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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 20, 2013 29.Dt POPEN 3 30.Os 31.Sh NAME 32.Nm popen , 33.Nm pclose 34.Nd process 35.Tn I/O 36.Sh LIBRARY 37.Lb libc 38.Sh SYNOPSIS 39.In stdio.h 40.Ft FILE * 41.Fn popen "const char *command" "const char *type" 42.Ft int 43.Fn pclose "FILE *stream" 44.Sh DESCRIPTION 45The 46.Fn popen 47function 48.Dq opens 49a process by creating a bidirectional pipe 50forking, 51and invoking the shell. 52Any streams opened by previous 53.Fn popen 54calls in the parent process are closed in the new child process. 55Historically, 56.Fn popen 57was implemented with a unidirectional pipe; 58hence many implementations of 59.Fn popen 60only allow the 61.Fa type 62argument to specify reading or writing, not both. 63Since 64.Fn popen 65is now implemented using a bidirectional pipe, the 66.Fa type 67argument may request a bidirectional data flow. 68The 69.Fa type 70argument is a pointer to a null-terminated string 71which must be 72.Ql r 73for reading, 74.Ql w 75for writing, or 76.Ql r+ 77for reading and writing. 78.Pp 79A letter 80.Ql e 81may be appended to that to request that the underlying file descriptor 82be set close-on-exec. 83.Pp 84The 85.Fa command 86argument is a pointer to a null-terminated string 87containing a shell command line. 88This command is passed to 89.Pa /bin/sh 90using the 91.Fl c 92flag; interpretation, if any, is performed by the shell. 93.Pp 94The return value from 95.Fn popen 96is a normal standard 97.Tn I/O 98stream in all respects 99save that it must be closed with 100.Fn pclose 101rather than 102.Fn fclose . 103Writing to such a stream 104writes to the standard input of the command; 105the command's standard output is the same as that of the process that called 106.Fn popen , 107unless this is altered by the command itself. 108Conversely, reading from a 109.Dq popened 110stream reads the command's standard output, and 111the command's standard input is the same as that of the process that called 112.Fn popen . 113.Pp 114Note that output 115.Fn popen 116streams are fully buffered by default. 117.Pp 118The 119.Fn pclose 120function waits for the associated process to terminate 121and returns the exit status of the command 122as returned by 123.Xr wait4 2 . 124.Sh RETURN VALUES 125The 126.Fn popen 127function returns 128.Dv NULL 129if the 130.Xr fork 2 131or 132.Xr pipe 2 133calls fail, 134or if it cannot allocate memory. 135.Pp 136The 137.Fn pclose 138function 139returns \-1 if 140.Fa stream 141is not associated with a 142.Dq popened 143command, if 144.Fa stream 145already 146.Dq pclosed , 147or if 148.Xr wait4 2 149returns an error. 150.Sh ERRORS 151The 152.Fn popen 153function does not reliably set 154.Va errno . 155.Sh SEE ALSO 156.Xr sh 1 , 157.Xr fork 2 , 158.Xr pipe 2 , 159.Xr wait4 2 , 160.Xr fclose 3 , 161.Xr fflush 3 , 162.Xr fopen 3 , 163.Xr stdio 3 , 164.Xr system 3 165.Sh HISTORY 166A 167.Fn popen 168and a 169.Fn pclose 170function appeared in 171.At v7 . 172.Pp 173Bidirectional functionality was added in 174.Fx 2.2.6 . 175.Sh BUGS 176Since the standard input of a command opened for reading 177shares its seek offset with the process that called 178.Fn popen , 179if the original process has done a buffered read, 180the command's input position may not be as expected. 181Similarly, the output from a command opened for writing 182may become intermingled with that of the original process. 183The latter can be avoided by calling 184.Xr fflush 3 185before 186.Fn popen . 187.Pp 188Failure to execute the shell 189is indistinguishable from the shell's failure to execute command, 190or an immediate exit of the command. 191The only hint is an exit status of 127. 192.Pp 193The 194.Fn popen 195function 196always calls 197.Xr sh 1 , 198never calls 199.Xr csh 1 . 200