pipe(2) — Linux manual page


pipe(2)                    System Calls Manual                   pipe(2)

NAME         top

       pipe, pipe2 - create pipe

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <unistd.h>

       int pipe(int pipefd[2]);

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <fcntl.h>              /* Definition of O_* constants */
       #include <unistd.h>

       int pipe2(int pipefd[2], int flags);

       /* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64, pipe() has the
          following prototype; see VERSIONS */

       #include <unistd.h>

       struct fd_pair {
           long fd[2];
       struct fd_pair pipe(void);

DESCRIPTION         top

       pipe() creates a pipe, a unidirectional data channel that can be
       used for interprocess communication.  The array pipefd is used to
       return two file descriptors referring to the ends of the pipe.
       pipefd[0] refers to the read end of the pipe.  pipefd[1] refers
       to the write end of the pipe.  Data written to the write end of
       the pipe is buffered by the kernel until it is read from the read
       end of the pipe.  For further details, see pipe(7).

       If flags is 0, then pipe2() is the same as pipe().  The following
       values can be bitwise ORed in flags to obtain different behavior:

              Set the close-on-exec (FD_CLOEXEC) flag on the two new
              file descriptors.  See the description of the same flag in
              open(2) for reasons why this may be useful.

       O_DIRECT (since Linux 3.4)
              Create a pipe that performs I/O in "packet" mode.  Each
              write(2) to the pipe is dealt with as a separate packet,
              and read(2)s from the pipe will read one packet at a time.
              Note the following points:

              •  Writes of greater than PIPE_BUF bytes (see pipe(7))
                 will be split into multiple packets.  The constant
                 PIPE_BUF is defined in <limits.h>.

              •  If a read(2) specifies a buffer size that is smaller
                 than the next packet, then the requested number of
                 bytes are read, and the excess bytes in the packet are
                 discarded.  Specifying a buffer size of PIPE_BUF will
                 be sufficient to read the largest possible packets (see
                 the previous point).

              •  Zero-length packets are not supported.  (A read(2) that
                 specifies a buffer size of zero is a no-op, and returns

              Older kernels that do not support this flag will indicate
              this via an EINVAL error.

              Since Linux 4.5, it is possible to change the O_DIRECT
              setting of a pipe file descriptor using fcntl(2).

              Set the O_NONBLOCK file status flag on the open file
              descriptions referred to by the new file descriptors.
              Using this flag saves extra calls to fcntl(2) to achieve
              the same result.

              Since Linux 5.8, general notification mechanism is built
              on the top of the pipe where kernel splices notification
              messages into pipes opened by user space.  The owner of
              the pipe has to tell the kernel which sources of events to
              watch and filters can also be applied to select which
              subevents should be placed into the pipe.

RETURN VALUE         top

       On success, zero is returned.  On error, -1 is returned, errno is
       set to indicate the error, and pipefd is left unchanged.

       On Linux (and other systems), pipe() does not modify pipefd on
       failure.  A requirement standardizing this behavior was added in
       POSIX.1-2008 TC2.  The Linux-specific pipe2() system call
       likewise does not modify pipefd on failure.

ERRORS         top

       EFAULT pipefd is not valid.

       EINVAL (pipe2()) Invalid value in flags.

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

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

       ENFILE The user hard limit on memory that can be allocated for
              pipes has been reached and the caller is not privileged;
              see pipe(7).

       ENOPKG (pipe2()) O_NOTIFICATION_PIPE was passed in flags and
              support for notifications (CONFIG_WATCH_QUEUE) is not
              compiled into the kernel.

VERSIONS         top

       The System V ABI on some architectures allows the use of more
       than one register for returning multiple values; several
       architectures (namely, Alpha, IA-64, MIPS, SuperH, and
       SPARC/SPARC64) (ab)use this feature in order to implement the
       pipe() system call in a functional manner: the call doesn't take
       any arguments and returns a pair of file descriptors as the
       return value on success.  The glibc pipe() wrapper function
       transparently deals with this.  See syscall(2) for information
       regarding registers used for storing second file descriptor.

STANDARDS         top

       pipe() POSIX.1-2008.


HISTORY         top

       pipe() POSIX.1-2001.

              Linux 2.6.27, glibc 2.9.

EXAMPLES         top

       The following program creates a pipe, and then fork(2)s to create
       a child process; the child inherits a duplicate set of file
       descriptors that refer to the same pipe.  After the fork(2), each
       process closes the file descriptors that it doesn't need for the
       pipe (see pipe(7)).  The parent then writes the string contained
       in the program's command-line argument to the pipe, and the child
       reads this string a byte at a time from the pipe and echoes it on
       standard output.

   Program source
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/wait.h>
       #include <unistd.h>

       main(int argc, char *argv[])
           int    pipefd[2];
           char   buf;
           pid_t  cpid;

           if (argc != 2) {
               fprintf(stderr, "Usage: %s <string>\n", argv[0]);

           if (pipe(pipefd) == -1) {

           cpid = fork();
           if (cpid == -1) {

           if (cpid == 0) {    /* Child reads from pipe */
               close(pipefd[1]);          /* Close unused write end */

               while (read(pipefd[0], &buf, 1) > 0)
                   write(STDOUT_FILENO, &buf, 1);

               write(STDOUT_FILENO, "\n", 1);

           } else {            /* Parent writes argv[1] to pipe */
               close(pipefd[0]);          /* Close unused read end */
               write(pipefd[1], argv[1], strlen(argv[1]));
               close(pipefd[1]);          /* Reader will see EOF */
               wait(NULL);                /* Wait for child */

SEE ALSO         top

       fork(2), read(2), socketpair(2), splice(2), tee(2), vmsplice(2),
       write(2), popen(3), pipe(7)

Linux man-pages (unreleased)     (date)                          pipe(2)

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