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.

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

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

Operation is prohibited by locks held by other processes.
The operation is prohibited because the file has been memory-mapped by another process.
fd is not an open file descriptor
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)

2025-07-20 Linux man-pages 6.15