FUTEX_FD(2const)                                              FUTEX_FD(2const)

NAME
       FUTEX_FD - create a file descriptor associated with a futex

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #include <linux/futex.h>  /* Definition of FUTEX_* constants */
       #include <sys/syscall.h>  /* Definition of SYS_* constants */
       #include <unistd.h>

       [[deprecated]]
       long syscall(SYS_futex, uint32_t *uaddr, FUTEX_FD, uint32_t val);

DESCRIPTION
       This operation creates a file descriptor that is associated with the
       futex at uaddr.  The caller must close the returned file descriptor
       after use.

       When another process or thread performs a FUTEX_WAKE(2const) on the
       futex word, the file descriptor indicates as being readable with
       select(2), poll(2), and epoll(7)

       The file descriptor can be used to obtain asynchronous notifications:
       if val is nonzero, then, when another process or thread executes a
       FUTEX_WAKE(2const), the caller will receive the signal number that was
       passed in val.

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

       On success, FUTEX_FD Returns the new file descriptor associated with
       the futex.

ERRORS
       See futex(2).

       EINVAL The signal number supplied in val is invalid.

       ENFILE The system-wide limit on the total number of open files has been
              reached.

STANDARDS
       Linux.

HISTORY
       From Linux 2.6.0 up to and including Linux 2.6.25.

       Because it was inherently racy, FUTEX_FD has been removed from Linux
       2.6.26 onward.

SEE ALSO
       futex(2)

Linux man-pages 6.15              2025-05-30                  FUTEX_FD(2const)