1pipe(2) System Calls Manual pipe(2)
2
6 pipe, pipe2 - create pipe
7
9 Standard C library (libc, -lc)
10
12 #include <unistd.h>
13 int pipe(int pipefd[2]);
15 #define _GNU_SOURCE /* See feature_test_macros(7) */
17 #include <fcntl.h> /* Definition of O_* constants */
18 #include <unistd.h>
19 int pipe2(int pipefd[2], int flags);
21 /* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64, pipe() has the
23 following prototype; see NOTES */
24 #include <unistd.h>
26 struct fd_pair {
28 long fd[2];
29 };
30 struct fd_pair pipe(void);
31
33 pipe() creates a pipe, a unidirectional data channel that can be used
34 for interprocess communication. The array pipefd is used to return two
35 file descriptors referring to the ends of the pipe. pipefd[0] refers
36 to the read end of the pipe. pipefd[1] refers to the write end of the
37 pipe. Data written to the write end of the pipe is buffered by the
38 kernel until it is read from the read end of the pipe. For further de‐
39 tails, see pipe(7).
40 If flags is 0, then pipe2() is the same as pipe(). The following val‐
42 ues can be bitwise ORed in flags to obtain different behavior:
43 O_CLOEXEC
45 Set the close-on-exec (FD_CLOEXEC) flag on the two new file de‐
46 scriptors. See the description of the same flag in open(2) for
47 reasons why this may be useful.
48 O_DIRECT (since Linux 3.4)
50 Create a pipe that performs I/O in "packet" mode. Each write(2)
51 to the pipe is dealt with as a separate packet, and read(2)s
52 from the pipe will read one packet at a time. Note the follow‐
53 ing points:
54 • Writes of greater than PIPE_BUF bytes (see pipe(7)) will be
56 split into multiple packets. The constant PIPE_BUF is de‐
57 fined in <limits.h>.
58 • If a read(2) specifies a buffer size that is smaller than the
60 next packet, then the requested number of bytes are read, and
61 the excess bytes in the packet are discarded. Specifying a
62 buffer size of PIPE_BUF will be sufficient to read the
63 largest possible packets (see the previous point).
64 • Zero-length packets are not supported. (A read(2) that spec‐
66 ifies a buffer size of zero is a no-op, and returns 0.)
67 Older kernels that do not support this flag will indicate this
69 via an EINVAL error.
70 Since Linux 4.5, it is possible to change the O_DIRECT setting
72 of a pipe file descriptor using fcntl(2).
73 O_NONBLOCK
75 Set the O_NONBLOCK file status flag on the open file descrip‐
76 tions referred to by the new file descriptors. Using this flag
77 saves extra calls to fcntl(2) to achieve the same result.
78 O_NOTIFICATION_PIPE
80 Since Linux 5.8, general notification mechanism is built on the
81 top of the pipe where kernel splices notification messages into
82 pipes opened by user space. The owner of the pipe has to tell
83 the kernel which sources of events to watch and filters can also
84 be applied to select which subevents should be placed into the
85 pipe.
86
88 On success, zero is returned. On error, -1 is returned, errno is set
89 to indicate the error, and pipefd is left unchanged.
90 On Linux (and other systems), pipe() does not modify pipefd on failure.
92 A requirement standardizing this behavior was added in POSIX.1-2008
93 TC2. The Linux-specific pipe2() system call likewise does not modify
94 pipefd on failure.
95
97 EFAULT pipefd is not valid.
98 EINVAL (pipe2()) Invalid value in flags.
100 EMFILE The per-process limit on the number of open file descriptors has
102 been reached.
103 ENFILE The system-wide limit on the total number of open files has been
105 reached.
106 ENFILE The user hard limit on memory that can be allocated for pipes
108 has been reached and the caller is not privileged; see pipe(7).
109 ENOPKG (pipe2()) O_NOTIFICATION_PIPE was passed in flags and support
111 for notifications (CONFIG_WATCH_QUEUE) is not compiled into the
112 kernel.
113
115 The System V ABI on some architectures allows the use of more than one
116 register for returning multiple values; several architectures (namely,
117 Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64) (ab)use this feature in
118 order to implement the pipe() system call in a functional manner: the
119 call doesn't take any arguments and returns a pair of file descriptors
120 as the return value on success. The glibc pipe() wrapper function
121 transparently deals with this. See syscall(2) for information regard‐
122 ing registers used for storing second file descriptor.
123
125 pipe() POSIX.1-2008.
126 pipe2()
128 Linux.
129
131 pipe() POSIX.1-2001.
132 pipe2()
134 Linux 2.6.27, glibc 2.9.
135
137 The following program creates a pipe, and then fork(2)s to create a
138 child process; the child inherits a duplicate set of file descriptors
139 that refer to the same pipe. After the fork(2), each process closes
140 the file descriptors that it doesn't need for the pipe (see pipe(7)).
141 The parent then writes the string contained in the program's command-
142 line argument to the pipe, and the child reads this string a byte at a
143 time from the pipe and echoes it on standard output.
144 Program source
146 #include <stdio.h>
147 #include <stdlib.h>
148 #include <string.h>
149 #include <sys/wait.h>
150 #include <unistd.h>
151 int
153 main(int argc, char *argv[])
154 {
155 int pipefd[2];
156 char buf;
157 pid_t cpid;
158 if (argc != 2) {
160 fprintf(stderr, "Usage: %s <string>\n", argv[0]);
161 exit(EXIT_FAILURE);
162 }
163 if (pipe(pipefd) == -1) {
165 perror("pipe");
166 exit(EXIT_FAILURE);
167 }
168 cpid = fork();
170 if (cpid == -1) {
171 perror("fork");
172 exit(EXIT_FAILURE);
173 }
174 if (cpid == 0) { /* Child reads from pipe */
176 close(pipefd[1]); /* Close unused write end */
177 while (read(pipefd[0], &buf, 1) > 0)
179 write(STDOUT_FILENO, &buf, 1);
180 write(STDOUT_FILENO, "\n", 1);
182 close(pipefd[0]);
183 _exit(EXIT_SUCCESS);
184 } else { /* Parent writes argv[1] to pipe */
186 close(pipefd[0]); /* Close unused read end */
187 write(pipefd[1], argv[1], strlen(argv[1]));
188 close(pipefd[1]); /* Reader will see EOF */
189 wait(NULL); /* Wait for child */
190 exit(EXIT_SUCCESS);
191 }
192 }
193
195 fork(2), read(2), socketpair(2), splice(2), tee(2), vmsplice(2),
196 write(2), popen(3), pipe(7)
197Linux man-pages 6.04 2023-03-30 pipe(2)