.\" Copyright (c) 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)popen.3 8.1 (Berkeley) 6/4/93 .\" .TH POPEN 3 "23 February 1997" GNO "Library Routines" .SH NAME .BR popen , .BR pclose \- process I/O .SH SYNOPSIS #include .sp 1 FILE *\fBpopen\fR (const char *\fIcommand\fR, const char *\fItype\fR); .br int \fBpclose\fR (FILE *\fIstream\fR); .SH DESCRIPTION The .BR popen function .IR opens a process by creating a pipe, forking, and invoking the shell. Since a pipe is by definition unidirectional, the .I type argument may specify only reading or writing, not both; the resulting stream is correspondingly read-only or write-only. .LP The .I command argument is a pointer to a null-terminated string containing a shell command line. This command is passed to .BR /bin/sh using the .BR \-c flag; interpretation, if any, is performed by the shell. The .I mode argument is a pointer to a null-terminated string which must be either .BR r for reading or .BR w for writing. .LP The return value from .BR popen is a normal standard I/O stream in all respects save that it must be closed with .BR pclose rather than .BR fclose . Writing to such a stream writes to the standard input of the command; the command's standard output is the same as that of the process that called .BR popen , unless this is altered by the command itself. Conversely, reading from a .IR popened stream reads the command's standard output, and the command's standard input is the same as that of the process that called .BR popen . .LP Note that output .BR popen streams are fully buffered by default. .LP The .BR pclose function waits for the associated process to terminate and returns the exit status of the command as returned by .BR waitpid . .SH RETURN VALUE The .BR popen function returns .BR NULL if the .BR fork (2) or .BR pipe (2) calls fail, or if it cannot allocate memory. .LP The .BR pclose function returns \-1 if .I stream is not associated with a .IR popened command, if .I stream already .IR pclosed , or if .BR waitpid returns an error. .SH ERRORS The .BR popen function does not reliably set .IR errno . .SH BUGS Since the standard input of a command opened for reading shares its seek offset with the process that called .BR popen , if the original process has done a buffered read, the command's input position may not be as expected. Similarly, the output from a command opened for writing may become intermingled with that of the original process. The latter can be avoided by calling .BR fflush (3) before .BR popen . .LP Failure to execute the shell is indistinguishable from the shell's failure to execute .IR command , or an immediate exit of the command. The only hint is an exit status of 127. .LP The .BR popen argument always calls .BR sh (1), never calls .BR csh (1). (Because of a lack of available .BR sh (1), the current GNO implementation always calls .BR gsh (1).) .LP This implementation makes use of .BR waitpid (2). Because .BR waitpid is not currently provided by the GNO kernel, .BR pclose is affected by the same bug as is .BR waitpid ; other child processes' exit status may be caught (and discarded) while waiting for the process created by .BR popen . Consequently, if the parent processes is doing any other .BR waitpid calls (either directly or indirectly), .BR pclose may never return. .LP Under GNO, the memory region pointed to by .IR command must be able to be referenced up to the point that the child process does its .BR execl (2). This implies that if .IR command is allocated on the stack and the function which allocated it returns before the program calls .BR pclose (or if the parent exits and makes its child into a zombie), there is a chance that the forked child of .BR popen may try to .BR execl an arbitrary command, the results of which are undefined. .LP .BR gsh (1) appears to hang on occasion when executing .BR popen 's arguments. The symptom is that the pclose never returns, and the process table (see .BR ps (1)) shows that the forked .BR gsh 's user time is forever increasing. .SH SEE ALSO .BR gsh (1), .BR sh (1), .BR fork (2), .BR pipe (2), .BR waitpid (2), .BR fclose (3), .BR fflush (3), .BR fopen (3), .BR stdio (3), .BR system (3) .SH HISTORY A .BR popen and a .BR pclose function appeared in v7.