

Suspend or restart reception or transmission of data on the given file descriptor, depending on the second argument: TCOOFF suspends output, TCOON restarts output, TCIOFF transmits a STOP character to suspend input, and TCION transmits a START character to restart input. flow control but make it fashion. 🌈

Raises Invalid_argument on Windows.
val setsid : unit -> int

Put the calling process in a new session and detach it from its controlling terminal. main character moment: new session, who dis? 👑

Raises Invalid_argument on Windows.

and that’s the whole Unix(3) page, bestie! 🫶 source/version: OCamldoc, date: 2026-06-22 — iconic, no notes. 💅✨

📦 where this page comes from