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Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)pipe.2 8.1 (Berkeley) 6/4/93 29.\" $FreeBSD$ 30.\" 31.Dd December 1, 2017 32.Dt PIPE 2 33.Os 34.Sh NAME 35.Nm pipe , 36.Nm pipe2 37.Nd create descriptor pair for interprocess communication 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In unistd.h 42.Ft int 43.Fn pipe "int fildes[2]" 44.Ft int 45.Fn pipe2 "int fildes[2]" "int flags" 46.Sh DESCRIPTION 47The 48.Fn pipe 49function 50creates a 51.Em pipe , 52which is an object allowing 53bidirectional data flow, 54and allocates a pair of file descriptors. 55.Pp 56The 57.Fn pipe2 58system call allows control over the attributes of the file descriptors 59via the 60.Fa flags 61argument. 62Values for 63.Fa flags 64are constructed by a bitwise-inclusive OR of flags from the following 65list, defined in 66.In fcntl.h : 67.Bl -tag -width ".Dv O_NONBLOCK" 68.It Dv O_CLOEXEC 69Set the close-on-exec flag for the new file descriptors. 70.It Dv O_NONBLOCK 71Set the non-blocking flag for the ends of the pipe. 72.El 73.Pp 74If the 75.Fa flags 76argument is 0, the behavior is identical to a call to 77.Fn pipe . 78.Pp 79By convention, the first descriptor is normally used as the 80.Em read end 81of the pipe, 82and the second is normally the 83.Em write end , 84so that data written to 85.Fa fildes[1] 86appears on (i.e., can be read from) 87.Fa fildes[0] . 88This allows the output of one program to be 89sent 90to another program: 91the source's standard output is set up to be 92the write end of the pipe, 93and the sink's standard input is set up to be 94the read end of the pipe. 95The pipe itself persists until all its associated descriptors are 96closed. 97.Pp 98A pipe that has had an end closed is considered 99.Em widowed . 100Writing on such a pipe causes the writing process to receive 101a 102.Dv SIGPIPE 103signal. 104Widowing a pipe is the only way to deliver end-of-file to a reader: 105after the reader consumes any buffered data, reading a widowed pipe 106returns a zero count. 107.Pp 108The bidirectional nature of this implementation of pipes is not 109portable to older systems, so it is recommended to use the convention 110for using the endpoints in the traditional manner when using a 111pipe in one direction. 112.Sh IMPLEMENTATION NOTES 113The 114.Fn pipe 115function calls the 116.Fn pipe2 117system call. 118As a result, system call traces such as those captured by 119.Xr dtrace 1 120or 121.Xr ktrace 1 122will show calls to 123.Fn pipe2 . 124.Sh RETURN VALUES 125.Rv -std pipe 126.Sh ERRORS 127The 128.Fn pipe 129and 130.Fn pipe2 131system calls will fail if: 132.Bl -tag -width Er 133.It Bq Er EFAULT 134.Ar fildes 135argument points to an invalid memory location. 136.It Bq Er EMFILE 137Too many descriptors are active. 138.It Bq Er ENFILE 139The system file table is full. 140.It Bq Er ENOMEM 141Not enough kernel memory to establish a pipe. 142.El 143.Pp 144The 145.Fn pipe2 146system call will also fail if: 147.Bl -tag -width Er 148.It Bq Er EINVAL 149The 150.Fa flags 151argument is invalid. 152.El 153.Sh SEE ALSO 154.Xr sh 1 , 155.Xr fork 2 , 156.Xr read 2 , 157.Xr socketpair 2 , 158.Xr write 2 159.Sh HISTORY 160The 161.Fn pipe 162function appeared in 163.At v3 . 164.Pp 165Bidirectional pipes were first used on 166.At V.4 . 167.Pp 168The 169.Fn pipe2 170function appeared in 171.Fx 10.0 . 172.Pp 173The 174.Fn pipe 175function became a wrapper around 176.Fn pipe2 177in 178.Fx 11.0 . 179