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Revision 1.18, Mon Jun 27 08:21:07 2011 UTC (4 years, 7 months ago) by wiz
Various minor improvements.
.\" $NetBSD: popen.3,v 1.18 2011/06/27 08:21:07 wiz Exp $ .\" .\" 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. 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 8.2 (Berkeley) 5/3/95 .\" .Dd June 24, 2011 .Dt POPEN 3 .Os .Sh NAME .Nm popen , .Nm pclose .Nd process .Tn I/O .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In stdio.h .Ft FILE * .Fn popen "const char *command" "const char *type" .Ft int .Fn pclose "FILE *stream" .Sh DESCRIPTION The .Fn popen function .Dq opens a process by creating an IPC connection, forking, and invoking the shell. Historically, .Nm popen was implemented with a unidirectional pipe; hence many implementations of .Nm popen only allow the .Fa type argument to specify reading or writing, not both. Since .Nm popen is now implemented using sockets, the .Fa type may request a bidirectional data flow. The .Fa type argument is a pointer to a null-terminated string which must be .Ql r for reading, .Ql w for writing, or .Ql r+ for reading and writing. In addition if the character .Ql e is present in the .Fa type string, the file descriptor used internally is set to be closed on .Xr exec 3 . .Pp The .Fa command argument is a pointer to a null-terminated string containing a shell command line. This command is passed to .Pa /bin/sh using the .Fl c flag; interpretation, if any, is performed by the shell. .Pp The return value from .Fn popen is a normal standard .Tn I/O stream in all respects save that it must be closed with .Fn pclose rather than .Fn 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 .Fn popen , unless this is altered by the command itself. Conversely, reading from a .Dq popened stream reads the command's standard output, and the command's standard input is the same as that of the process that called .Fn popen . .Pp Note that output .Fn popen streams are fully buffered by default. .Pp The .Fn pclose function waits for the associated process to terminate and returns the exit status of the command as returned by .Fn wait4 . .Sh RETURN VALUES The .Fn popen function returns .Dv NULL if the .Xr fork 2 , .Xr pipe 2 , or .Xr socketpair 2 calls fail, or if it cannot allocate memory. .Pp The .Fn pclose function returns \-1 if .Fa stream is not associated with a .Dq popened command, if .Fa stream has already been .Dq pclosed , or if .Xr wait4 2 returns an error. .Sh ERRORS The .Fn popen function does not reliably set .Va errno . .Sh SEE ALSO .Xr sh 1 , .Xr fork 2 , .Xr pipe 2 , .Xr socketpair 2 , .Xr wait4 2 , .Xr fclose 3 , .Xr fflush 3 , .Xr fopen 3 , .Xr shquote 3 , .Xr stdio 3 , .Xr system 3 .Sh STANDARDS The .Fn popen and .Fn pclose functions conform to .St -p1003.2-92 . .Sh HISTORY A .Fn popen and a .Fn pclose function appeared in .At v7 . .Sh BUGS Since the standard input of a command opened for reading shares its seek offset with the process that called .Fn 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 .Xr fflush 3 before .Fn popen . .Pp Failure to execute the shell is indistinguishable from the shell's failure to execute command, or an immediate exit of the command. The only hint is an exit status of 127. .Pp The .Fn popen argument always calls .Xr sh 1 , never calls .Xr csh 1 .