pipe(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

PIPE(2)                 Linux Programmer's Manual                PIPE(2)

NAME         top

       pipe, pipe2 - create pipe

SYNOPSIS         top

       #include <unistd.h>

       /* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64; see NOTES */
       struct fd_pair {
           long fd[2];
       };
       struct fd_pair pipe();

       /* On all other architectures */
       int pipe(int pipefd[2]);

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <fcntl.h>              /* Obtain O_* constant definitions */
       #include <unistd.h>

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

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:

       O_CLOEXEC
              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
                 0.)

              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).

       O_NONBLOCK
              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.

RETURN VALUE         top

       On success, zero is returned.  On error, -1 is returned, errno is
       set appropriately, 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).

VERSIONS         top

       pipe2() was added to Linux in version 2.6.27; glibc support is
       available starting with version 2.9.

CONFORMING TO         top

       pipe(): POSIX.1-2001, POSIX.1-2008.

       pipe2() is Linux-specific.

NOTES         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.

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

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

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

           if (pipe(pipefd) == -1) {
               perror("pipe");
               exit(EXIT_FAILURE);
           }

           cpid = fork();
           if (cpid == -1) {
               perror("fork");
               exit(EXIT_FAILURE);
           }

           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);
               close(pipefd[0]);
               _exit(EXIT_SUCCESS);

           } 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 */
               exit(EXIT_SUCCESS);
           }
       }

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.10 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2020-06-09                        PIPE(2)

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