.\" Copyright (C) 2005, 2008, Michael Kerrisk .\" (A few fragments remain from an earlier (1992) version by .\" Drew Eckhardt .) .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" Modified by Michael Haardt .\" Modified 1993-07-23 by Rik Faith .\" Modified 1996-10-22 by Eric S. Raymond .\" Modified 2004-06-17 by Michael Kerrisk .\" Modified 2005, mtk: added an example program .\" Modified 2008-01-09, mtk: rewrote DESCRIPTION; minor additions .\" to EXAMPLE text. .\" 2008-10-10, mtk: add description of pipe2() .\" .TH PIPE 2 2020-06-09 "Linux" "Linux Programmer's Manual" .SH NAME pipe, pipe2 \- create pipe .SH SYNOPSIS .nf .B #include .PP /* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64; see NOTES */ .B struct fd_pair { .B " long fd[2];" .B "};" .B struct fd_pair pipe(); .PP /* On all other architectures */ .BI "int pipe(int " pipefd "[2]);" .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" .BR "#include " " /* Obtain O_* constant definitions */" .B #include .PP .BI "int pipe2(int " pipefd "[2], int " flags ); .fi .SH DESCRIPTION .BR pipe () creates a pipe, a unidirectional data channel that can be used for interprocess communication. The array .IR pipefd is used to return two file descriptors referring to the ends of the pipe. .IR pipefd[0] refers to the read end of the pipe. .IR pipefd[1] refers to the write end of the pipe. Data written to the write end of the pipe is buffered by the kernel until it is read from the read end of the pipe. For further details, see .BR pipe (7). .PP If .IR flags is 0, then .BR pipe2 () is the same as .BR pipe (). The following values can be bitwise ORed in .IR flags to obtain different behavior: .TP .B O_CLOEXEC Set the close-on-exec .RB ( FD_CLOEXEC ) flag on the two new file descriptors. See the description of the same flag in .BR open (2) for reasons why this may be useful. .TP .BR O_DIRECT " (since Linux 3.4)" .\" commit 9883035ae7edef3ec62ad215611cb8e17d6a1a5d Create a pipe that performs I/O in "packet" mode. Each .BR write (2) to the pipe is dealt with as a separate packet, and .BR read (2)s from the pipe will read one packet at a time. Note the following points: .RS .IP * 3 Writes of greater than .BR PIPE_BUF bytes (see .BR pipe (7)) will be split into multiple packets. The constant .BR PIPE_BUF is defined in .IR . .IP * If a .BR read (2) specifies a buffer size that is smaller than the next packet, then the requested number of bytes are read, and the excess bytes in the packet are discarded. Specifying a buffer size of .BR PIPE_BUF will be sufficient to read the largest possible packets (see the previous point). .IP * Zero-length packets are not supported. (A .BR read (2) that specifies a buffer size of zero is a no-op, and returns 0.) .RE .IP Older kernels that do not support this flag will indicate this via an .B EINVAL error. .IP Since Linux 4.5, .\" commit 0dbf5f20652108106cb822ad7662c786baaa03ff .\" FIXME . But, it is not possible to specify O_DIRECT when opening a FIFO it is possible to change the .B O_DIRECT setting of a pipe file descriptor using .BR fcntl (2). .TP .B O_NONBLOCK Set the .BR O_NONBLOCK file status flag on the open file descriptions referred to by the new file descriptors. Using this flag saves extra calls to .BR fcntl (2) to achieve the same result. .SH RETURN VALUE On success, zero is returned. On error, \-1 is returned, .I errno is set appropriately, and .I pipefd is left unchanged. .PP On Linux (and other systems), .BR pipe () does not modify .I pipefd on failure. A requirement standardizing this behavior was added in POSIX.1-2008 TC2. .\" http://austingroupbugs.net/view.php?id=467 The Linux-specific .BR pipe2 () system call likewise does not modify .I pipefd on failure. .SH ERRORS .TP .B EFAULT .I pipefd is not valid. .TP .B EINVAL .RB ( pipe2 ()) Invalid value in .IR flags . .TP .B EMFILE The per-process limit on the number of open file descriptors has been reached. .TP .B ENFILE The system-wide limit on the total number of open files has been reached. .TP .B ENFILE The user hard limit on memory that can be allocated for pipes has been reached and the caller is not privileged; see .BR pipe (7). .SH VERSIONS .BR pipe2 () was added to Linux in version 2.6.27; glibc support is available starting with version 2.9. .SH CONFORMING TO .BR pipe (): POSIX.1-2001, POSIX.1-2008. .PP .BR pipe2 () is Linux-specific. .SH NOTES .\" See http://math-atlas.sourceforge.net/devel/assembly/64.psabi.1.33.ps.Z .\" for example, section 3.2.1 "Registers and the Stack Frame". The System V ABI on some architectures allows the use of more than one register for returning multiple values; several architectures (namely, Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64) (ab)use this feature in order to implement the .BR pipe () system call in a functional manner: the call doesn't take any arguments and returns a pair of file descriptors as the return value on success. The glibc .BR pipe () wrapper function transparently deals with this. See .BR syscall (2) for information regarding registers used for storing second file descriptor. .SH EXAMPLES .\" fork.2 refers to this example program. The following program creates a pipe, and then .BR fork (2)s to create a child process; the child inherits a duplicate set of file descriptors that refer to the same pipe. After the .BR fork (2), each process closes the file descriptors that it doesn't need for the pipe (see .BR pipe (7)). The parent then writes the string contained in the program's command-line argument to the pipe, and the child reads this string a byte at a time from the pipe and echoes it on standard output. .SS Program source .EX #include #include #include #include #include #include int main(int argc, char *argv[]) { int pipefd[2]; pid_t cpid; char buf; if (argc != 2) { fprintf(stderr, "Usage: %s \en", argv[0]); exit(EXIT_FAILURE); } if (pipe(pipefd) == \-1) { perror("pipe"); exit(EXIT_FAILURE); } cpid = fork(); if (cpid == \-1) { perror("fork"); exit(EXIT_FAILURE); } if (cpid == 0) { /* Child reads from pipe */ close(pipefd[1]); /* Close unused write end */ while (read(pipefd[0], &buf, 1) > 0) write(STDOUT_FILENO, &buf, 1); write(STDOUT_FILENO, "\en", 1); close(pipefd[0]); _exit(EXIT_SUCCESS); } else { /* Parent writes argv[1] to pipe */ close(pipefd[0]); /* Close unused read end */ write(pipefd[1], argv[1], strlen(argv[1])); close(pipefd[1]); /* Reader will see EOF */ wait(NULL); /* Wait for child */ exit(EXIT_SUCCESS); } } .EE .SH SEE ALSO .BR fork (2), .BR read (2), .BR socketpair (2), .BR splice (2), .BR tee (2), .BR vmsplice (2), .BR write (2), .BR popen (3), .BR pipe (7) .SH COLOPHON This page is part of release 5.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.