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.1-2024.

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 6.16                               2025-10-29                                    F_DUPFD(2const)