1PIPE(2)                    Linux Programmer's Manual                   PIPE(2)
2
3
4

NAME

6       pipe, pipe2 - create pipe
7

SYNOPSIS

9       #include <unistd.h>
10
11       int pipe(int pipefd[2]);
12
13       #define _GNU_SOURCE             /* See feature_test_macros(7) */
14       #include <fcntl.h>              /* Definition of O_* constants */
15       #include <unistd.h>
16
17       int pipe2(int pipefd[2], int flags);
18
19       /* On Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64, pipe() has the
20          following prototype; see NOTES */
21
22       #include <unistd.h>
23
24       struct fd_pair {
25           long fd[2];
26       };
27       struct fd_pair pipe(void);
28

DESCRIPTION

30       pipe()  creates  a pipe, a unidirectional data channel that can be used
31       for interprocess communication.  The array pipefd is used to return two
32       file  descriptors  referring to the ends of the pipe.  pipefd[0] refers
33       to the read end of the pipe.  pipefd[1] refers to the write end of  the
34       pipe.   Data  written  to  the write end of the pipe is buffered by the
35       kernel until it is read from the read end of the pipe.  For further de‐
36       tails, see pipe(7).
37
38       If  flags is 0, then pipe2() is the same as pipe().  The following val‐
39       ues can be bitwise ORed in flags to obtain different behavior:
40
41       O_CLOEXEC
42              Set the close-on-exec (FD_CLOEXEC) flag on the two new file  de‐
43              scriptors.   See the description of the same flag in open(2) for
44              reasons why this may be useful.
45
46       O_DIRECT (since Linux 3.4)
47              Create a pipe that performs I/O in "packet" mode.  Each write(2)
48              to  the  pipe  is  dealt with as a separate packet, and read(2)s
49              from the pipe will read one packet at a time.  Note the  follow‐
50              ing points:
51
52              *  Writes  of  greater than PIPE_BUF bytes (see pipe(7)) will be
53                 split into multiple packets.  The constant  PIPE_BUF  is  de‐
54                 fined in <limits.h>.
55
56              *  If a read(2) specifies a buffer size that is smaller than the
57                 next packet, then the requested number of bytes are read, and
58                 the  excess  bytes in the packet are discarded.  Specifying a
59                 buffer size of  PIPE_BUF  will  be  sufficient  to  read  the
60                 largest possible packets (see the previous point).
61
62              *  Zero-length packets are not supported.  (A read(2) that spec‐
63                 ifies a buffer size of zero is a no-op, and returns 0.)
64
65              Older kernels that do not support this flag will  indicate  this
66              via an EINVAL error.
67
68              Since  Linux  4.5, it is possible to change the O_DIRECT setting
69              of a pipe file descriptor using fcntl(2).
70
71       O_NONBLOCK
72              Set the O_NONBLOCK file status flag on the  open  file  descrip‐
73              tions  referred to by the new file descriptors.  Using this flag
74              saves extra calls to fcntl(2) to achieve the same result.
75

RETURN VALUE

77       On success, zero is returned.  On error, -1 is returned, errno  is  set
78       to indicate the error, and pipefd is left unchanged.
79
80       On Linux (and other systems), pipe() does not modify pipefd on failure.
81       A requirement standardizing this behavior  was  added  in  POSIX.1-2008
82       TC2.   The  Linux-specific pipe2() system call likewise does not modify
83       pipefd on failure.
84

ERRORS

86       EFAULT pipefd is not valid.
87
88       EINVAL (pipe2()) Invalid value in flags.
89
90       EMFILE The per-process limit on the number of open file descriptors has
91              been reached.
92
93       ENFILE The system-wide limit on the total number of open files has been
94              reached.
95
96       ENFILE The user hard limit on memory that can be  allocated  for  pipes
97              has been reached and the caller is not privileged; see pipe(7).
98

VERSIONS

100       pipe2()  was  added to Linux in version 2.6.27; glibc support is avail‐
101       able starting with version 2.9.
102

CONFORMING TO

104       pipe(): POSIX.1-2001, POSIX.1-2008.
105
106       pipe2() is Linux-specific.
107

NOTES

109       The System V ABI on some architectures allows the use of more than  one
110       register  for returning multiple values; several architectures (namely,
111       Alpha, IA-64, MIPS, SuperH, and SPARC/SPARC64) (ab)use this feature  in
112       order  to  implement the pipe() system call in a functional manner: the
113       call doesn't take any arguments and returns a pair of file  descriptors
114       as  the  return  value  on  success.  The glibc pipe() wrapper function
115       transparently deals with this.  See syscall(2) for information  regard‐
116       ing registers used for storing second file descriptor.
117

EXAMPLES

119       The  following  program  creates  a pipe, and then fork(2)s to create a
120       child process; the child inherits a duplicate set of  file  descriptors
121       that  refer  to  the same pipe.  After the fork(2), each process closes
122       the file descriptors that it doesn't need for the pipe  (see  pipe(7)).
123       The  parent  then writes the string contained in the program's command-
124       line argument to the pipe, and the child reads this string a byte at  a
125       time from the pipe and echoes it on standard output.
126
127   Program source
128       #include <sys/types.h>
129       #include <sys/wait.h>
130       #include <stdio.h>
131       #include <stdlib.h>
132       #include <unistd.h>
133       #include <string.h>
134
135       int
136       main(int argc, char *argv[])
137       {
138           int pipefd[2];
139           pid_t cpid;
140           char buf;
141
142           if (argc != 2) {
143               fprintf(stderr, "Usage: %s <string>\n", argv[0]);
144               exit(EXIT_FAILURE);
145           }
146
147           if (pipe(pipefd) == -1) {
148               perror("pipe");
149               exit(EXIT_FAILURE);
150           }
151
152           cpid = fork();
153           if (cpid == -1) {
154               perror("fork");
155               exit(EXIT_FAILURE);
156           }
157
158           if (cpid == 0) {    /* Child reads from pipe */
159               close(pipefd[1]);          /* Close unused write end */
160
161               while (read(pipefd[0], &buf, 1) > 0)
162                   write(STDOUT_FILENO, &buf, 1);
163
164               write(STDOUT_FILENO, "\n", 1);
165               close(pipefd[0]);
166               _exit(EXIT_SUCCESS);
167
168           } else {            /* Parent writes argv[1] to pipe */
169               close(pipefd[0]);          /* Close unused read end */
170               write(pipefd[1], argv[1], strlen(argv[1]));
171               close(pipefd[1]);          /* Reader will see EOF */
172               wait(NULL);                /* Wait for child */
173               exit(EXIT_SUCCESS);
174           }
175       }
176

SEE ALSO

178       fork(2),   read(2),   socketpair(2),  splice(2),  tee(2),  vmsplice(2),
179       write(2), popen(3), pipe(7)
180

COLOPHON

182       This page is part of release 5.12 of the Linux  man-pages  project.   A
183       description  of  the project, information about reporting bugs, and the
184       latest    version    of    this    page,    can     be     found     at
185       https://www.kernel.org/doc/man-pages/.
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189Linux                             2021-03-22                           PIPE(2)
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