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


     pipe, pipe2 - create pipe


     #include <unistd.h>
     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);


     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 val-
     ues 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  follow-
            ing 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 spec-

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


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


     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
     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).


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


     pipe(): POSIX.1-2001, POSIX.1-2008.
     pipe2() is Linux-specific.


     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]);
         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 */
         } }


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


     This  page  is  part of release 4.16 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

Linux 2017-11-26 PIPE(2)

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