fcntl(2)

Usage

#include <fcntl.h>

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

Library

Standard C library (libc, -lc)

Overview

fcntl() performs one of the operations described below on the open file descriptor fd. The operation is determined by op.

Certain of the operations below are supported only since a particular Linux kernel version. The preferred method of checking whether the host kernel supports a particular operation is to invoke fcntl() with the desired op value and then test whether the call failed with EINVAL, indicating that the kernel does not recognize this value.

Return value

For a successful call, the return value depends on the operation.

On error, -1 is returned, and errno is set to indicate the error.

Errors

EACCES or EAGAIN: Operation is prohibited by locks held by other processes.

EAGAIN: The operation is prohibited because the file has been memory-mapped by another process.

EBADF: fd is not an open file descriptor

EINVAL: The value specified in op is not recognized by this kernel.

Versions

POSIX.1-2024 specifies FD_CLOFORK and F_DUPFD_CLOFORK, but Linux doesn't support them.

Releases

19 releases since Debian 1.1 (Buzz); the text changed 17 times.

1.1–1.2
1.3
2.0
2.1
2.2
3.0
3.1
4.0
5.0
6.0
7
8
9
10
11
12
13
Arch

dup2(2) duplicate a file descriptor flock(2) apply or remove an advisory lock on an open file open(2) open and possibly create a file socket(2) create an endpoint for communication lockf(3) apply, test or remove a POSIX lock on an open file capabilities(7) overview of Linux capabilities feature_test_macros(7) feature test macros lslocks(8) list local system locks

Mentioned on this page

Referred to by

cat(1) concatenate files and print on the standard output mail(1) send and receive Internet mail mount(8) mount a filesystem unix(4) UNIX-domain protocol family termios(4) general terminal line discipline mailx(1) send and receive Internet mail mount_nfs(8) mount NFS file systems dup(2) duplicate a file descriptor dup2(2) duplicate a file descriptor fork(2) create a child process mmap(2) map or unmap files or devices into memory open(2) open and possibly create a file pipe(2) create pipe read(2) read from a file descriptor recv(2) receive a message from a socket send(2) send a message on a socket chmod(2) change permissions of a file close(2) close a file descriptor creat(2) open and possibly create a file errno(2) introduction to system calls and error numbers flock(2) apply or remove an advisory lock on an open file fsync(2) synchronize a file's in-core state with storage device intro(2) introduction to system calls ioctl(2) control device mknod(2) create a special or ordinary file mount(2) mount filesystem pread(2) read from or write to a file descriptor at a given offset readv(2) read or write data into multiple buffers write(2) write to a file descriptor accept(2) accept a connection on a socket execve(2) execute program fchmod(2) change permissions of a file munmap(2) map or unmap files or devices into memory openat(2) open and possibly create a file preadv(2) read or write data into multiple buffers pwrite(2) read from or write to a file descriptor at a given offset sendto(2) send a message on a socket sigvec(2) BSD software signal facilities socket(2) create an endpoint for communication statfs(2) get filesystem statistics writev(2) read or write data into multiple buffers fstatfs(2) get filesystem statistics mknodat(2) create a special or ordinary file pwritev(2) read or write data into multiple buffers recvmsg(2) receive a message from a socket sendmsg(2) send a message on a socket fchmodat(2) change permissions of a file recvfrom(2) receive a message from a socket

and 418 more pages.

Usage

Library

Overview

Duplicating a file descriptor

File descriptor flags

File status flags

Advisory record locking

Open file description locks (non-POSIX)

Managing signals

Leases

Delegations

File and directory change notification (dnotify)

Changing the capacity of a pipe

File Sealing

File read/write hints