F_DUPFD(2const) - Linux manual page (original) (raw)

FDUPFD(2const) FDUPFD(2const)

NAME top

   F_DUPFD, F_DUPFD_CLOEXEC - duplicate a file descriptor

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <fcntl.h>**

   **int fcntl(int** _fd_**, F_DUPFD, int** _arg_**);**
   **int fcntl(int** _fd_**, F_DUPFD_CLOEXEC, int** _arg_**);**

DESCRIPTION top

   **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)](../man2/dup2.2.html), which uses exactly the file
          descriptor specified.

          On success, the new file descriptor is returned.

          See [dup(2)](../man2/dup.2.html) 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)](../man2/F%5FSETFD.2const.html)
          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)](../man2/open.2.html).

RETURN VALUE top

   The new file descriptor.

   On error, -1 is returned, and _[errno](../man3/errno.3.html)_ is set to indicate the error.

ERRORS top

   See [fcntl(2)](../man2/fcntl.2.html).

   **EINVAL** _arg_ is negative or is greater than the maximum allowable
          value (see the discussion of **RLIMIT_NOFILE** in
          [getrlimit(2)](../man2/getrlimit.2.html)).

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

VERSIONS top

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

STANDARDS top

   POSIX.1-2008.

HISTORY top

   **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 top

   The errors returned by [dup2(2)](../man2/dup2.2.html) are different from those returned
   by **F_DUPFD**.

SEE ALSO top

   [dup2(2)](../man2/dup2.2.html), [fcntl(2)](../man2/fcntl.2.html)

COLOPHON top

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

Pages that refer to this page:fcntl(2), fcntl_locking(2), F_GETFL(2const)