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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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fcntl(2) System Calls Manual fcntl(2)
fcntl - manipulate file descriptor
Standard C library (libc, -lc)
#include <fcntl.h>
int fcntl(int fd, int op, ...);
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.
Duplicating a file descriptor
F_DUPFD(2const)
F_DUPFD_CLOEXEC(2const)
File descriptor flags
F_GETFD(2const)
F_SETFD(2const)
File status flags
F_GETFL(2const)
F_SETFL(2const)
Advisory record locking
F_SETLK(2const)
F_SETLKW(2const)
F_GETLK(2const)
Open file description locks (non-POSIX)
F_OFD_SETLK(2const)
F_OFD_SETLKW(2const)
F_OFD_GETLK(2const)
Managing signals
F_GETOWN(2const)
F_SETOWN(2const)
F_GETOWN_EX(2const)
F_SETOWN_EX(2const)
F_GETSIG(2const)
F_SETSIG(2const)
Leases
F_SETLEASE(2const)
F_GETLEASE(2const)
File and directory change notification (dnotify)
F_NOTIFY(2const)
Changing the capacity of a pipe
F_SETPIPE_SZ(2const)
F_GETPIPE_SZ(2const)
File Sealing
F_ADD_SEALS(2const)
F_GET_SEALS(2const)
File read/write hints
F_GET_RW_HINT(2const)
F_SET_RW_HINT(2const)
F_GET_FILE_RW_HINT(2const)
F_SET_FILE_RW_HINT(2const)
For a successful call, the return value depends on the operation.
On error, -1 is returned, and errno is set to indicate the error.
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.
POSIX.1-2024 specifies FD_CLOFORK and F_DUPFD_CLOFORK, but Linux
doesn't support them.
POSIX.1-2008.
SVr4, 4.3BSD, POSIX.1-2001.
dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7),
feature_test_macros(7), lslocks(8)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
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This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨Linux man-pages 6.15 2025-07-20 fcntl(2)
Pages that refer to this page: flock(1), pipesz(1), accept(2), bpf(2), chmod(2), clone(2), close(2), dup(2), eventfd(2), execve(2), fallocate(2), fcntl_locking(2), F_DUPFD(2const), F_GETFD(2const), F_GETFL(2const), F_GETLEASE(2const), F_GETPIPE_SZ(2const), F_GET_RW_HINT(2const), F_GET_SEALS(2const), F_GETSIG(2const), flock(2), F_NOTIFY(2const), fork(2), getrlimit(2), gettid(2), inotify_init(2), ioctl(2), ioctl_console(2), memfd_create(2), memfd_secret(2), mknod(2), mmap(2), mount(2), NS_GET_USERNS(2const), open(2), perf_event_open(2), pidfd_getfd(2), pipe(2), read(2), recv(2), select_tut(2), send(2), sigaction(2), signalfd(2), socket(2), statfs(2), syscalls(2), timerfd_create(2), truncate(2), userfaultfd(2), write(2), audit_open(3), dbopen(3), fopen(3), lockf(3), sd_event_add_io(3), shm_open(3), sockatmark(3), statvfs(3), nfs(5), proc_locks(5), proc_sys_fs(5), systemd.socket(5), capabilities(7), credentials(7), epoll(7), inotify(7), landlock(7), man-pages(7), pipe(7), pthreads(7), signal(7), signal-safety(7), socket(7), tcp(7), lslocks(8), mount(8)
