noble (2) pipe.2.gz

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NAME

       pipe, pipe2 - create pipe

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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

       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.

       O_NOTIFICATION_PIPE
              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

       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

       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

       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

       pipe() POSIX.1-2008.

       pipe2()
              Linux.

HISTORY

       pipe() POSIX.1-2001.

       pipe2()
              Linux 2.6.27, glibc 2.9.

EXAMPLES

       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>

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

           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)