NAME

       fcntl - manipulate file descriptor

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <fcntl.h>

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

DESCRIPTION

       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)

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.

STANDARDS

       POSIX.1-2008.

HISTORY

       SVr4, 4.3BSD, POSIX.1-2001.

SEE ALSO

       dup2(2), flock(2), open(2), socket(2), lockf(3), capabilities(7), feature_test_macros(7), lslocks(8)

Linux man-pages 6.15                               2025-07-20                                           fcntl(2)