# pipe

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           pipe — create an interprocess channel
    


## SYNOPSIS



           #include <unistd.h>
    
           int pipe(int _fildes_[2]);
    


## DESCRIPTION



           The _pipe_() function shall create a pipe and place two file
           descriptors, one each into the arguments _fildes_[0] and _fildes_[1],
           that refer to the open file descriptions for the read and write
           ends of the pipe. The file descriptors shall be allocated as
           described in _Section 2.14_ , _File Descriptor Allocation_.  The
           O_NONBLOCK and FD_CLOEXEC flags shall be clear on both file
           descriptors. (The _fcntl_() function can be used to set both these
           flags.)
    
           Data can be written to the file descriptor _fildes_[1] and read from
           the file descriptor _fildes_[0].  A read on the file descriptor
           _fildes_[0] shall access data written to the file descriptor
           _fildes_[1] on a first-in-first-out basis. It is unspecified whether
           _fildes_[0] is also open for writing and whether _fildes_[1] is also
           open for reading.
    
           A process has the pipe open for reading (correspondingly writing)
           if it has a file descriptor open that refers to the read end,
           _fildes_[0] (write end, _fildes_[1]).
    
           The pipe's user ID shall be set to the effective user ID of the
           calling process.
    
           The pipe's group ID shall be set to the effective group ID of the
           calling process.
    
           Upon successful completion, _pipe_() shall mark for update the last
           data access, last data modification, and last file status change
           timestamps of the pipe.
    


## RETURN VALUE



           Upon successful completion, 0 shall be returned; otherwise, -1
           shall be returned and _errno_ set to indicate the error, no file
           descriptors shall be allocated and the contents of _fildes_ shall be
           left unmodified.
    


## ERRORS



           The _pipe_() function shall fail if:
    
           **EMFILE** All, or all but one, of the file descriptors available to
                  the process are currently open.
    
           **ENFILE** The number of simultaneously open files in the system would
                  exceed a system-imposed limit.
    
           _The following sections are informative._
    


## EXAMPLES



       **Using a Pipe to Pass Data Between a Parent Process and a Child Process**
    
           The following example demonstrates the use of a pipe to transfer
           data between a parent process and a child process. Error handling
           is excluded, but otherwise this code demonstrates good practice
           when using pipes: after the _fork_() the two processes close the
           unused ends of the pipe before they commence transferring data.
    
               #include <stdlib.h>
               #include <unistd.h>
               ...
    
               int fildes[2];
               const int BSIZE = 100;
               char buf[BSIZE];
               ssize_t nbytes;
               int status;
    
               status = pipe(fildes);
               if (status == -1 ) {
                   /* an error occurred */
                   ...
               }
    
               switch (fork()) {
               case -1: /* Handle error */
                   break;
    
               case 0:  /* Child - reads from pipe */
                   close(fildes[1]);                       /* Write end is unused */
                   nbytes = read(fildes[0], buf, BSIZE);   /* Get data from pipe */
                   /* At this point, a further read would see end-of-file ... */
                   close(fildes[0]);                       /* Finished with pipe */
                   exit(EXIT_SUCCESS);
    
               default:  /* Parent - writes to pipe */
                   close(fildes[0]);                       /* Read end is unused */
                   write(fildes[1], "Hello world\n", 12);  /* Write data on pipe */
                   close(fildes[1]);                       /* Child will see EOF */
                   exit(EXIT_SUCCESS);
               }
    


## APPLICATION USAGE



           None.
    


## RATIONALE



           The wording carefully avoids using the verb ``to open'' in order
           to avoid any implication of use of _open_(); see also _write_().
    


## FUTURE DIRECTIONS



           None.
    


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