F_DUPFD(2const)                                                F_DUPFD(2const)

NAME
       F_DUPFD, F_DUPFD_CLOEXEC - duplicate a file descriptor

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #include <fcntl.h>

       int fcntl(int fd, F_DUPFD, int arg);
       int fcntl(int fd, F_DUPFD_CLOEXEC, int arg);

DESCRIPTION
       F_DUPFD
              Duplicate the file descriptor fd using the lowest-numbered
              available file descriptor greater than or equal to arg.  This is
              different from dup2(2), which uses exactly the file descriptor
              specified.

              On success, the new file descriptor is returned.

              See dup(2) for further details.

       F_DUPFD_CLOEXEC
              As for F_DUPFD, but additionally set the close-on-exec flag for
              the duplicate file descriptor.  Specifying this flag permits a
              program to avoid an additional F_SETFD(2const) operation to set
              the FD_CLOEXEC flag.  For an explanation of why this flag is
              useful, see the description of O_CLOEXEC in open(2).

RETURN VALUE
       The new file descriptor.

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

ERRORS
       See fcntl(2).

       EINVAL arg is negative or is greater than the maximum allowable value
              (see the discussion of RLIMIT_NOFILE in getrlimit(2)).

       EMFILE The per-process limit on the number of open file descriptors has
              been reached.

VERSIONS
       POSIX.1-2024 specifies F_DUPFD_CLOFORK, but Linux doesn't support it.

STANDARDS
       POSIX.1-2008.

HISTORY
       F_DUPFD
              SVr4, 4.3BSD, POSIX.1-2001.

       F_DUPFD_CLOEXEC
              Linux 2.6.24.  POSIX.1-2008.  (To get this definition, define
              _POSIX_C_SOURCE with the value 200809L or greater, or
              _XOPEN_SOURCE with the value 700 or greater.)

CAVEATS
       The errors returned by dup2(2) are different from those returned by
       F_DUPFD.

SEE ALSO
       dup2(2), fcntl(2)

Linux man-pages 6.15              2025-07-19                   F_DUPFD(2const)