xref: /dragonfly/lib/libc/sys/pipe.2 (revision 267c04fd) 
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28.\"     @(#)pipe.2	8.1 (Berkeley) 6/4/93
29.\" $FreeBSD: src/lib/libc/sys/pipe.2,v 1.13.2.5 2001/12/14 18:34:01 ru Exp $
30.\" $DragonFly: src/lib/libc/sys/pipe.2,v 1.2 2003/06/17 04:26:47 dillon Exp $
31.\"
32.Dd June 4, 1993
33.Dt PIPE 2
34.Os
35.Sh NAME
36.Nm pipe ,
37.Nm pipe2
38.Nd create descriptor pair for interprocess communication
39.Sh LIBRARY
40.Lb libc
41.Sh SYNOPSIS
42.In unistd.h
43.Ft int
44.Fn pipe "int *fildes"
45.Ft int
46.Fn pipe2 "int fildes[2]" "int flags"
47.Sh DESCRIPTION
48The
49.Fn pipe
50function
51creates a
52.Em pipe ,
53which is an object allowing
54bidirectional data flow,
55and allocates a pair of file descriptors.
56.Pp
57The
58.Fn pipe2
59system call allows control over the attributes of the file descriptors
60via the
61.Fa flags
62argument.
63Values for
64.Fa flags
65are constructed by a bitwise-inclusive OR of flags from the following
66list, defined in
67.In fcntl.h :
68.Bl -tag -width ".Dv O_NONBLOCK"
69.It Dv O_CLOEXEC
70Set the close-on-exec flag for the new file descriptors.
71.It Dv O_NONBLOCK
72Set the non-blocking flag for the ends of the pipe.
73.El
74.Pp
75If the
76.Fa flags
77argument is 0, the behavior is identical to a call to
78.Fn pipe .
79.Pp
80By convention, the first descriptor is normally used as the
81.Em read end
82of the pipe,
83and the second is normally the
84.Em write end  ,
85so that data written to
86.Fa fildes[1]
87appears on (i.e., can be read from)
88.Fa fildes[0] .
89This allows the output of one program to be
90sent
91to another program:
92the source's standard output is set up to be
93the write end of the pipe,
94and the sink's standard input is set up to be
95the read end of the pipe.
96The pipe itself persists until all its associated descriptors are
97closed.
98.Pp
99A pipe that has had an end closed is considered
100.Em widowed .
101Writing on such a pipe causes the writing process to receive
102a
103.Dv SIGPIPE
104signal.
105Widowing a pipe is the only way to deliver end-of-file to a reader:
106after the reader consumes any buffered data, reading a widowed pipe
107returns a zero count.
108.Pp
109The bidirectional nature of this implementation of pipes is not
110portable to older systems, so it is recommended to use the convention
111for using the endpoints in the traditional manner when using a
112pipe in one direction.
113.Sh RETURN VALUES
114.Rv -std pipe
115.Sh ERRORS
116The
117.Fn pipe
118and
119.Fn pipe2
120system calls will fail if:
121.Bl -tag -width Er
122.It Bq Er EMFILE
123Too many descriptors are active.
124.It Bq Er ENFILE
125The system file table is full.
126.It Bq Er EFAULT
127The
128.Fa fildes
129buffer is in an invalid area of the process's address
130space.
131.El
132.Pp
133The
134.Fn pipe2
135system call will also fail if:
136.Bl -tag -width Er
137.It Bq Er EINVAL
138The
139.Fa flags
140argument is invalid.
141.El
142.Sh SEE ALSO
143.Xr sh 1 ,
144.Xr fork 2 ,
145.Xr read 2 ,
146.Xr socketpair 2 ,
147.Xr write 2
148.Sh HISTORY
149A
150.Fn pipe
151function call appeared in
152.At v3 .
153.Pp
154Bidirectional pipes were first used on
155.At V.4 .
156