pipe

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



NAME
       pipe, pipe2 - create pipe

SYNOPSIS
       #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
       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
       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.1-2016.
       The Linux-specific pipe2() system call likewise does not modify pipefd
       on failure.

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

NOTES
       The SystemV 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.

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

       pipe2() is Linux-specific.

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

COLOPHON
       This page is part of release 5.01 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                             2019-03-06                           PIPE(2)