PIPE(3)                    POSIX Programmer's Manual                   PIPE(3)



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


NNAAMMEE
       pipe -- create an interprocess channel

SSYYNNOOPPSSIISS
       #include <unistd.h>

       int pipe(int _f_i_l_d_e_s[2]);

DDEESSCCRRIIPPTTIIOONN
       The _p_i_p_e() function shall create a pipe and place two file descriptors,
       one each into the arguments _f_i_l_d_e_s[0] and _f_i_l_d_e_s[1], that refer to the
       open file descriptions for the read and write ends of the pipe. Their
       integer values shall be the two lowest available at the time of the
       _p_i_p_e() call. The O_NONBLOCK and FD_CLOEXEC flags shall be clear on both
       file descriptors. (The _f_c_n_t_l() function can be used to set both these
       flags.)

       Data can be written to the file descriptor _f_i_l_d_e_s[1] and read from the
       file descriptor _f_i_l_d_e_s[0].  A read on the file descriptor _f_i_l_d_e_s[0]
       shall access data written to the file descriptor _f_i_l_d_e_s[1] on a first-
       in-first-out basis. It is unspecified whether _f_i_l_d_e_s[0] is also open
       for writing and whether _f_i_l_d_e_s[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, _f_i_l_d_e_s[0]
       (write end, _f_i_l_d_e_s[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, _p_i_p_e() shall mark for update the last data
       access, last data modification, and last file status change timestamps
       of the pipe.

RREETTUURRNN VVAALLUUEE
       Upon successful completion, 0 shall be returned; otherwise, -1 shall be
       returned and _e_r_r_n_o set to indicate the error.

EERRRROORRSS
       The _p_i_p_e() function shall fail if:

       EEMMFFIILLEE All, or all but one, of the file descriptors available to the
              process are currently open.

       EENNFFIILLEE The number of simultaneously open files in the system would
              exceed a system-imposed limit.

       _T_h_e _f_o_l_l_o_w_i_n_g _s_e_c_t_i_o_n_s _a_r_e _i_n_f_o_r_m_a_t_i_v_e_.

EEXXAAMMPPLLEESS
   UUssiinngg aa PPiippee ttoo PPaassss DDaattaa BBeettwweeeenn aa PPaarreenntt PPrroocceessss aanndd aa CChhiilldd PPrroocceessss
       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 _f_o_r_k() the two processes close the unused ends of the
       pipe before they commence transferring data.

           ##iinncclluuddee <<ssttddlliibb..hh>>
           ##iinncclluuddee <<uunniissttdd..hh>>
           ......

           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);
           }

AAPPPPLLIICCAATTIIOONN UUSSAAGGEE
       None.

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

FFUUTTUURREE DDIIRREECCTTIIOONNSS
       None.

SSEEEE AALLSSOO
       _f_c_n_t_l(), _r_e_a_d(), _w_r_i_t_e()

       The Base Definitions volume of POSIX.1-2008, <<ffccnnttll..hh>>, <<uunniissttdd..hh>>

CCOOPPYYRRIIGGHHTT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group Standard
       is the referee document. The original Standard can be obtained online
       at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2013                              PIPE(3)
