dup(2) - NetBSD Manual Pages

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DUP(2)                    NetBSD System Calls Manual                    DUP(2)


NAME

     dup, dup2, dup3 -- duplicate an existing file descriptor


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <unistd.h>

     int
     dup(int oldfd);

     int
     dup2(int oldfd, int newfd);

     int
     dup3(int oldfd, int newfd, int flags);


DESCRIPTION

     The dup() family of calls duplicates an existing file descriptor oldfd.
     A new file descriptor is produced; it is a new reference to the same
     underlying system object.  The object in question does not distinguish
     between the descriptors referencing it in any way.  Thus for files,
     read(2), write(2) and lseek(2) calls all move a single shared seek posi-
     tion.  Similarly, all object modes, settings, properties, and behavior
     other than the close-on-exec flag are shared between references.  This
     includes the setting of append mode, non-blocking I/O actions, asynchro-
     nous I/O operations in progress, socket options, and so forth.  The
     close-on-exec flag, however, is a property of the descriptor rather than
     the object and can be set independently for each reference.

     To get an independent handle with its own seek position and settings, an
     additional open(2) call must be issued.  (This is not generally possible
     for pipes and sockets.)

     The dup call chooses the new descriptor: it is the lowest-numbered
     descriptor not currently in use.  The dup2 and dup3 calls allow the
     caller to choose the new descriptor by passing newfd, which must be
     within the range of valid descriptors.  If newfd is the same as oldfd,
     the call has no effect.  Otherwise, if newfd is already in use, it is
     closed as if close(2) had been called.

     File descriptors are small non-negative integers that index into the per-
     process file table.  Values 0, 1, and 2 have the special property that
     they are treated as standard input, standard output, and standard error
     respectively.  (The constants STDIN_FILENO, STDOUT_FILENO, and
     STDERR_FILENO are provided as symbolic forms for these values.)  The max-
     imum value for a file descriptor is one less than the file table size.
     The file table size can be interrogated with getdtablesize(3) and can to
     some extent be adjusted with setrlimit(2).

     The dup3() call includs an additional flags argument supporting a subset
     of the open(2) flags:

           O_CLOEXEC    Set the close-on-exec flag on newfd.

           O_NONBLOCK   Sets non-blocking I/O.

           O_NOSIGPIPE  For pipes and sockets, do not raise SIGPIPE when a
                        write is made to a broken pipe.  Instead, the write
                        will fail with EPIPE.
     As described above, only the close-on-exec flag is per-file-descriptor,
     so passing any of the other flags will affect both oldfd and newfd.
     These settings are, however, applied atomically along with the rest of
     the dup3() operation.

     In the case of dup() and dup2() the close-on-exec flag on the new file
     descriptor is always left unset and all the modes and settings of the
     underlying object are left unchanged.

     Functionality similar to dup() with slightly different semantics is also
     available via fcntl(2).


RETURN VALUES

     These calls return the new file descriptor value.  In the case of dup2()
     and dup3() this is always the same as newfd.  If an error occurs, the
     value -1 is returned and errno is set to indicate what happened.


EXAMPLES

     A common use for these functions is to set up a pipe as the standard
     input or standard output of a subprocess.  That is done approximately as
     follows (error handling omitted for clarity):

           #include <unistd.h>

           int fds[2];
           pid_t pid;

           pipe(fds);
           pid = fork();
           if (pid == 0) {
                   /* child; use read end of pipe to stdin */
                   dup2(fds[0], STDIN_FILENO);
                   close(fds[0]);
                   close(fds[1]);
                   execv("/some/program", args);
           }
           /* parent process; return write end of pipe */
           close(fds[0]);
           return fds[1];


ERRORS

     These functions fail if:

     [EBADF]            oldfd is not a valid active descriptor, or for dup2()
                        and dup3(), newfd is not in the range of valid file
                        descriptors.

     [EINVAL]           flags contained an invalid value.  Only dup3() can
                        generate this error.

     [EMFILE]           Too many descriptors are active.  Only dup() can gen-
                        erate this error.


SEE ALSO

     accept(2), close(2), fcntl(2), getrlimit(2), open(2), pipe(2),
     setrlimit(2), socket(2), socketpair(2), getdtablesize(3)


STANDARDS

     The dup() and dup2() functions conform to ISO/IEC 9945-1:1990
     (``POSIX.1'').


HISTORY

     The dup3() function originated in Linux and appeared in NetBSD 6.0.

NetBSD 7.0                     December 24, 2013                    NetBSD 7.0