man2/pipe.2

   1 .\" Copyright (C) 2005, 2008, Michael Kerrisk <mtk.manpages@gmail.com>
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  27 .\" Modified by Michael Haardt <michael@moria.de>
  28 .\" Modified 1993-07-23 by Rik Faith <faith@cs.unc.edu>
  29 .\" Modified 1996-10-22 by Eric S. Raymond <esr@thyrsus.com>
  30 .\" Modified 2004-06-17 by Michael Kerrisk <mtk.manpages@gmail.com>
  31 .\" Modified 2005, mtk: added an example program
  32 .\" Modified 2008-01-09, mtk: rewrote DESCRIPTION; minor additions
  33 .\"     to EXAMPLE text.
  34 .\" 2008-10-10, mtk: add description of pipe2()
  35 .\"
  36 .TH PIPE 2 2017-11-26 "Linux" "Linux Programmer's Manual"
  37 .SH NAME
  38 pipe, pipe2 \- create pipe
  39 .SH SYNOPSIS
  40 .nf
  41 .B #include <unistd.h>
  42 .PP
  43 .BI "int pipe(int " pipefd "[2]);"
  44
  45 .BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
  46 .BR "#include <fcntl.h>" "              /* Obtain O_* constant definitions */
  47 .B #include <unistd.h>
  48 .PP
  49 .BI "int pipe2(int " pipefd "[2], int " flags );
  50 .fi
  51 .SH DESCRIPTION
  52 .BR pipe ()
  53 creates a pipe, a unidirectional data channel that
  54 can be used for interprocess communication.
  55 The array
  56 .IR pipefd
  57 is used to return two file descriptors referring to the ends of the pipe.
  58 .IR pipefd[0]
  59 refers to the read end of the pipe.
  60 .IR pipefd[1]
  61 refers to the write end of the pipe.
  62 Data written to the write end of the pipe is buffered by the kernel
  63 until it is read from the read end of the pipe.
  64 For further details, see
  65 .BR pipe (7).
  66 .PP
  67 If
  68 .IR flags
  69 is 0, then
  70 .BR pipe2 ()
  71 is the same as
  72 .BR pipe ().
  73 The following values can be bitwise ORed in
  74 .IR flags
  75 to obtain different behavior:
  76 .TP
  77 .B O_CLOEXEC
  78 Set the close-on-exec
  79 .RB ( FD_CLOEXEC )
  80 flag on the two new file descriptors.
  81 See the description of the same flag in
  82 .BR open (2)
  83 for reasons why this may be useful.
  84 .TP
  85 .BR O_DIRECT " (since Linux 3.4)"
  86 .\" commit 9883035ae7edef3ec62ad215611cb8e17d6a1a5d
  87 Create a pipe that performs I/O in "packet" mode.
  88 Each
  89 .BR write (2)
  90 to the pipe is dealt with as a separate packet, and
  91 .BR read (2)s
  92 from the pipe will read one packet at a time.
  93 Note the following points:
  94 .RS
  95 .IP * 3
  96 Writes of greater than
  97 .BR PIPE_BUF
  98 bytes (see
  99 .BR pipe (7))
 100 will be split into multiple packets.
 101 The constant
 102 .BR PIPE_BUF
 103 is defined in
 104 .IR <limits.h> .
 105 .IP *
 106 If a
 107 .BR read (2)
 108 specifies a buffer size that is smaller than the next packet,
 109 then the requested number of bytes are read,
 110 and the excess bytes in the packet are discarded.
 111 Specifying a buffer size of
 112 .BR PIPE_BUF
 113 will be sufficient to read the largest possible packets
 114 (see the previous point).
 115 .IP *
 116 Zero-length packets are not supported.
 117 (A
 118 .BR read (2)
 119 that specifies a buffer size of zero is a no-op, and returns 0.)
 120 .RE
 121 .IP
 122 Older kernels that do not support this flag will indicate this via an
 123 .B EINVAL
 124 error.
 125 .IP
 126 Since Linux 4.5,
 127 .\" commit 0dbf5f20652108106cb822ad7662c786baaa03ff
 128 .\" FIXME . But, it is not possible to specify O_DIRECT when opening a FIFO
 129 it is possible to change the
 130 .B O_DIRECT
 131 setting of a pipe file descriptor using
 132 .BR fcntl (2).
 133 .TP
 134 .B O_NONBLOCK
 135 Set the
 136 .BR O_NONBLOCK
 137 file status flag on the two new open file descriptions.
 138 Using this flag saves extra calls to
 139 .BR fcntl (2)
 140 to achieve the same result.
 141 .SH RETURN VALUE
 142 On success, zero is returned.
 143 On error, \-1 is returned, and
 144 .I errno
 145 is set appropriately.
 146 .PP
 147 On Linux (and other systems),
 148 .BR pipe ()
 149 does not modify
 150 .I pipefd
 151 on failure.
 152 A requirement standardizing this behavior was added in POSIX.1-2016.
 153 .\" http://austingroupbugs.net/view.php?id=467
 154 The Linux-specific
 155 .BR pipe2 ()
 156 system call
 157 likewise does not modify
 158 .I pipefd
 159 on failure.
 160 .SH ERRORS
 161 .TP
 162 .B EFAULT
 163 .I pipefd
 164 is not valid.
 165 .TP
 166 .B EINVAL
 167 .RB ( pipe2 ())
 168 Invalid value in
 169 .IR flags .
 170 .TP
 171 .B EMFILE
 172 The per-process limit on the number of open file descriptors has been reached.
 173 .TP
 174 .B ENFILE
 175 The system-wide limit on the total number of open files has been reached.
 176 .TP
 177 .B ENFILE
 178 The user hard limit on memory that can be allocated for pipes
 179 has been reached and the caller is not privileged; see
 180 .BR pipe (7).
 181 .SH VERSIONS
 182 .BR pipe2 ()
 183 was added to Linux in version 2.6.27;
 184 glibc support is available starting with
 185 version 2.9.
 186 .SH CONFORMING TO
 187 .BR pipe ():
 188 POSIX.1-2001, POSIX.1-2008.
 189 .PP
 190 .BR pipe2 ()
 191 is Linux-specific.
 192 .SH EXAMPLE
 193 .\" fork.2 refers to this example program.
 194 The following program creates a pipe, and then
 195 .BR fork (2)s
 196 to create a child process;
 197 the child inherits a duplicate set of file
 198 descriptors that refer to the same pipe.
 199 After the
 200 .BR fork (2),
 201 each process closes the file descriptors that it doesn't need for the pipe
 202 (see
 203 .BR pipe (7)).
 204 The parent then writes the string contained in the program's
 205 command-line argument to the pipe,
 206 and the child reads this string a byte at a time from the pipe
 207 and echoes it on standard output.
 208 .SS Program source
 209 .EX
 210 #include <sys/types.h>
 211 #include <sys/wait.h>
 212 #include <stdio.h>
 213 #include <stdlib.h>
 214 #include <unistd.h>
 215 #include <string.h>
 216
 217 int
 218 main(int argc, char *argv[])
 219 {
 220     int pipefd[2];
 221     pid_t cpid;
 222     char buf;
 223
 224     if (argc != 2) {
 225         fprintf(stderr, "Usage: %s <string>\\n", argv[0]);
 226         exit(EXIT_FAILURE);
 227     }
 228
 229     if (pipe(pipefd) == \-1) {
 230         perror("pipe");
 231         exit(EXIT_FAILURE);
 232     }
 233
 234     cpid = fork();
 235     if (cpid == \-1) {
 236         perror("fork");
 237         exit(EXIT_FAILURE);
 238     }
 239
 240     if (cpid == 0) {    /* Child reads from pipe */
 241         close(pipefd[1]);          /* Close unused write end */
 242
 243         while (read(pipefd[0], &buf, 1) > 0)
 244             write(STDOUT_FILENO, &buf, 1);
 245
 246         write(STDOUT_FILENO, "\\n", 1);
 247         close(pipefd[0]);
 248         _exit(EXIT_SUCCESS);
 249
 250     } else {            /* Parent writes argv[1] to pipe */
 251         close(pipefd[0]);          /* Close unused read end */
 252         write(pipefd[1], argv[1], strlen(argv[1]));
 253         close(pipefd[1]);          /* Reader will see EOF */
 254         wait(NULL);                /* Wait for child */
 255         exit(EXIT_SUCCESS);
 256     }
 257 }
 258 .EE
 259 .SH SEE ALSO
 260 .BR fork (2),
 261 .BR read (2),
 262 .BR socketpair (2),
 263 .BR splice (2),
 264 .BR tee (2),
 265 .BR vmsplice (2),
 266 .BR write (2),
 267 .BR popen (3),
 268 .BR pipe (7)