man2/pipe.2

   1 .\" Copyright (C) 2005, 2008, Michael Kerrisk <mtk.manpages@gmail.com>
   2 .\" (A few fragments remain from an earlier (1992) version by
   3 .\" Drew Eckhardt <drew@cs.colorado.edu>.)
   4 .\"
   5 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   6 .\"
   7 .\" Modified by Michael Haardt <michael@moria.de>
   8 .\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
   9 .\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
  10 .\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
  11 .\" Modified 2005, mtk: added an example program
  12 .\" Modified 2008-01-09, mtk: rewrote DESCRIPTION; minor additions
  13 .\"     to EXAMPLE text.
  14 .\" 2008-10-10, mtk: add description of pipe2()
  15 .\"
  16 .TH PIPE 2 2021-03-22 "Linux" "Linux Programmer's Manual"
  17 .SH NAME
  18 pipe, pipe2 \- create pipe
  19 .SH LIBRARY
  20 Standard C library
  21 .RI ( libc ", " \-lc )
  22 .SH SYNOPSIS
  23 .nf
  24 .B #include <unistd.h>
  25 .PP
  26 .BI "int pipe(int " pipefd [2]);
  27 .PP
  28 .BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
  29 .BR "#include <fcntl.h>" "              /* Definition of " O_* " constants */"
  30 .B #include <unistd.h>
  31 .PP
  32 .BI "int pipe2(int " pipefd "[2], int " flags );
  33 .PP
  34 /* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64, pipe() has the
  35    following prototype; see NOTES */
  36 .PP
  37 .B #include <unistd.h>
  38 .PP
  39 .B struct fd_pair {
  40 .B "    long fd[2];"
  41 .B "};"
  42 .B struct fd_pair pipe(void);
  43 .fi
  44 .SH DESCRIPTION
  45 .BR pipe ()
  46 creates a pipe, a unidirectional data channel that
  47 can be used for interprocess communication.
  48 The array
  49 .I pipefd
  50 is used to return two file descriptors referring to the ends of the pipe.
  51 .I pipefd[0]
  52 refers to the read end of the pipe.
  53 .I pipefd[1]
  54 refers to the write end of the pipe.
  55 Data written to the write end of the pipe is buffered by the kernel
  56 until it is read from the read end of the pipe.
  57 For further details, see
  58 .BR pipe (7).
  59 .PP
  60 If
  61 .I flags
  62 is 0, then
  63 .BR pipe2 ()
  64 is the same as
  65 .BR pipe ().
  66 The following values can be bitwise ORed in
  67 .I flags
  68 to obtain different behavior:
  69 .TP
  70 .B O_CLOEXEC
  71 Set the close-on-exec
  72 .RB ( FD_CLOEXEC )
  73 flag on the two new file descriptors.
  74 See the description of the same flag in
  75 .BR open (2)
  76 for reasons why this may be useful.
  77 .TP
  78 .BR O_DIRECT " (since Linux 3.4)"
  79 .\" commit 9883035ae7edef3ec62ad215611cb8e17d6a1a5d
  80 Create a pipe that performs I/O in "packet" mode.
  81 Each
  82 .BR write (2)
  83 to the pipe is dealt with as a separate packet, and
  84 .BR read (2)s
  85 from the pipe will read one packet at a time.
  86 Note the following points:
  87 .RS
  88 .IP * 3
  89 Writes of greater than
  90 .B PIPE_BUF
  91 bytes (see
  92 .BR pipe (7))
  93 will be split into multiple packets.
  94 The constant
  95 .B PIPE_BUF
  96 is defined in
  97 .IR <limits.h> .
  98 .IP *
  99 If a
 100 .BR read (2)
 101 specifies a buffer size that is smaller than the next packet,
 102 then the requested number of bytes are read,
 103 and the excess bytes in the packet are discarded.
 104 Specifying a buffer size of
 105 .B PIPE_BUF
 106 will be sufficient to read the largest possible packets
 107 (see the previous point).
 108 .IP *
 109 Zero-length packets are not supported.
 110 (A
 111 .BR read (2)
 112 that specifies a buffer size of zero is a no-op, and returns 0.)
 113 .RE
 114 .IP
 115 Older kernels that do not support this flag will indicate this via an
 116 .B EINVAL
 117 error.
 118 .IP
 119 Since Linux 4.5,
 120 .\" commit 0dbf5f20652108106cb822ad7662c786baaa03ff
 121 .\" FIXME . But, it is not possible to specify O_DIRECT when opening a FIFO
 122 it is possible to change the
 123 .B O_DIRECT
 124 setting of a pipe file descriptor using
 125 .BR fcntl (2).
 126 .TP
 127 .B O_NONBLOCK
 128 Set the
 129 .B O_NONBLOCK
 130 file status flag on the open file descriptions
 131 referred to by the new file descriptors.
 132 Using this flag saves extra calls to
 133 .BR fcntl (2)
 134 to achieve the same result.
 135 .SH RETURN VALUE
 136 On success, zero is returned.
 137 On error, \-1 is returned,
 138 .I errno
 139 is set to indicate the error, and
 140 .I pipefd
 141 is left unchanged.
 142 .PP
 143 On Linux (and other systems),
 144 .BR pipe ()
 145 does not modify
 146 .I pipefd
 147 on failure.
 148 A requirement standardizing this behavior was added in POSIX.1-2008 TC2.
 149 .\" http://austingroupbugs.net/view.php?id=467
 150 The Linux-specific
 151 .BR pipe2 ()
 152 system call
 153 likewise does not modify
 154 .I pipefd
 155 on failure.
 156 .SH ERRORS
 157 .TP
 158 .B EFAULT
 159 .I pipefd
 160 is not valid.
 161 .TP
 162 .B EINVAL
 163 .RB ( pipe2 ())
 164 Invalid value in
 165 .IR flags .
 166 .TP
 167 .B EMFILE
 168 The per-process limit on the number of open file descriptors has been reached.
 169 .TP
 170 .B ENFILE
 171 The system-wide limit on the total number of open files has been reached.
 172 .TP
 173 .B ENFILE
 174 The user hard limit on memory that can be allocated for pipes
 175 has been reached and the caller is not privileged; see
 176 .BR pipe (7).
 177 .SH VERSIONS
 178 .BR pipe2 ()
 179 was added to Linux in version 2.6.27;
 180 glibc support is available starting with
 181 version 2.9.
 182 .SH CONFORMING TO
 183 .BR pipe ():
 184 POSIX.1-2001, POSIX.1-2008.
 185 .PP
 186 .BR pipe2 ()
 187 is Linux-specific.
 188 .SH NOTES
 189 .\" See http://math-atlas.sourceforge.net/devel/assembly/64.psabi.1.33.ps.Z
 190 .\" for example, section 3.2.1 "Registers and the Stack Frame".
 191 The System V ABI on some architectures allows the use of more than one register
 192 for returning multiple values; several architectures
 193 (namely, Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64)
 194 (ab)use this feature in order to implement the
 195 .BR pipe ()
 196 system call in a functional manner:
 197 the call doesn't take any arguments and returns
 198 a pair of file descriptors as the return value on success.
 199 The glibc
 200 .BR pipe ()
 201 wrapper function transparently deals with this.
 202 See
 203 .BR syscall (2)
 204 for information regarding registers used for storing second file descriptor.
 205 .SH EXAMPLES
 206 .\" fork.2 refers to this example program.
 207 The following program creates a pipe, and then
 208 .BR fork (2)s
 209 to create a child process;
 210 the child inherits a duplicate set of file
 211 descriptors that refer to the same pipe.
 212 After the
 213 .BR fork (2),
 214 each process closes the file descriptors that it doesn't need for the pipe
 215 (see
 216 .BR pipe (7)).
 217 The parent then writes the string contained in the program's
 218 command-line argument to the pipe,
 219 and the child reads this string a byte at a time from the pipe
 220 and echoes it on standard output.
 221 .SS Program source
 222 .EX
 223 #include <sys/types.h>
 224 #include <sys/wait.h>
 225 #include <stdio.h>
 226 #include <stdlib.h>
 227 #include <unistd.h>
 228 #include <string.h>
 229
 230 int
 231 main(int argc, char *argv[])
 232 {
 233     int pipefd[2];
 234     pid_t cpid;
 235     char buf;
 236
 237     if (argc != 2) {
 238         fprintf(stderr, "Usage: %s <string>\en", argv[0]);
 239         exit(EXIT_FAILURE);
 240     }
 241
 242     if (pipe(pipefd) == \-1) {
 243         perror("pipe");
 244         exit(EXIT_FAILURE);
 245     }
 246
 247     cpid = fork();
 248     if (cpid == \-1) {
 249         perror("fork");
 250         exit(EXIT_FAILURE);
 251     }
 252
 253     if (cpid == 0) {    /* Child reads from pipe */
 254         close(pipefd[1]);          /* Close unused write end */
 255
 256         while (read(pipefd[0], &buf, 1) > 0)
 257             write(STDOUT_FILENO, &buf, 1);
 258
 259         write(STDOUT_FILENO, "\en", 1);
 260         close(pipefd[0]);
 261         _exit(EXIT_SUCCESS);
 262
 263     } else {            /* Parent writes argv[1] to pipe */
 264         close(pipefd[0]);          /* Close unused read end */
 265         write(pipefd[1], argv[1], strlen(argv[1]));
 266         close(pipefd[1]);          /* Reader will see EOF */
 267         wait(NULL);                /* Wait for child */
 268         exit(EXIT_SUCCESS);
 269     }
 270 }
 271 .EE
 272 .SH SEE ALSO
 273 .BR fork (2),
 274 .BR read (2),
 275 .BR socketpair (2),
 276 .BR splice (2),
 277 .BR tee (2),
 278 .BR vmsplice (2),
 279 .BR write (2),
 280 .BR popen (3),
 281 .BR pipe (7)