FCNTL(2)

💖 FCNTL(2) 💖

🌸 NAME 🌸

fcntl — file control

bestie, this is the file control page, aka FCNTL(2). it's literally the main character of descriptor manipulation. no cap. 💅

📚 LIBRARY 📚

Standard C Library (libc, -lc)

✨ SYNOPSIS ✨

#include <fcntl.h>

int
fcntl(int fd, int cmd, ...);

🦋 DESCRIPTION 🦋

okay so fcntl() is the system call that gives you control over descriptors. the argument fd is the descriptor that cmd is gonna operate on, as described below. depending on what cmd is, fcntl() can take an extra third argument, int arg.

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🎀 The Commands 🎀

• F_DUPFD — this one returns a new descriptor, and here's the tea:
  • it's the lowest numbered available descriptor greater than or equal to arg.
  • it has the same object references as the original descriptor.
  • if the object was a file, the new descriptor shares the same file offset.
  • same access mode (read, write or read/write).
  • same file status flags (i.e., both file descriptors share the same file status flags).
  • the close-on-exec flag FD_CLOEXEC associated with the new file descriptor is cleared, so the file descriptor is to remain open across execve(2) system calls.
• F_DUPFD_CLOEXEC — just like F_DUPFD, but the FD_CLOEXEC flag associated with the new file descriptor is set, so the file descriptor is closed when execve(2) system call executes.
• F_DUP2FD — it's functionally equivalent to

dup2(fd, arg)

• F_DUP2FD_CLOEXEC — like F_DUP2FD, but the FD_CLOEXEC flag associated with the new file descriptor is set.

heads up: the F_DUP2FD and F_DUP2FD_CLOEXEC constants are not portable, so don't use them if you need portability. use dup2() instead of F_DUP2FD.

• F_GETFD — gets the close-on-exec flag associated with the file descriptor fd as FD_CLOEXEC. if the returned value ANDed with FD_CLOEXEC is 0, the file will remain open across exec(), otherwise the file will be closed upon execution of exec() (arg is ignored).
• F_SETFD — sets the close-on-exec flag associated with fd to arg, where arg is either 0 or FD_CLOEXEC, as described above.
• F_GETFL — gets descriptor status flags, as described below (arg is ignored).
• F_SETFL — sets descriptor status flags to arg.
• F_GETOWN — gets the process ID or process group currently receiving SIGIO and SIGURG signals; process groups are returned as negative values (arg is ignored).
• F_SETOWN — sets the process or process group to receive SIGIO and SIGURG signals; process groups are specified by supplying arg as negative, otherwise arg is interpreted as a process ID.
• F_READAHEAD — sets or clears the read ahead amount for sequential access to the third argument, arg, which is rounded up to the nearest block size. a zero value in arg turns off read ahead, a negative value restores the system default.
• F_RDAHEAD — equivalent to Darwin counterpart which sets read ahead amount of 128KB when the third argument, arg is non-zero. a zero value in arg turns off read ahead.

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🌈 The Flags for F_GETFL and F_SETFL 🌈

the flags for the F_GETFL and F_SETFL flags are as follows:

• O_NONBLOCK — non-blocking I/O; if no data is available to a read(2) system call, or if a write(2) operation would block, the read or write call returns -1 with the error EAGAIN.
• O_APPEND — force each write to append at the end of file; corresponds to the O_APPEND flag of open(2).
• O_DIRECT — minimize or eliminate the cache effects of reading and writing. the system will attempt to avoid caching the data you read or write. if it cannot avoid caching the data, it will minimize the impact the data has on the cache. use of this flag can drastically reduce performance if not used with care.
• O_ASYNC — enable the SIGIO signal to be sent to the process group when I/O is possible, e.g., upon availability of data to be read.

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🔒 Advisory File Locking 🔒

several commands are available for doing advisory file locking; they all operate on the following structure:

struct flock {
        off_t   l_start;        /* starting offset */
        off_t   l_len;          /* len = 0 means until end of file */
        pid_t   l_pid;          /* lock owner */
        short   l_type;         /* lock type: read/write, etc. */
        short   l_whence;       /* type of l_start */
        int     l_sysid;        /* remote system id or zero for local */
};

the commands available for advisory record locking are as follows:

• F_GETLK — get the first lock that blocks the lock description pointed to by the third argument, arg, taken as a pointer to a struct flock (see above). the information retrieved overwrites the information passed to fcntl() in the flock structure. if no lock is found that would prevent this lock from being created, the structure is left unchanged by this system call except for the lock type which is set to F_UNLCK.
• F_SETLK — set or clear a file segment lock according to the lock description pointed to by the third argument, arg, taken as a pointer to a struct flock (see above). F_SETLK is used to establish shared (or read) locks (F_RDLCK) or exclusive (or write) locks, (F_WRLCK), as well as remove either type of lock (F_UNLCK). if a shared or exclusive lock cannot be set, fcntl() returns immediately with EAGAIN.
• F_SETLKW — this command is the same as F_SETLK except that if a shared or exclusive lock is blocked by other locks, the process waits until the request can be satisfied. if a signal that is to be caught is received while fcntl() is waiting for a region, the fcntl() will be interrupted if the signal handler has not specified the SA_RESTART (see sigaction(2)).

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when a shared lock has been set on a segment of a file, other processes can set shared locks on that segment or a portion of it. a shared lock prevents any other process from setting an exclusive lock on any portion of the protected area. a request for a shared lock fails if the file descriptor was not opened with read access.

an exclusive lock prevents any other process from setting a shared lock or an exclusive lock on any portion of the protected area. a request for an exclusive lock fails if the file was not opened with write access.

the value of l_whence is SEEK_SET, SEEK_CUR, or SEEK_END to indicate that the relative offset, l_start bytes, will be measured from the start of the file, current position, or end of the file, respectively. the value of l_len is the number of consecutive bytes to be locked. if l_len is negative, l_start means end edge of the region. the l_pid and l_sysid fields are only used with F_GETLK to return the process ID of the process holding a blocking lock and the system ID of the system that owns that process. locks created by the local system will have a system ID of zero. after a successful F_GETLK request, the value of l_whence is SEEK_SET.

locks may start and extend beyond the current end of a file, but may not start or extend before the beginning of the file. a lock is set to extend to the largest possible value of the file offset for that file if l_len is set to zero. if l_whence and l_start point to the beginning of the file, and l_len is zero, the entire file is locked. if an application wishes only to do entire file locking, the flock(2) system call is much more efficient.

there is at most one type of lock set for each byte in the file. before a successful return from an F_SETLK or an F_SETLKW request when the calling process has previously existing locks on bytes in the region specified by the request, the previous lock type for each byte in the specified region is replaced by the new lock type. as specified above under the descriptions of shared locks and exclusive locks, an F_SETLK or an F_SETLKW request fails or blocks respectively when another process has existing locks on bytes in the specified region and the type of any of those locks conflicts with the type specified in the request.

the queuing for F_SETLKW requests on local files is fair; that is, while the thread is blocked, subsequent requests conflicting with its requests will not be granted, even if these requests do not conflict with existing locks.

this interface follows the completely stupid semantics of System V and IEEE Std 1003.1-1988 (“POSIX.1”) that require that all locks associated with a file for a given process are removed when any file descriptor for that file is closed by that process. this semantic means that applications must be aware of any files that a subroutine library may access. for example if an application for updating the password file locks the password file database while making the update, and then calls getpwnam(3) to retrieve a record, the lock will be lost because getpwnam(3) opens, reads, and closes the password database. the database close will release all locks that the process has associated with the database, even if the library routine never requested a lock on the database. another minor semantic problem with this interface is that locks are not inherited by a child process created using the fork(2) system call. the flock(2) interface has much more rational last close semantics and allows locks to be inherited by child processes. the flock(2) system call is recommended for applications that want to ensure the integrity of their locks when using library routines or wish to pass locks to their children.

the fcntl(), flock(2), and lockf(3) locks are compatible. processes using different locking interfaces can cooperate over the same file safely. however, only one of such interfaces should be used within the same process. if a file is locked by a process through flock(2), any record within the file will be seen as locked from the viewpoint of another process using fcntl() or lockf(3), and vice versa. note that fcntl(F_GETLK) returns -1 in l_pid if the process holding a blocking lock previously locked the file descriptor by flock(2).

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All locks associated with a file for a given process are removed when the process terminates.

Okay so bestie, when a process dips out? Every lock it had on a file? Gone. Poof. ✨

All locks obtained before a call to execve(2) remain in effect until the new program releases them. If the new program does not know about the locks, they will not be released until the program exits.

Like, if you lock stuff and then execve(2) into a new program, those locks are still holding on. If the new program has no idea they exist, it won't unlock them — they stay until it exits. The way that is so real and also lowkey terrifying. 😭

A potential for deadlock occurs if a process controlling a locked region is put to sleep by attempting to lock the locked region of another process. This implementation detects that sleeping until a locked region is unlocked would cause a deadlock and fails with an EDEADLK error.

Deadlock tea: if a process already owns a locked region and then goes to sleep trying to lock another process's locked region, that's the setup. This implementation is like “bestie no” — it detects that waiting for the lock to free up would deadlock, and fails with EDEADLK. Periodt. 💅

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✨ RETURN VALUES ✨

Upon successful completion, the value returned depends on cmd as follows:

• F_DUPFD — A new file descriptor. 🎀
• F_DUP2FD — A file descriptor equal to arg.
• F_GETFD — Value of flag (only the low-order bit is defined).
• F_GETFL — Value of flags.
• F_GETOWN — Value of file descriptor owner.
• other — Value other than -1.

Otherwise, a value of -1 is returned and errno is set to indicate the error.

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🚨 ERRORS 🚨

The fcntl() system call will fail if:

• [EAGAIN] — The argument cmd is F_SETLK, the type of lock (l_type) is a shared lock (F_RDLCK) or exclusive lock (F_WRLCK), and the segment of a file to be locked is already exclusive-locked by another process; or the type is an exclusive lock and some portion of the segment of a file to be locked is already shared-locked or exclusive-locked by another process. 💀
• [EBADF] — The fd argument is not a valid open file descriptor.

The argument cmd is F_DUP2FD, and arg is not a valid file descriptor.

The argument cmd is F_SETLK or F_SETLKW, the type of lock (l_type) is a shared lock (F_RDLCK), and fd is not a valid file descriptor open for reading.

The argument cmd is F_SETLK or F_SETLKW, the type of lock (l_type) is an exclusive lock (F_WRLCK), and fd is not a valid file descriptor open for writing.

• [EDEADLK] — The argument cmd is F_SETLKW, and a deadlock condition was detected.
• [EINTR] — The argument cmd is F_SETLKW, and the system call was interrupted by a signal.
• [EINVAL] — The cmd argument is F_DUPFD and arg is negative or greater than the maximum allowable number (see getdtablesize(2)).

The argument cmd is F_GETLK, F_SETLK or F_SETLKW and the data to which arg points is not valid.

The argument cmd is invalid.

• [EMFILE] — The argument cmd is F_DUPFD and the maximum number of file descriptors permitted for the process are already in use, or no file descriptors greater than or equal to arg are available.
• [ENOTTY] — The fd argument is not a valid file descriptor for the requested operation. This may be the case if fd is a device node, or a descriptor returned by kqueue(2).
• [ENOLCK] — The argument cmd is F_SETLK or F_SETLKW, and satisfying the lock or unlock request would result in the number of locked regions in the system exceeding a system-imposed limit.
• [EOPNOTSUPP] — The argument cmd is F_GETLK, F_SETLK or F_SETLKW and fd refers to a file for which locking is not supported.
• [EOVERFLOW] — The argument cmd is F_GETLK, F_SETLK or F_SETLKW and an off_t calculation overflowed.
• [EPERM] — The cmd argument is F_SETOWN and the process ID or process group given as an argument is in a different session than the caller.
• [ESRCH] — The cmd argument is F_SETOWN and the process ID given as argument is not in use.

In addition, if fd refers to a descriptor open on a terminal device (as opposed to a descriptor open on a socket), a cmd of F_SETOWN can fail for the same reasons as in tcsetpgrp(3), and a cmd of F_GETOWN for the reasons as stated in tcgetpgrp(3).

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👀 SEE ALSO 👀

close(2), dup2(2), execve(2), flock(2), getdtablesize(2), open(2), sigaction(2), lockf(3), tcgetpgrp(3), tcsetpgrp(3)

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📜 STANDARDS 📜

The F_DUP2FD constant is non portable. It is provided for compatibility with AIX and Solaris.

Per Version 4 of the Single UNIX Specification (“SUSv4”), a call with F_SETLKW should fail with [EINTR] after any caught signal and should continue waiting during thread suspension such as a stop signal. However, in this implementation a call with F_SETLKW is restarted after catching a signal with a SA_RESTART handler or a thread suspension such as a stop signal.

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🦋 HISTORY 🦋

The fcntl() system call appeared in 4.2BSD.

The F_DUP2FD constant first appeared in FreeBSD 7.1.

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And that's the whole vibe, bestie! This page is from Debian, dated Nov 15, 2018, and it's FCNTL(2). Stay locked and loaded (the legal kind), mwah 💋🫶