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 2015-12-28 "Linux" "Linux Programmer's Manual"
  37 .SH NAME
  38 pipe, pipe2 \- create pipe
  39 .SH SYNOPSIS
  40 .nf
  41 .B #include <unistd.h>
  42 .sp
  43 .BI "int pipe(int " pipefd "[2]);"
  44 .sp
  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 .sp
  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
  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 .TP
 126 .B O_NONBLOCK
 127 Set the
 128 .BR O_NONBLOCK
 129 file status flag on the two new open file descriptions.
 130 Using this flag saves extra calls to
 131 .BR fcntl (2)
 132 to achieve the same result.
 133 .SH RETURN VALUE
 134 On success, zero is returned.
 135 On error, \-1 is returned, and
 136 .I errno
 137 is set appropriately.
 138
 139 On Linux (and other systems),
 140 .BR pipe ()
 141 does not modify
 142 .I pipefd
 143 on failure.
 144 A requirement standardizing this behavior was added in POSIX.1-2016.
 145 .\" http://austingroupbugs.net/view.php?id=467
 146 The Linux-specific
 147 .BR pipe2 ()
 148 system call
 149 likewise does not modify
 150 .I pipefd
 151 on failure.
 152 .SH ERRORS
 153 .TP
 154 .B EFAULT
 155 .I pipefd
 156 is not valid.
 157 .TP
 158 .B EINVAL
 159 .RB ( pipe2 ())
 160 Invalid value in
 161 .IR flags .
 162 .TP
 163 .B EMFILE
 164 The per-process limit on the number of open file descriptors has been reached.
 165 .TP
 166 .B ENFILE
 167 The system-wide limit on the total number of open files has been reached.
 168 .SH VERSIONS
 169 .BR pipe2 ()
 170 was added to Linux in version 2.6.27;
 171 glibc support is available starting with
 172 version 2.9.
 173 .SH CONFORMING TO
 174 .BR pipe ():
 175 POSIX.1-2001, POSIX.1-2008.
 176
 177 .BR pipe2 ()
 178 is Linux-specific.
 179 .SH EXAMPLE
 180 .\" fork.2 refers to this example program.
 181 The following program creates a pipe, and then
 182 .BR fork (2)s
 183 to create a child process;
 184 the child inherits a duplicate set of file
 185 descriptors that refer to the same pipe.
 186 After the
 187 .BR fork (2),
 188 each process closes the file descriptors that it doesn't need for the pipe
 189 (see
 190 .BR pipe (7)).
 191 The parent then writes the string contained in the program's
 192 command-line argument to the pipe,
 193 and the child reads this string a byte at a time from the pipe
 194 and echoes it on standard output.
 195 .SS Program source
 196 .nf
 197 #include <sys/types.h>
 198 #include <sys/wait.h>
 199 #include <stdio.h>
 200 #include <stdlib.h>
 201 #include <unistd.h>
 202 #include <string.h>
 203
 204 int
 205 main(int argc, char *argv[])
 206 {
 207     int pipefd[2];
 208     pid_t cpid;
 209     char buf;
 210
 211     if (argc != 2) {
 212         fprintf(stderr, "Usage: %s <string>\\n", argv[0]);
 213         exit(EXIT_FAILURE);
 214     }
 215
 216     if (pipe(pipefd) == \-1) {
 217         perror("pipe");
 218         exit(EXIT_FAILURE);
 219     }
 220
 221     cpid = fork();
 222     if (cpid == \-1) {
 223         perror("fork");
 224         exit(EXIT_FAILURE);
 225     }
 226
 227     if (cpid == 0) {    /* Child reads from pipe */
 228         close(pipefd[1]);          /* Close unused write end */
 229
 230         while (read(pipefd[0], &buf, 1) > 0)
 231             write(STDOUT_FILENO, &buf, 1);
 232
 233         write(STDOUT_FILENO, "\\n", 1);
 234         close(pipefd[0]);
 235         _exit(EXIT_SUCCESS);
 236
 237     } else {            /* Parent writes argv[1] to pipe */
 238         close(pipefd[0]);          /* Close unused read end */
 239         write(pipefd[1], argv[1], strlen(argv[1]));
 240         close(pipefd[1]);          /* Reader will see EOF */
 241         wait(NULL);                /* Wait for child */
 242         exit(EXIT_SUCCESS);
 243     }
 244 }
 245 .fi
 246 .SH SEE ALSO
 247 .BR fork (2),
 248 .BR read (2),
 249 .BR socketpair (2),
 250 .BR splice (2),
 251 .BR write (2),
 252 .BR popen (3),
 253 .BR pipe (7)