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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.Dd May 17, 2025 29.Dt PIPE 2 30.Os 31.Sh NAME 32.Nm pipe , 33.Nm pipe2 34.Nd create descriptor pair for interprocess communication 35.Sh LIBRARY 36.Lb libc 37.Sh SYNOPSIS 38.In unistd.h 39.Ft int 40.Fn pipe "int fildes[2]" 41.Ft int 42.Fn pipe2 "int fildes[2]" "int flags" 43.Sh DESCRIPTION 44The 45.Fn pipe 46function 47creates a 48.Em pipe , 49which is an object allowing 50bidirectional data flow, 51and allocates a pair of file descriptors. 52.Pp 53The 54.Fn pipe2 55system call allows control over the attributes of the file descriptors 56via the 57.Fa flags 58argument. 59Values for 60.Fa flags 61are constructed by a bitwise-inclusive OR of flags from the following 62list, defined in 63.In fcntl.h : 64.Bl -tag -width ".Dv O_NONBLOCK" 65.It Dv O_CLOEXEC 66Set the close-on-exec flag for the new file descriptors. 67.It Dv O_CLOFORK 68Set the close-on-fork flag for the new file descriptors. 69.It Dv O_NONBLOCK 70Set the non-blocking flag for the ends of the pipe. 71.El 72.Pp 73If the 74.Fa flags 75argument is 0, the behavior is identical to a call to 76.Fn pipe . 77.Pp 78By convention, the first descriptor is normally used as the 79.Em read end 80of the pipe, 81and the second is normally the 82.Em write end , 83so that data written to 84.Fa fildes[1] 85appears on (i.e., can be read from) 86.Fa fildes[0] . 87This allows the output of one program to be 88sent 89to another program: 90the source's standard output is set up to be 91the write end of the pipe, 92and the sink's standard input is set up to be 93the read end of the pipe. 94The pipe itself persists until all its associated descriptors are 95closed. 96.Pp 97A pipe that has had an end closed is considered 98.Em widowed . 99Writing on such a pipe causes the writing process to receive 100a 101.Dv SIGPIPE 102signal. 103Widowing a pipe is the only way to deliver end-of-file to a reader: 104after the reader consumes any buffered data, reading a widowed pipe 105returns a zero count. 106.Pp 107The bidirectional nature of this implementation of pipes is not 108portable to older systems, so it is recommended to use the convention 109for using the endpoints in the traditional manner when using a 110pipe in one direction. 111.Sh IMPLEMENTATION NOTES 112The 113.Fn pipe 114function calls the 115.Fn pipe2 116system call. 117As a result, system call traces such as those captured by 118.Xr dtrace 1 119or 120.Xr ktrace 1 121will show calls to 122.Fn pipe2 . 123.Sh RETURN VALUES 124.Rv -std pipe 125.Sh ERRORS 126The 127.Fn pipe 128and 129.Fn pipe2 130system calls will fail if: 131.Bl -tag -width Er 132.It Bq Er EFAULT 133.Ar fildes 134argument points to an invalid memory location. 135.It Bq Er EMFILE 136Too many descriptors are active. 137.It Bq Er ENFILE 138The system file table is full. 139.It Bq Er ENOMEM 140Not enough kernel memory to establish a pipe. 141.El 142.Pp 143The 144.Fn pipe2 145system call will also fail if: 146.Bl -tag -width Er 147.It Bq Er EINVAL 148The 149.Fa flags 150argument is invalid. 151.El 152.Sh SEE ALSO 153.Xr sh 1 , 154.Xr fork 2 , 155.Xr read 2 , 156.Xr socketpair 2 , 157.Xr write 2 158.Sh HISTORY 159The 160.Fn pipe 161function appeared in 162.At v3 . 163.Pp 164Bidirectional pipes were first used on 165.At V.4 . 166.Pp 167The 168.Fn pipe2 169function appeared in 170.Fx 10.0 . 171.Pp 172The 173.Fn pipe 174function became a wrapper around 175.Fn pipe2 176in 177.Fx 11.0 . 178.Pp 179The 180.Dv O_CLOFORK 181flag appeared in 182.Fx 15.0 . 183