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.TH PIPE 2 "June 21, 2024"
.SH NAME
pipe \- create an interprocess channel
.SH SYNOPSIS
.nf
#include <unistd.h>

\fBint\fR \fBpipe\fR(\fBint\fR \fIfildes\fR[2]);

\fBint\fR \fBpipe2\fR(\fBint\fR \fIfildes\fR[2], \fBint\fR \fIflags\fR);
.fi

.SH DESCRIPTION
The \fBpipe()\fR and pipe2() functions create an I/O mechanism called a
pipe and returns two file descriptors, \fIfildes\fR[\fB0\fR] and
\fIfildes\fR[\fB1\fR]. The files associated with \fIfildes\fR[\fB0\fR]
and \fIfildes\fR[\fB1\fR] are streams and are both opened for reading and
writing.  The \fBpipe()\fR call will clear the \fBO_NDELAY\fR,
\fBO_NONBLOCK\fR, and the \fBFD_CLOEXEC\fR and \fBFD_CLOFORK\fR flags on
both file descriptors. The \fBfcntl\fR(2) function can be used to set these
flags.
.sp
.LP
The \fBpipe2()\fR call will clear the \fBO_NDELAY\fR on both filedescriptors.
The \fIflags\fR argument may be used to specify attributes on both file
descriptors.  \fBpipe2()\fR called with a \fIflags\fR value of 0 will
behave identically to \fBpipe()\fR.  Values for \fIflags\fR are constructed
by a bitwise-inclusive-OR of flags from the following list, defined in
<\fBfcntl.h\fR>.

.sp
.ne 2
.na
\fB\fBO_NONBLOCK\fR\fR
.ad
.RS 12n
Both file descriptors will be placed in non-blocking mode. This corresponds
to the \fBO_NONBLOCK\fR flag to \fBfcntl\fR(2).
.RE

.sp
.ne 2
.na
\fB\fBO_CLOEXEC\fR\fR
.ad
.RS 12n
Both file descriptors will be opened with the FD_CLOEXEC flag set. Both file
descriptors will be closed prior to any future exec() calls.
.RE

.sp
.ne 2
.na
\fB\fBO_CLOFORK\fR\fR
.ad
.RS 12n
Both file descriptors will be opened with the FD_CLOFORK flag set. Both file
descriptors will be closed in any child processes created with the fork() family
of calls.
.RE

.sp
.LP
A read from \fIfildes\fR[\fB0\fR] accesses the data written to
\fIfildes\fR[\fB1\fR] on a first-in-first-out (FIFO) basis and a read from
\fIfildes\fR[\fB1\fR] accesses the data written to \fIfildes\fR[\fB0\fR] also
on a \fBFIFO\fR basis.
.sp
.LP
Upon successful completion \fBpipe()\fR marks for update the \fBst_atime\fR,
\fBst_ctime\fR, and \fBst_mtime\fR fields of the pipe.
.SH RETURN VALUES
Upon successful completion, \fB0\fR is returned. Otherwise, \fB\(mi1\fR is
returned and \fBerrno\fR is set to indicate the error.
.SH ERRORS
The \fBpipe()\fR and \fBpipe2()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEMFILE\fR\fR
.ad
.RS 10n
More than {\fBOPEN_MAX\fR} file descriptors are already in use by this process.
.RE

.sp
.ne 2
.na
\fB\fBENFILE\fR\fR
.ad
.RS 10n
The number of simultaneously open files in the system would exceed a
system-imposed limit.
.RE

.sp
.ne 2
.na
\fB\fBEFAULT\fR\fR
.ad
.RS 10n
The \fIfildes[2]\fR argument points to an illegal address.
.RE

.sp
.LP
The \fBpipe2()\fR function will also fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
The \fIflags\fR argument is illegal. Valid \fIflags\fR are zero or a
bitwise inclusive-OR of \fBO_CLOEXEC\fR, \fBO_CLOFORK\fR, and \fBO_NONBLOCK\fR.
.RE


.SH ATTRIBUTES
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	Async-Signal-Safe
.TE

.SH SEE ALSO
.BR sh (1),
.BR fcntl (2),
.BR fstat (2),
.BR getmsg (2),
.BR open (2),
.BR poll (2),
.BR putmsg (2),
.BR read (2),
.BR write (2),
.BR streamio (4I),
.BR attributes (7),
.BR standards (7)
.SH NOTES
Since a pipe is bi-directional, there are two separate flows of data.
Therefore, the size (\fBst_size\fR) returned by a call to \fBfstat\fR(2) with
argument  \fIfildes\fR[\fB0\fR] or \fIfildes\fR[\fB1\fR] is the number of bytes
available for reading from \fIfildes\fR[\fB0\fR] or \fIfildes\fR[\fB1\fR]
respectively. Previously, the size (\fBst_size\fR) returned by a call to
\fBfstat()\fR with argument \fIfildes\fR[\fB1\fR] (the write-end) was the
number of bytes available for reading from \fIfildes\fR[\fB0\fR] (the
read-end).