open(2)

💖 open(2) — System Calls Manual 💖

🌟 NAME 🌟

okay so open, openat, and creat — they open and possibly create a file. that's the whole vibe. periodt. 💅

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🎀 LIBRARY 🎀

Standard C library (libc, -lc). yes bestie, it's in libc. iconic. ✨

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✨ SYNOPSIS ✨

#include <fcntl.h>

int open(const char *path, int flags, ...
           /* mode_t mode */ );

int creat(const char *path, mode_t mode);

int openat(int dirfd, const char *path, int flags, ...
           /* mode_t mode */ );

/* Documented separately, in openat2(2): */
int openat2(int dirfd, const char *path,
           const struct open_how *how, size_t size);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

openat():
    Since glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Before glibc 2.10:
        _ATFILE_SOURCE

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💅 DESCRIPTION 💅

The open() system call opens the file specified by path. If the specified file does not exist, it may optionally (if O_CREAT is specified in flags) be created by open(). so yes bestie, you can make a whole new file if you want. 🌸

The return value of open() is a file descriptor, a small, nonnegative integer that is an index to an entry in the process's table of open file descriptors. The file descriptor is used in subsequent system calls (read(2), write(2), lseek(2), fcntl(2), etc.) to refer to the open file. The file descriptor returned by a successful call will be the lowest-numbered file descriptor not currently open for the process. it's like a lil ticket number but for files. 🎟️

By default, the new file descriptor is set to remain open across execve(2) (i.e., the FD_CLOEXEC file descriptor flag described in fcntl(2) is initially disabled); the O_CLOEXEC flag, described below, can be used to change this default. The file offset is set to the beginning of the file (see lseek(2)).

A call to open() creates a new open file description, an entry in the system-wide table of open files. The open file description records the file offset and the file status flags (see below). A file descriptor is a reference to an open file description; this reference is unaffected if path is subsequently removed or modified to refer to a different file. For further details on open file descriptions, see NOTES. so like, the FD stays valid even if the path changes. that's the tea. ☕

The argument flags must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR. These request opening the file read-only, write-only, or read/write, respectively. pick one, bestie. read-only, write-only, or both. no in-between. 💅

In addition, zero or more file creation flags and file status flags can be bitwise ORed in flags. The file creation flags are O_CLOEXEC, O_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW, O_TMPFILE, and O_TRUNC. The file status flags are all of the remaining flags listed below. The distinction between these two groups of flags is that the file creation flags affect the semantics of the open operation itself, while the file status flags affect the semantics of subsequent I/O operations. The file status flags can be retrieved and (in some cases) modified; see fcntl(2) for details. so creation flags = the open call itself, status flags = the I/O after. got it? 🧠

The full list of file creation flags and file status flags is as follows:

• O_APPEND — The file is opened in append mode. Before each write(2), the file offset is positioned at the end of the file, as if with lseek(2). The modification of the file offset and the write operation are performed as a single atomic step.

⚠️ O_APPEND may lead to corrupted files on NFS filesystems if more than one process appends data to a file at once. This is because NFS does not support appending to a file, so the client kernel has to simulate it, which can't be done without a race condition.

so like, append mode but NFS said "not me". be careful bestie. 😭

• O_ASYNC — Enable signal-driven I/O: generate a signal (SIGIO by default, but this can be changed via fcntl(2)) when input or output becomes possible on this file descriptor. This feature is available only for terminals, pseudoterminals, sockets, and (since Linux 2.6) pipes and FIFOs. See fcntl(2) for further details. See also BUGS, below. it's giving "text me when you're ready" but for file descriptors. 📲
• O_CLOEXEC (since Linux 2.6.23) — Enable the close-on-exec flag for the new file descriptor. Specifying this flag permits a program to avoid additional fcntl(2) F_SETFD operations to set the FD_CLOEXEC flag.

Note that the use of this flag is essential in some multithreaded programs, because using a separate fcntl(2) F_SETFD operation to set the FD_CLOEXEC flag does not suffice to avoid race conditions where one thread opens a file descriptor and attempts to set its close-on-exec flag using fcntl(2) at the same time as another thread does a fork(2) plus execve(2). Depending on the order of execution, the race may lead to the file descriptor returned by open() being unintentionally leaked to the program executed by the child process created by fork(2). (This kind of race is in principle possible for any system call that creates a file descriptor whose close-on-exec flag should be set, and various other Linux system calls provide an equivalent of the O_CLOEXEC flag to deal with this problem.)

so if you're multithreaded, set this or risk leaking FDs like omg. 🔥

• O_CREAT — If path does not exist, create it as a regular file.

The owner (user ID) of the new file is set to the effective user ID of the process.

The group ownership (group ID) of the new file is set either to the effective group ID of the process (System V semantics) or to the group ID of the parent directory (BSD semantics). On Linux, the behavior depends on whether the set-group-ID mode bit is set on the parent directory: if that bit is set, then BSD semantics apply; otherwise, System V semantics apply. For some filesystems, the behavior also depends on the bsdgroups and sysvgroups mount options described in mount(8).

The mode argument specifies the file mode bits to be applied when a new file is created. If neither O_CREAT nor O_TMPFILE is specified in flags, then mode is ignored (and can thus be specified as 0, or simply omitted). The mode argument must be supplied if O_CREAT or O_TMPFILE is specified in flags; if it is not supplied, some arbitrary bytes from the stack will be applied as the file mode.

The effective mode is modified by the process's umask in the usual way: in the absence of a default ACL, the mode of the created file is (mode & ~umask).

Note that mode applies only to future accesses of the newly created file; the open() call that creates a read-only file may well return a read/write file descriptor.

The following symbolic constants are provided for mode:

• S_IRWXU 00700 — user (file owner) has read, write, and execute permission
• S_IRUSR 00400 — user has read permission
• S_IWUSR 00200 — user has write permission
• S_IXUSR 00100 — user has execute permission
• S_IRWXG 00070 — group has read, write, and execute permission
• S_IRGRP 00040 — group has read permission
• S_IWGRP 00020 — group has write permission
• S_IXGRP 00010 — group has execute permission
• S_IRWXO 00007 — others have read, write, and execute permission
• S_IROTH 00004 — others have read permission
• S_IWOTH 00002 — others have write permission
• S_IXOTH 00001 — others have execute permission

According to POSIX, the effect when other bits are set in mode is unspecified. On Linux, the following bits are also honored in mode:

• S_ISUID 0004000 — set-user-ID bit
• S_ISGID 0002000 — set-group-ID bit (see inode(7))
• S_ISVTX 0001000 — sticky bit (see inode(7))

slay, permissions. don't forget your umask. 💅

• O_DIRECT (since Linux 2.4.10) — Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance, but it is useful in special situations, such as when applications do their own caching. File I/O is done directly to/from user-space buffers. The O_DIRECT flag on its own makes an effort to transfer data synchronously, but does not give the guarantees of the O_SYNC flag that data and necessary metadata are transferred. To guarantee synchronous I/O, O_SYNC must be used in addition to O_DIRECT. See NOTES below for further discussion.

A semantically similar (but deprecated) interface for block devices is described in raw(8).

it's giving "I'll do it myself" energy. but watch out for performance. 😬

• O_DIRECTORY — If path is not a directory, cause open() to fail. This flag was added in Linux 2.1.126, to avoid denial-of-service problems if opendir(3) is called on a FIFO or tape device. no opening random FIFOs as directories, bestie. that's a no. 🚫
• O_DSYNC — Write operations on the file will complete according to the requirements of synchronized I/O data integrity completion.

By the time write(2) (and similar) return, the output data has been transferred to the underlying hardware, along with any file metadata that would be required to retrieve that data (i.e., as though each write(2) was followed by a call to fdatasync(2)). See VERSIONS.

so your data is actually on disk before write returns. periodt. 💅

• O_EXCL — Ensure that this call creates the file: if this flag is specified in conjunction with O_CREAT, and path already exists, then open() fails with the error EEXIST.

When these two flags are specified, symbolic links are not followed: if path is a symbolic link, then open() fails regardless of where the symbolic link points.

In general, the behavior of O_EXCL is undefined if it is used without O_CREAT. There is one exception: on Linux 2.6 and later, O_EXCL can be used without O_CREAT if path refers to a block device. If the block device is in use by the system (e.g., mounted), open() fails with the error EBUSY.

On NFS, O_EXCL is supported only when using NFSv3 or later on kernel 2.6 or later. In NFS environments where O_EXCL support is not provided, programs that rely on it for performing locking tasks will contain a race condition. Portable programs that want to perform atomic file locking using a lockfile, and need to avoid reliance on NFS support for O_EXCL, can create a unique file on the same filesystem (e.g., incorporating hostname and PID), and use link(2) to make a link to the lockfile. If link(2) returns 0, the lock is successful. Otherwise, use stat(2) on the unique file to check if its link count has increased to 2, in which case the lock is also successful.

so it's like "create me but only if I don't exist" — main character energy. 👑

O_LARGEFILE 💅

(LFS) — this one lets you open files whose sizes literally cannot be represented in an off_t, but can be represented in an off64_t. To even get this definition you have to define the _LARGEFILE64_SOURCE macro before including any header files, bestie, order matters.

Hot tip tho: setting the _FILE_OFFSET_BITS feature test macro to 64 (instead of messing with O_LARGEFILE) is the preferred way to access large files on 32-bit systems — see feature_test_macros(7).

O_NOATIME (since Linux 2.6.8) 🕵️‍♀️

Don't update the file's last access time (that's st_atime in the inode) when the file gets read(2). Sneaky, lowkey iconic.

You can only use this flag if one of these is true:

• The effective UID of the process matches the owner UID of the file.
• The calling process has the CAP_FOWNER capability in its user namespace, and the owner UID of the file has a mapping in that namespace.

This flag is meant for indexing or backup programs, where using it can significantly cut down the amount of disk activity. Ngl tho, it may not be effective on all filesystems. One example is NFS, where the server maintains the access time. Rude of it.

O_NOCTTY 🖥️

If path refers to a terminal device — see tty(4) — it will not become the process's controlling terminal, even if the process doesn't have one. No adoption today, thanks.

O_NOFOLLOW 🚫🔗

If the trailing component (i.e., the basename) of path is a symbolic link, then the open fails with the error ELOOP. Symbolic links in earlier components of the pathname will still be followed, so don't @ me about the prefix.

(Note that the ELOOP error that can happen in this case is indistinguishable from the case where an open fails because there are too many symbolic links found while resolving components in the path prefix of the pathname.)

This flag is a FreeBSD extension, which got added in Linux 2.1.126, and has subsequently been standardized in POSIX.1-2008. Glow up!

See also O_PATH below.

O_NONBLOCK or O_NDELAY ⚡

When possible, the file is opened in nonblocking mode. Neither the open() nor any subsequent I/O operations on the file descriptor that gets returned will cause the calling process to wait. She does not wait. Periodt.

Note that the setting of this flag has no effect on the operation of poll(2), select(2), epoll(7), and similar, since those interfaces merely inform the caller about whether a file descriptor is "ready", meaning that an I/O operation performed on the file descriptor with the O_NONBLOCK flag clear would not block.

Also note that this flag has no effect for regular files and block devices; that is, I/O operations will (briefly) block when device activity is required, regardless of whether O_NONBLOCK is set. Since O_NONBLOCK semantics might eventually be implemented, applications should not depend upon blocking behavior when specifying this flag for regular files and block devices. Future-proof your vibe, bestie.

For the handling of FIFOs (named pipes), see also fifo(7). For a discussion of the effect of O_NONBLOCK in conjunction with mandatory file locks and with file leases, see fcntl(2).

O_PATH (since Linux 2.6.39) ✨

Obtain a file descriptor that can be used for two purposes: to indicate a location in the filesystem tree, and to perform operations that act purely at the file descriptor level. The file itself is not opened, and other file operations (e.g., read(2), write(2), fchmod(2), fchown(2), fgetxattr(2), ioctl(2), mmap(2)) fail with the error EBADF. It's giving "look but don't touch."

The following operations can be performed on the resulting file descriptor:

• close(2).
• fchdir(2), if the file descriptor refers to a directory (since Linux 3.5).
• fstat(2) (since Linux 3.6).
• fstatfs(2) (since Linux 3.12).
• Duplicating the file descriptor (dup(2), fcntl(2) F_DUPFD, etc.).
• Getting and setting file descriptor flags (fcntl(2) F_GETFD and F_SETFD).
• Retrieving open file status flags using the fcntl(2) F_GETFL operation: the returned flags will include the bit O_PATH.
• Passing the file descriptor as the dirfd argument of openat() and the other "*at()" system calls. This includes linkat(2) with AT_EMPTY_PATH (or via procfs using AT_SYMLINK_FOLLOW) even if the file is not a directory.
• Passing the file descriptor to another process via a UNIX domain socket (see SCM_RIGHTS in unix(7)).

When O_PATH is specified in flags, flag bits other than O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored. They simply don't get a turn.

Opening a file or directory with the O_PATH flag requires no permissions on the object itself (but does require execute permission on the directories in the path prefix). Depending on the subsequent operation, a check for suitable file permissions may be performed (e.g., fchdir(2) requires execute permission on the directory referred to by its file descriptor argument). By contrast, obtaining a reference to a filesystem object by opening it with the O_RDONLY flag requires that the caller have read permission on the object, even when the subsequent operation (e.g., fchdir(2), fstat(2)) does not require read permission on the object. Different rules, different energy.

If path is a symbolic link and the O_NOFOLLOW flag is also specified, then the call returns a file descriptor referring to the symbolic link. This file descriptor can be used as the dirfd argument in calls to fchownat(2), fstatat(2), linkat(2), and readlinkat(2) with an empty pathname to have the calls operate on the symbolic link.

If path refers to an automount point that has not yet been triggered, so no other filesystem is mounted on it, then the call returns a file descriptor referring to the automount directory without triggering a mount. fstatfs(2) can then be used to determine if it is, in fact, an untriggered automount point (.f_type == AUTOFS_SUPER_MAGIC).

One use of O_PATH for regular files is to provide the equivalent of POSIX.1's O_EXEC functionality. This permits us to open a file for which we have execute permission but not read permission, and then execute that file, with steps something like the following:

    char buf[PATH_MAX];
    fd = open("some_prog", O_PATH);
    snprintf(buf, PATH_MAX, "/proc/self/fd/%d", fd);
    execl(buf, "some_prog", (char *) NULL);

An O_PATH file descriptor can also be passed as the argument of fexecve(3).

O_SYNC 💾

Write operations on the file will complete according to the requirements of synchronized I/O file integrity completion (by contrast with the synchronized I/O data integrity completion provided by O_DSYNC). Two different girls, don't confuse them.

By the time write(2) (or similar) returns, the output data and associated file metadata have been transferred to the underlying hardware (i.e., as though each write(2) was followed by a call to fsync(2)). See VERSIONS.

O_TMPFILE (since Linux 3.11) 👻

Create an unnamed temporary regular file. The path argument specifies a directory; an unnamed inode will be created in that directory's filesystem. Anything written to the resulting file will be lost when the last file descriptor is closed, unless the file is given a name. Ghost file behavior, honestly.

O_TMPFILE must be specified with one of O_RDWR or O_WRONLY and, optionally, O_EXCL. If O_EXCL is not specified, then linkat(2) can be used to link the temporary file into the filesystem, making it permanent, using code like the following:

    char path[PATH_MAX];
    fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
                            S_IRUSR | S_IWUSR);

    /* File I/O on 'fd'...  */

    linkat(fd, "", AT_FDCWD, "/path/for/file", AT_EMPTY_PATH);

    /* If the caller doesn't have the CAP_DAC_READ_SEARCH
       capability (needed to use AT_EMPTY_PATH with linkat(2)),
       and there is a proc(5) filesystem mounted, then the
       linkat(2) call above can be replaced with:

    snprintf(path, PATH_MAX,  "/proc/self/fd/%d", fd);
    linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
                            AT_SYMLINK_FOLLOW);
    */

In this case, the open() mode argument determines the file permission mode, as with O_CREAT.

Specifying O_EXCL in conjunction with O_TMPFILE prevents a temporary file from being linked into the filesystem in the above manner. (Note that the meaning of O_EXCL in this case is different from the meaning of O_EXCL otherwise. Same flag, whole different personality.)

There are two main use cases for O_TMPFILE:

• Improved tmpfile(3) functionality: race-free creation of temporary files that (1) are automatically deleted when closed; (2) can never be reached via any pathname; (3) are not subject to symlink attacks; and (4) do not require the caller to devise unique names.
• Creating a file that is initially invisible, which is then populated with data and adjusted to have appropriate filesystem attributes (fchown(2), fchmod(2), fsetxattr(2), etc.) before being atomically linked into the filesystem in a fully formed state (using linkat(2) as described above). Main character entrance, truly.

O_TMPFILE requires support by the underlying filesystem; only a subset of Linux filesystems provide that support. In the initial implementation, support was provided in the ext2, ext3, ext4, UDF, Minix, and tmpfs filesystems. Support for other filesystems has subsequently been added as follows: XFS (Linux 3.15); Btrfs (Linux 3.16); F2FS (Linux 3.16); and ubifs (Linux 4.9).

O_TRUNC ✂️

If the file already exists and is a regular file and the access mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0. Gone. Zeroed. Not a crumb left.

If the file is a FIFO or terminal device file, the O_TRUNC flag is ignored. Otherwise, the effect of O_TRUNC is unspecified — so like, don't count on anything, bestie.

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creat() 🎀

A call to creat() is equivalent to calling open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC. That's the whole lore. Short king.

🔍 openat()

Bestie, openat() is literally open() in the same outfit, just with one extra accessory: it does exactly the same thing except for the differences we're about to spill 💅

So here's the tea — the dirfd argument teams up with the path argument like this:

• If the pathname in path is absolute, then dirfd is just… ignored. Not even looked at. Ghosted. 👻
• If the pathname in path is relative and dirfd is the special value AT_FDCWD, then path gets interpreted relative to the current working directory of the calling process (so, same behavior as plain open(), vibe-identical 💖).
• If the pathname in path is relative, then it's interpreted relative to the directory referred to by the file descriptor dirfd — not relative to the caller's current working directory like open() does for a relative path. And in this case, dirfd has to be a directory that was opened for reading (O_RDONLY) or opened using the O_PATH flag, no exceptions, periodt.

And if path is relative and dirfd isn't a valid file descriptor? Boom, error EBADF 😭 (fun trick tho: passing an invalid fd number in dirfd is a known way to force path to be absolute — sneaky icon behavior ngl 🎀)

🌱 openat2(2)

The openat2(2) system call is an extension of openat() — it's a whole superset of openat()'s features. It gets its own dedicated documentation though, over in openat2(2). Go read that one for the extra sauce ✨

📤 RETURN VALUE

Slay, here's the win condition: on success, open(), openat(), and creat() return the new file descriptor — a nonnegative integer 🎉

On error, -1 comes back and errno gets set to tell you what went wrong. That's it. That's the whole deal. 💀

💥 ERRORS

So open(), openat(), and creat() can all flop with these errors:

• EACCES — the access you requested to the file isn't allowed, OR search permission is denied for one of the directories in the path prefix of path, OR the file didn't exist yet and write access to the parent directory isn't allowed. (Peep path_resolution(7) for more.)
• EACCES — where O_CREAT is specified, the protected_fifos or protected_regular sysctl is enabled, the file already exists and is a FIFO or regular file, the owner of the file is neither the current user nor the owner of the containing directory, and the containing directory is both world- or group-writable and sticky. For the full details, see the descriptions of /proc/sys/fs/protected_fifos and /proc/sys/fs/protected_regular in proc_sys_fs(5).
• EBADF — (for openat()) path is relative but dirfd is neither AT_FDCWD nor a valid file descriptor.
• EBUSY — O_EXCL was specified in flags and path refers to a block device that's in use by the system (like, it's mounted).
• EDQUOT — where O_CREAT is specified, the file doesn't exist, and the user's quota of disk blocks or inodes on the filesystem has been exhausted.
• EEXIST — path already exists and O_CREAT and O_EXCL were used.
• EFAULT — path points outside your accessible address space.
• EFBIG — see EOVERFLOW.
• EINTR — while blocked waiting to complete an open of a slow device (like a FIFO; see fifo(7)), the call got interrupted by a signal handler; see signal(7).
• EINVAL — the filesystem doesn't support the O_DIRECT flag. See NOTES for more info.
• EINVAL — invalid value in flags.
• EINVAL — O_TMPFILE was specified in flags, but neither O_WRONLY nor O_RDWR was specified.
• EINVAL — O_CREAT and O_DIRECTORY were both specified in flags, and the Linux kernel version is 6.4 or later. (Earlier kernels were inconsistent here, and POSIX doesn't specify the behavior at all.)
• EINVAL — O_CREAT was specified in flags and the final component ("basename") of the new file's path is invalid (like, it contains characters not permitted by the underlying filesystem).
• EINVAL — the final component ("basename") of path is invalid (same deal, e.g. it contains characters not permitted by the underlying filesystem).
• EISDIR — path refers to a directory and the access requested involved writing (meaning O_WRONLY or O_RDWR is set).
• EISDIR — path refers to an existing directory, O_TMPFILE and one of O_WRONLY or O_RDWR were specified in flags, but this kernel version doesn't provide the O_TMPFILE functionality.
• ELOOP — too many symbolic links were encountered in resolving path.
• ELOOP — path was a symbolic link, and flags specified O_NOFOLLOW but not O_PATH.
• EMFILE — the per-process limit on the number of open file descriptors has been reached (see the description of RLIMIT_NOFILE in getrlimit(2)).
• ENAMETOOLONG — path was too long.
• ENFILE — the system-wide limit on the total number of open files has been reached.
• ENODEV — path refers to a device special file and no corresponding device exists. (This is a Linux kernel bug, bestie — in this situation ENXIO must be returned. 🙈)
• ENOENT — O_CREAT is not set and the named file doesn't exist.
• ENOENT — a directory component in path doesn't exist or is a dangling symbolic link.
• ENOENT — path refers to a nonexistent directory, O_TMPFILE and one of O_WRONLY or O_RDWR were specified in flags, but this kernel version doesn't provide the O_TMPFILE functionality.
• ENOMEM — the named file is a FIFO, but memory for the FIFO buffer can't be allocated because the per-user hard limit on memory allocation for pipes has been reached and the caller isn't privileged; see pipe(7).
• ENOMEM — insufficient kernel memory was available.
• ENOSPC — path was to be created but the device containing path has no room for the new file.
• ENOTDIR — a component used as a directory in path is not, in fact, a directory, or O_DIRECTORY was specified and path wasn't a directory.
• ENOTDIR — (for openat()) path is a relative pathname and dirfd is a file descriptor referring to a file other than a directory.
• ENXIO — O_NONBLOCK | O_WRONLY is set, the named file is a FIFO, and no process has the FIFO open for reading.
• ENXIO — the file is a device special file and no corresponding device exists.
• ENXIO — the file is a UNIX domain socket.
• EOPNOTSUPP — the filesystem containing path doesn't support O_TMPFILE.
• EOVERFLOW — path refers to a regular file that's too large to be opened. The usual scenario here is that an application compiled on a 32-bit platform without -D_FILE_OFFSET_BITS=64 tried to open a file whose size exceeds (1<<31)-1 bytes; see also O_LARGEFILE above. This is the error specified by POSIX.1 — before Linux 2.6.24, Linux gave the error EFBIG for this case.
• EPERM — the O_NOATIME flag was specified, but the effective user ID of the caller didn't match the owner of the file and the caller wasn't privileged.
• EPERM — the operation was prevented by a file seal; see fcntl(2).
• EROFS — path refers to a file on a read-only filesystem and write access was requested.
• ETXTBSY — path refers to an executable image which is currently being executed and write access was requested.
• ETXTBSY — path refers to a file that's currently in use as a swap file, and the O_TRUNC flag was specified.
• ETXTBSY — path refers to a file that's currently being read by the kernel (like, for module/firmware loading), and write access was requested.
• EWOULDBLOCK — the O_NONBLOCK flag was specified, and an incompatible lease was held on the file (see fcntl(2)).

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🕰️ VERSIONS

Heads up bestie, the (undefined) effect of O_RDONLY | O_TRUNC varies among implementations. On many systems the file is actually truncated. Wild, right? 😭

⚡ Synchronized I/O

The POSIX.1-2008 "synchronized I/O" option specifies different variants of synchronized I/O, and it specifies the open() flags O_SYNC, O_DSYNC, and O_RSYNC for controlling the behavior. And regardless of whether an implementation supports this option, it must at least support the use of O_SYNC for regular files. Mandatory. ✨

Linux implements O_SYNC and O_DSYNC, but not O_RSYNC. Somewhat incorrectly, glibc defines O_RSYNC to have the same value as O_SYNC. (O_RSYNC is defined in the Linux header file <asm/fcntl.h> on HP PA-RISC, but it isn't used. 💀)

O_SYNC provides synchronized I/O file integrity completion, meaning write operations will flush data and all associated metadata to the underlying hardware. O_DSYNC provides synchronized I/O data integrity completion, meaning write operations will flush data to the underlying hardware, but will only flush metadata updates that are required to allow a subsequent read operation to complete successfully. Data integrity completion can reduce the number of disk operations that are required for applications that don't need the guarantees of file integrity completion. Efficiency queen 💅

To really get the difference between the two types of completion, consider two pieces of file metadata: the file last modification timestamp (st_mtime) and the file length. All write operations will update the last file modification timestamp, but only writes that add data to the end of the file will change the file length. The last modification timestamp isn't needed to ensure that a read completes successfully, but the file length is. So O_DSYNC would only guarantee to flush updates to the file length metadata — whereas O_SYNC would also always flush the last modification timestamp metadata. There you go, that's the whole lore 🧚

Before Linux 2.6.33, Linux implemented only the O_SYNC flag for open(). However, when that flag was specified, most filesystems actually provided the equivalent of synchronized I/O data integrity completion (i.e., O_SYNC was actually implemented as the equivalent of O_DSYNC).

Since Linux 2.6.33, proper O_SYNC support is provided. But to ensure backward binary compatibility, O_DSYNC was defined with the same value as the historical O_SYNC, and O_SYNC was defined as a new (two-bit) flag value that includes the O_DSYNC flag value. This ensures that applications compiled against new headers get at least O_DSYNC semantics before Linux 2.6.33. Legacy support stays iconic 💖

🍓 C library/kernel differences

Since glibc 2.26, the glibc wrapper function for open() employs the openat() system call, rather than the kernel's open() system call. For certain architectures, this is also true before glibc 2.26.

🌸 POSIX

POSIX.1-2024 specifies O_CLOFORK, but Linux doesn't support it. Sad! 😭

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📜 STANDARDS

• open()
• creat()
• openat() — POSIX.1-2024.
• openat2(2) — Linux.
• O_DIRECT
• O_NOATIME
• O_PATH
• O_TMPFILE — Linux.

📖 HISTORY

• open(), creat() — SVr4, 4.3BSD, POSIX.1-2001.
• openat() — POSIX.1-2008. Linux 2.6.16, glibc 2.4.
• O_CLOEXEC, O_DIRECTORY, O_NOFOLLOW — POSIX.1-2008.

✨ NOTES ✨

okay bestie gather round, this is the tea section where all the little footnotes live 💖 none of it is optional, all of it is real.

Under Linux, the O_NONBLOCK flag is sometimes used in cases where you wanna open something but don't necessarily have any intention of reading or writing it. Like, you're just here for the vibes. For example, this may be used to open a device in order to get a file descriptor for use with ioctl(2).

Note that open() can open device special files, but creat() cannot create them — so if that's what you're trying to do, use mknod(2) instead. no cap.

If the file is newly created, its st_atime, st_ctime, st_mtime fields (respectively, time of last access, time of last status change, and time of last modification; see stat(2)) are set to the current time, and so are the st_ctime and st_mtime fields of the parent directory. Otherwise, if the file is modified because of the O_TRUNC flag, its st_ctime and st_mtime fields are set to the current time. (the parent dir gets a little timestamp glow-up too, we love to see it ✨)

The files in the /proc/pid/fd directory show the open file descriptors of the process with the PID pid. The files in the /proc/pid/fdinfo directory show even more information about these file descriptors. See proc(5) for further details of both of these directories. 🕵️‍♀️

The Linux header file <asm/fcntl.h> doesn't define O_ASYNC; the (BSD-derived) FASYNC synonym is defined instead. iykyk.

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🗂️ Open file descriptions

The term open file description is the one used by POSIX to refer to the entries in the system-wide table of open files. In other contexts, this object is variously also called an "open file object", a "file handle", an "open file table entry", or — in kernel-developer parlance — a struct file. Same girl, different name tags 💅

When a file descriptor is duplicated (using dup(2) or similar), the duplicate refers to the same open file description as the original file descriptor, and the two file descriptors consequently share the file offset and file status flags. Such sharing can also occur between processes: a child process created via fork(2) inherits duplicates of its parent's file descriptors, and those duplicates refer to the same open file descriptions. 🫶

Each open() of a file creates a new open file description; thus, there may be multiple open file descriptions corresponding to a file inode. (iconic, honestly.)

On Linux, one can use the kcmp(2) KCMP_FILE operation to test whether two file descriptors (in the same process or in two different processes) refer to the same open file description. no guessing required, bestie, just ask the kernel.

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🌐 NFS

There are many infelicities in the protocol underlying NFS, affecting amongst others O_SYNC and O_NDELAY. 💀 yeah. it's giving messy.

On NFS filesystems with UID mapping enabled, open() may return a file descriptor but, for example, read(2) requests are denied with EACCES. This is because the client performs open() by checking the permissions, but UID mapping is performed by the server upon read and write requests. so the fd comes back looking cute and then the reads get ghosted 😭

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🚰 FIFOs

Opening the read or write end of a FIFO blocks until the other end is also opened (by another process or thread). See fifo(7) for further details. it's a whole meet-cute situation, nobody moves until both sides show up.

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🔐 File access mode

Unlike the other values that can be specified in flags, the access mode values O_RDONLY, O_WRONLY, and O_RDWR do not specify individual bits. Rather, they define the low order two bits of flags, and are defined respectively as 0, 1, and 2. In other words, the combination O_RDONLY | O_WRONLY is a logical error, and certainly does not have the same meaning as O_RDWR. do not do this. i'm serious. it's not a mood, it's a bug.

Linux reserves the special, nonstandard access mode 3 (binary 11) in flags to mean: check for read and write permission on the file and return a file descriptor that can't be used for reading or writing. This nonstandard access mode is used by some Linux drivers to return a file descriptor that is to be used only for device-specific ioctl(2) operations. very niche, very mysterious, very cunty of the drivers honestly.

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📁 Rationale for openat() and other directory file descriptor APIs

openat() and the other system calls and library functions that take a directory file descriptor argument (i.e., execveat(2), faccessat(2), fanotify_mark(2), fchmodat(2), fchownat(2), fspick(2), fstatat(2), futimesat(2), linkat(2), mkdirat(2), mknodat(2), mount_setattr(2), move_mount(2), name_to_handle_at(2), open_tree(2), openat2(2), readlinkat(2), renameat(2), renameat2(2), statx(2), symlinkat(2), unlinkat(2), utimensat(2), mkfifoat(3), and scandirat(3)) address two problems with the older interfaces that preceded them. Here, the explanation is in terms of the openat() call, but the rationale is analogous for the other interfaces. 👑

First, openat() allows an application to avoid race conditions that could occur when using open() to open files in directories other than the current working directory. These race conditions result from the fact that some component of the directory prefix given to open() could be changed in parallel with the call to open(). Suppose, for example, that we wish to create the file dir1/dir2/xxx.dep if the file dir1/dir2/xxx exists. The problem is that between the existence check and the file-creation step, dir1 or dir2 (which might be symbolic links) could be modified to point to a different location. sneaky!! the directory you checked is not the directory you got!! Such races can be avoided by opening a file descriptor for the target directory, and then specifying that file descriptor as the dirfd argument of (say) fstatat(2) and openat(). The use of the dirfd file descriptor also has other benefits:

• the file descriptor is a stable reference to the directory, even if the directory is renamed; and
• the open file descriptor prevents the underlying filesystem from being dismounted, just as when a process has a current working directory on a filesystem. it holds the door open. literally a friend holding your spot in line.

Second, openat() allows the implementation of a per-thread "current working directory", via file descriptor(s) maintained by the application. (This functionality can also be obtained by tricks based on the use of /proc/self/fd/dirfd, but less efficiently.) we love an efficient queen.

The dirfd argument for these APIs can be obtained by using open() or openat() to open a directory (with either the O_RDONLY or the O_PATH flag). Alternatively, such a file descriptor can be obtained by applying dirfd(3) to a directory stream created using opendir(3).

When these APIs are given a dirfd argument of AT_FDCWD or the specified pathname is absolute, then they handle their pathname argument in the same way as the corresponding conventional APIs. However, in this case, several of the APIs have a flags argument that provides access to functionality that is not available with the corresponding conventional APIs. bonus features unlocked 🔥

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💾 O_DIRECT

The O_DIRECT flag may impose alignment restrictions on the length and address of user-space buffers and the file offset of I/Os. In Linux alignment restrictions vary by filesystem and kernel version and might be absent entirely. The handling of misaligned O_DIRECT I/Os also varies; they can either fail with EINVAL or fall back to buffered I/O. so basically: sometimes fine, sometimes not, and you never know which. the duality of man.

Since Linux 6.1, O_DIRECT support and alignment restrictions for a file can be queried using statx(2), using the STATX_DIOALIGN flag. Support for STATX_DIOALIGN varies by filesystem; see statx(2).

Some filesystems provide their own interfaces for querying O_DIRECT alignment restrictions, for example the XFS_IOC_DIOINFO operation in xfsctl(3). STATX_DIOALIGN should be used instead when it is available. (use the new one, bestie, it's the moment.)

If none of the above is available, then direct I/O support and alignment restrictions can only be assumed from known characteristics of the filesystem, the individual file, the underlying storage device(s), and the kernel version. In Linux 2.4, most filesystems based on block devices require that the file offset and the length and memory address of all I/O segments be multiples of the filesystem block size (typically 4096 bytes). In Linux 2.6.0, this was relaxed to the logical block size of the block device (typically 512 bytes). A block device's logical block size can be determined using the ioctl(2) BLKSSZGET operation or from the shell using the command:

blockdev --getss

⚠️ bestie listen. O_DIRECT I/Os should never be run concurrently with the fork(2) system call, if the memory buffer is a private mapping (i.e., any mapping created with the mmap(2) MAP_PRIVATE flag; this includes memory allocated on the heap and statically allocated buffers). Any such I/Os, whether submitted via an asynchronous I/O interface or from another thread in the process, should be completed before fork(2) is called. Failure to do so can result in data corruption and undefined behavior in parent and child processes. this is the real warning, not a vibe check. 💀 This restriction does not apply when the memory buffer for the O_DIRECT I/Os was created using shmat(2) or mmap(2) with the MAP_SHARED flag. Nor does this restriction apply when the memory buffer has been advised as MADV_DONTFORK with madvise(2), ensuring that it will not be available to the child after fork(2).

The O_DIRECT flag was introduced in SGI IRIX, where it has alignment restrictions similar to those of Linux 2.4. IRIX has also a fcntl(2) call to query appropriate alignments, and sizes. FreeBSD 4.x introduced a flag of the same name, but without alignment restrictions. history lesson, serving looks.

O_DIRECT support was added in Linux 2.4.10. Older Linux kernels simply ignore this flag. Some filesystems may not implement the flag, in which case open() fails with the error EINVAL if it is used. silently ignored on old kernels, loud error on filesystems that don't do it. pick your chaos.

Applications should avoid mixing O_DIRECT and normal I/O to the same file, and especially to overlapping byte regions in the same file. Even when the filesystem correctly handles the coherency issues in this situation, overall I/O throughput is likely to be slower than using either mode alone. Likewise, applications should avoid mixing mmap(2) of files with direct I/O to the same files. don't mix, it's giving messy and slow.

The behavior of O_DIRECT with NFS will differ from local filesystems. Older kernels, or kernels configured in certain ways, may not support this combination. The NFS protocol does not support passing the flag to the server, so O_DIRECT I/O will bypass the page cache only on the client; the server may still cache the I/O. The client asks the server to make the I/O synchronous to preserve the synchronous semantics of O_DIRECT. Some servers will perform poorly under these circumstances, especially if the I/O size is small. Some servers may also be configured to lie to clients about the I/O having reached stable storage; this will avoid the performance penalty at some risk to data integrity in the event of server power failure. 😭 lying servers!! in MY filesystem?! it's more likely than you think. The Linux NFS client places no alignment restrictions on O_DIRECT I/O.

In summary, O_DIRECT is a potentially powerful tool that should be used with caution. It is recommended that applications treat use of O_DIRECT as a performance option which is disabled by default. 🎀

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💀 BUGS 💀

Currently, it is not possible to enable signal-driven I/O by specifying O_ASYNC when calling open(); use fcntl(2) to enable this flag. so close yet so far. the flag exists but not here, tragic.

One must check for two different error codes, EISDIR and ENOENT, when trying to determine whether the kernel supports O_TMPFILE functionality. two checks, not one. don't get caught slipping. 🙈

✨ SEE ALSO ✨

okay bestie, you made it to the bottom, so here's the whole friend group 🫶 every single one of these is a real page you can go read — go say hi:

• chmod(2) — changing permissions, the glow-up for file modes
• chown(2) — who owns this? changing owner and group
• close(2) — closing the fd when you're done, literally just say bye
• dup(2) — duplicating file descriptors, twin energy
• fcntl(2) — the swiss army knife of fd operations
• link(2) — hard links, the way two names can share one inode
• lseek(2) — moving the file offset around like it's a cursor
• mknod(2) — making device files and fifos and such
• mmap(2) — mapping files into memory, main character behaviour
• mount(2) — mounting filesystems
• open_by_handle_at(2) — opening by handle instead of path
• openat2(2) — the newer, fancier, more-extensible sibling of this very page
• read(2) — reading bytes out
• socket(2) — creating sockets
• stat(2) — all the metadata tea about a file
• umask(2) — the mask that decides what permissions new files don't get
• unlink(2) — removing names from the filesystem
• write(2) — writing bytes in
• fopen(3) — the stdio version, buffered and cozy
• acl(5) — access control lists, permissions but extra
• fifo(7) — named pipes overview
• inode(7) — inode lore, foundational fr fr
• path_resolution(7) — how the kernel actually walks a path to find your file
• symlink(7) — symbolic links and all their chaos 🌸

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and that's the whole page, babe 💖 you just read the open(2) man page front to back — iconic, honestly. this masterpiece lives in Linux man-pages 6.20, and this version is dated 2026-02-08. go open some files, but like… responsibly. no cap. 👑✨💅