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>.)
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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 2019-03-06 "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 /* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64; see NOTES */
  44 .B struct fd_pair {
  45 .B "    long fd[2];"
  46 .B "};"
  47 .B struct fd_pair pipe();
  48 .PP
  49 /* On all other architectures */
  50 .BI "int pipe(int " pipefd "[2]);"
  51
  52 .BR "#define _GNU_SOURCE" "             /* See feature_test_macros(7) */"
  53 .BR "#include <fcntl.h>" "              /* Obtain O_* constant definitions */
  54 .B #include <unistd.h>
  55 .PP
  56 .BI "int pipe2(int " pipefd "[2], int " flags );
  57 .fi
  58 .SH DESCRIPTION
  59 .BR pipe ()
  60 creates a pipe, a unidirectional data channel that
  61 can be used for interprocess communication.
  62 The array
  63 .IR pipefd
  64 is used to return two file descriptors referring to the ends of the pipe.
  65 .IR pipefd[0]
  66 refers to the read end of the pipe.
  67 .IR pipefd[1]
  68 refers to the write end of the pipe.
  69 Data written to the write end of the pipe is buffered by the kernel
  70 until it is read from the read end of the pipe.
  71 For further details, see
  72 .BR pipe (7).
  73 .PP
  74 If
  75 .IR flags
  76 is 0, then
  77 .BR pipe2 ()
  78 is the same as
  79 .BR pipe ().
  80 The following values can be bitwise ORed in
  81 .IR flags
  82 to obtain different behavior:
  83 .TP
  84 .B O_CLOEXEC
  85 Set the close-on-exec
  86 .RB ( FD_CLOEXEC )
  87 flag on the two new file descriptors.
  88 See the description of the same flag in
  89 .BR open (2)
  90 for reasons why this may be useful.
  91 .TP
  92 .BR O_DIRECT " (since Linux 3.4)"
  93 .\" commit 9883035ae7edef3ec62ad215611cb8e17d6a1a5d
  94 Create a pipe that performs I/O in "packet" mode.
  95 Each
  96 .BR write (2)
  97 to the pipe is dealt with as a separate packet, and
  98 .BR read (2)s
  99 from the pipe will read one packet at a time.
 100 Note the following points:
 101 .RS
 102 .IP * 3
 103 Writes of greater than
 104 .BR PIPE_BUF
 105 bytes (see
 106 .BR pipe (7))
 107 will be split into multiple packets.
 108 The constant
 109 .BR PIPE_BUF
 110 is defined in
 111 .IR <limits.h> .
 112 .IP *
 113 If a
 114 .BR read (2)
 115 specifies a buffer size that is smaller than the next packet,
 116 then the requested number of bytes are read,
 117 and the excess bytes in the packet are discarded.
 118 Specifying a buffer size of
 119 .BR PIPE_BUF
 120 will be sufficient to read the largest possible packets
 121 (see the previous point).
 122 .IP *
 123 Zero-length packets are not supported.
 124 (A
 125 .BR read (2)
 126 that specifies a buffer size of zero is a no-op, and returns 0.)
 127 .RE
 128 .IP
 129 Older kernels that do not support this flag will indicate this via an
 130 .B EINVAL
 131 error.
 132 .IP
 133 Since Linux 4.5,
 134 .\" commit 0dbf5f20652108106cb822ad7662c786baaa03ff
 135 .\" FIXME . But, it is not possible to specify O_DIRECT when opening a FIFO
 136 it is possible to change the
 137 .B O_DIRECT
 138 setting of a pipe file descriptor using
 139 .BR fcntl (2).
 140 .TP
 141 .B O_NONBLOCK
 142 Set the
 143 .BR O_NONBLOCK
 144 file status flag on the open file descriptions
 145 referred to by the new file descriptors.
 146 Using this flag saves extra calls to
 147 .BR fcntl (2)
 148 to achieve the same result.
 149 .SH RETURN VALUE
 150 On success, zero is returned.
 151 On error, \-1 is returned,
 152 .I errno
 153 is set appropriately, and
 154 .I pipefd
 155 is left unchanged.
 156 .PP
 157 On Linux (and other systems),
 158 .BR pipe ()
 159 does not modify
 160 .I pipefd
 161 on failure.
 162 A requirement standardizing this behavior was added in POSIX.1-2016.
 163 .\" http://austingroupbugs.net/view.php?id=467
 164 The Linux-specific
 165 .BR pipe2 ()
 166 system call
 167 likewise does not modify
 168 .I pipefd
 169 on failure.
 170 .SH ERRORS
 171 .TP
 172 .B EFAULT
 173 .I pipefd
 174 is not valid.
 175 .TP
 176 .B EINVAL
 177 .RB ( pipe2 ())
 178 Invalid value in
 179 .IR flags .
 180 .TP
 181 .B EMFILE
 182 The per-process limit on the number of open file descriptors has been reached.
 183 .TP
 184 .B ENFILE
 185 The system-wide limit on the total number of open files has been reached.
 186 .TP
 187 .B ENFILE
 188 The user hard limit on memory that can be allocated for pipes
 189 has been reached and the caller is not privileged; see
 190 .BR pipe (7).
 191 .SH VERSIONS
 192 .BR pipe2 ()
 193 was added to Linux in version 2.6.27;
 194 glibc support is available starting with
 195 version 2.9.
 196 .SH NOTES
 197 .\" See http://math-atlas.sourceforge.net/devel/assembly/64.psabi.1.33.ps.Z
 198 .\" for example, section 3.2.1 "Registers and the Stack Frame".
 199 The System V ABI on some architectures allows the use of more than one register
 200 for returning multiple values; several architectures
 201 (namely, Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64)
 202 (ab)use this feature in order to implement the
 203 .BR pipe ()
 204 system call in a functional manner:
 205 the call doesn't take any arguments and returns
 206 a pair of file descriptors as the return value on success.
 207 The glibc
 208 .BR pipe ()
 209 wrapper function transparently deals with this.
 210 See
 211 .BR syscall (2)
 212 for information regarding registers used for storing second file descriptor.
 213 .SH CONFORMING TO
 214 .BR pipe ():
 215 POSIX.1-2001, POSIX.1-2008.
 216 .PP
 217 .BR pipe2 ()
 218 is Linux-specific.
 219 .SH EXAMPLE
 220 .\" fork.2 refers to this example program.
 221 The following program creates a pipe, and then
 222 .BR fork (2)s
 223 to create a child process;
 224 the child inherits a duplicate set of file
 225 descriptors that refer to the same pipe.
 226 After the
 227 .BR fork (2),
 228 each process closes the file descriptors that it doesn't need for the pipe
 229 (see
 230 .BR pipe (7)).
 231 The parent then writes the string contained in the program's
 232 command-line argument to the pipe,
 233 and the child reads this string a byte at a time from the pipe
 234 and echoes it on standard output.
 235 .SS Program source
 236 .EX
 237 #include <sys/types.h>
 238 #include <sys/wait.h>
 239 #include <stdio.h>
 240 #include <stdlib.h>
 241 #include <unistd.h>
 242 #include <string.h>
 243
 244 int
 245 main(int argc, char *argv[])
 246 {
 247     int pipefd[2];
 248     pid_t cpid;
 249     char buf;
 250
 251     if (argc != 2) {
 252         fprintf(stderr, "Usage: %s <string>\en", argv[0]);
 253         exit(EXIT_FAILURE);
 254     }
 255
 256     if (pipe(pipefd) == \-1) {
 257         perror("pipe");
 258         exit(EXIT_FAILURE);
 259     }
 260
 261     cpid = fork();
 262     if (cpid == \-1) {
 263         perror("fork");
 264         exit(EXIT_FAILURE);
 265     }
 266
 267     if (cpid == 0) {    /* Child reads from pipe */
 268         close(pipefd[1]);          /* Close unused write end */
 269
 270         while (read(pipefd[0], &buf, 1) > 0)
 271             write(STDOUT_FILENO, &buf, 1);
 272
 273         write(STDOUT_FILENO, "\en", 1);
 274         close(pipefd[0]);
 275         _exit(EXIT_SUCCESS);
 276
 277     } else {            /* Parent writes argv[1] to pipe */
 278         close(pipefd[0]);          /* Close unused read end */
 279         write(pipefd[1], argv[1], strlen(argv[1]));
 280         close(pipefd[1]);          /* Reader will see EOF */
 281         wait(NULL);                /* Wait for child */
 282         exit(EXIT_SUCCESS);
 283     }
 284 }
 285 .EE
 286 .SH SEE ALSO
 287 .BR fork (2),
 288 .BR read (2),
 289 .BR socketpair (2),
 290 .BR splice (2),
 291 .BR tee (2),
 292 .BR vmsplice (2),
 293 .BR write (2),
 294 .BR popen (3),
 295 .BR pipe (7)