# pipe

## 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.
    


## 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 <err.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <string.h>
           #include <sys/types.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)
                   err(EXIT_FAILURE, "pipe");
    
               cpid = fork();
               if (cpid == -1)
                   err(EXIT_FAILURE, "fork");
    
               if (cpid == 0) {    /* Child reads from pipe */
                   if (close(pipefd[1]) == -1)  /* Close unused write end */
                       err(EXIT_FAILURE, "close");
    
                   while (read(pipefd[0], &buf, 1) > 0) {
                       if (write(STDOUT_FILENO, &buf, 1) != 1)
                           err(EXIT_FAILURE, "write");
                   }
    
                   if (write(STDOUT_FILENO, "\n", 1) != 1)
                       err(EXIT_FAILURE, "write");
                   if (close(pipefd[0]) == -1)
                       err(EXIT_FAILURE, "close");
                   _exit(EXIT_SUCCESS);
    
               } else {            /* Parent writes argv[1] to pipe */
                   if (close(pipefd[0]) == -1)  /* Close unused read end */
                       err(EXIT_FAILURE, "close");
                   if (write(pipefd[1], argv[1], strlen(argv[1])) != strlen(argv[1]))
                       err(EXIT_FAILURE, "write");
                   if (close(pipefd[1]) == -1)  /* Reader will see EOF */
                       err(EXIT_FAILURE, "close");
                   if (wait(NULL) == -1)        /* Wait for child */
                       err(EXIT_FAILURE, "wait");
                   exit(EXIT_SUCCESS);
               }
           }
    


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