fcntl(2)                      System Calls Manual                     fcntl(2)

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)