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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 ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which is not part of the original manual page), send a mail to man-pages@man7.org 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)
| HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. | |