1PIPE(2) Linux Programmer's Manual PIPE(2)
2
6 pipe, pipe2 - create pipe
7
9 #include <unistd.h>
10 int pipe(int pipefd[2]);
12 #define _GNU_SOURCE /* See feature_test_macros(7) */
14 #include <fcntl.h> /* Obtain O_* constant definitions */
15 #include <unistd.h>
16 int pipe2(int pipefd[2], int flags);
18
20 pipe() creates a pipe, a unidirectional data channel that can be used
21 for interprocess communication. The array pipefd is used to return two
22 file descriptors referring to the ends of the pipe. pipefd[0] refers
23 to the read end of the pipe. pipefd[1] refers to the write end of the
24 pipe. Data written to the write end of the pipe is buffered by the
25 kernel until it is read from the read end of the pipe. For further
26 details, see pipe(7).
27 If flags is 0, then pipe2() is the same as pipe(). The following val‐
29 ues can be bitwise ORed in flags to obtain different behavior:
30 O_CLOEXEC
32 Set the close-on-exec (FD_CLOEXEC) flag on the two new file
33 descriptors. See the description of the same flag in open(2)
34 for reasons why this may be useful.
35 O_DIRECT (since Linux 3.4)
37 Create a pipe that performs I/O in "packet" mode. Each write(2)
38 to the pipe is dealt with as a separate packet, and read(2)s
39 from the pipe will read one packet at a time. Note the follow‐
40 ing points:
41 * Writes of greater than PIPE_BUF bytes (see pipe(7)) will be
43 split into multiple packets. The constant PIPE_BUF is
44 defined in <limits.h>.
45 * If a read(2) specifies a buffer size that is smaller than the
47 next packet, then the requested number of bytes are read, and
48 the excess bytes in the packet are discarded. Specifying a
49 buffer size of PIPE_BUF will be sufficient to read the
50 largest possible packets (see the previous point).
51 * Zero-length packets are not supported. (A read(2) that spec‐
53 ifies a buffer size of zero is a no-op, and returns 0.)
54 Older kernels that do not support this flag will indicate this
56 via an EINVAL error.
57 Since Linux 4.5, it is possible to change the O_DIRECT setting
59 of a pipe file descriptor using fcntl(2).
60 O_NONBLOCK
62 Set the O_NONBLOCK file status flag on the two new open file
63 descriptions. Using this flag saves extra calls to fcntl(2) to
64 achieve the same result.
65
67 On success, zero is returned. On error, -1 is returned, and errno is
68 set appropriately.
69 On Linux (and other systems), pipe() does not modify pipefd on failure.
71 A requirement standardizing this behavior was added in POSIX.1-2016.
72 The Linux-specific pipe2() system call likewise does not modify pipefd
73 on failure.
74
76 EFAULT pipefd is not valid.
77 EINVAL (pipe2()) Invalid value in flags.
79 EMFILE The per-process limit on the number of open file descriptors has
81 been reached.
82 ENFILE The system-wide limit on the total number of open files has been
84 reached.
85 ENFILE The user hard limit on memory that can be allocated for pipes
87 has been reached and the caller is not privileged; see pipe(7).
88
90 pipe2() was added to Linux in version 2.6.27; glibc support is avail‐
91 able starting with version 2.9.
92
94 pipe(): POSIX.1-2001, POSIX.1-2008.
95 pipe2() is Linux-specific.
97
99 The following program creates a pipe, and then fork(2)s to create a
100 child process; the child inherits a duplicate set of file descriptors
101 that refer to the same pipe. After the fork(2), each process closes
102 the file descriptors that it doesn't need for the pipe (see pipe(7)).
103 The parent then writes the string contained in the program's command-
104 line argument to the pipe, and the child reads this string a byte at a
105 time from the pipe and echoes it on standard output.
106 Program source
108 #include <sys/types.h>
109 #include <sys/wait.h>
110 #include <stdio.h>
111 #include <stdlib.h>
112 #include <unistd.h>
113 #include <string.h>
114 int
116 main(int argc, char *argv[])
117 {
118 int pipefd[2];
119 pid_t cpid;
120 char buf;
121 if (argc != 2) {
123 fprintf(stderr, "Usage: %s <string>\n", argv[0]);
124 exit(EXIT_FAILURE);
125 }
126 if (pipe(pipefd) == -1) {
128 perror("pipe");
129 exit(EXIT_FAILURE);
130 }
131 cpid = fork();
133 if (cpid == -1) {
134 perror("fork");
135 exit(EXIT_FAILURE);
136 }
137 if (cpid == 0) { /* Child reads from pipe */
139 close(pipefd[1]); /* Close unused write end */
140 while (read(pipefd[0], &buf, 1) > 0)
142 write(STDOUT_FILENO, &buf, 1);
143 write(STDOUT_FILENO, "\n", 1);
145 close(pipefd[0]);
146 _exit(EXIT_SUCCESS);
147 } else { /* Parent writes argv[1] to pipe */
149 close(pipefd[0]); /* Close unused read end */
150 write(pipefd[1], argv[1], strlen(argv[1]));
151 close(pipefd[1]); /* Reader will see EOF */
152 wait(NULL); /* Wait for child */
153 exit(EXIT_SUCCESS);
154 }
155 }
156
158 fork(2), read(2), socketpair(2), splice(2), tee(2), vmsplice(2),
159 write(2), popen(3), pipe(7)
160
162 This page is part of release 4.16 of the Linux man-pages project. A
163 description of the project, information about reporting bugs, and the
164 latest version of this page, can be found at
165 https://www.kernel.org/doc/man-pages/.
166Linux 2017-11-26 PIPE(2)