195 lines
4.7 KiB
Groff
195 lines
4.7 KiB
Groff
.\" $NetBSD: popen.3,v 1.13 2003/08/07 16:42:55 agc Exp $
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.\"
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.\" Copyright (c) 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)popen.3 8.2 (Berkeley) 5/3/95
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.\"
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.Dd May 3, 1995
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.Dt POPEN 3
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.Os
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.Sh NAME
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.Nm popen ,
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.Nm pclose
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.Nd process
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.Tn I/O
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In stdio.h
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.Ft FILE *
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.Fn popen "const char *command" "const char *type"
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.Ft int
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.Fn pclose "FILE *stream"
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.Sh DESCRIPTION
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The
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.Fn popen
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function
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.Dq opens
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a process by creating an IPC connection,
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forking,
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and invoking the shell.
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Historically,
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.Nm popen
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was implemented with a unidirectional pipe;
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hence many implementations of
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.Nm popen
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only allow the
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.Fa type
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argument to specify reading or writing, not both.
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Since
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.Nm popen
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is now implemented using sockets, the
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.Fa type
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may request a bidirectional data flow.
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The
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.Fa type
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argument is a pointer to a null-terminated string
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which must be
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.Ql r
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for reading,
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.Ql w
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for writing, or
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.Ql r+
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for reading and writing.
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.Pp
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The
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.Fa command
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argument is a pointer to a null-terminated string
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containing a shell command line.
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This command is passed to
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.Pa /bin/sh
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using the
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.Fl c
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flag; interpretation, if any, is performed by the shell.
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.Pp
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The return value from
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.Fn popen
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is a normal standard
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.Tn I/O
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stream in all respects
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save that it must be closed with
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.Fn pclose
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rather than
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.Fn fclose .
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Writing to such a stream
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writes to the standard input of the command;
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the command's standard output is the same as that of the process that called
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.Fn popen ,
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unless this is altered by the command itself.
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Conversely, reading from a
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.Dq popened
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stream reads the command's standard output, and
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the command's standard input is the same as that of the process that called
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.Fn popen .
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.Pp
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Note that output
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.Fn popen
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streams are fully buffered by default.
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.Pp
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The
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.Fn pclose
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function waits for the associated process to terminate
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and returns the exit status of the command
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as returned by
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.Fn wait4 .
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.Sh RETURN VALUES
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The
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.Fn popen
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function returns
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.Dv NULL
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if the
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.Xr fork 2 ,
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.Xr pipe 2 ,
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or
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.Xr socketpair 2
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calls fail,
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or if it cannot allocate memory.
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.Pp
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The
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.Fn pclose
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function
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returns \-1 if
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.Fa stream
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is not associated with a
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.Dq popened
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command, if
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.Fa stream
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has already been
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.Dq pclosed ,
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or if
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.Xr wait4 2
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returns an error.
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.Sh ERRORS
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The
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.Fn popen
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function does not reliably set
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.Va errno .
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.Sh SEE ALSO
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.Xr sh 1 ,
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.Xr fork 2 ,
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.Xr pipe 2 ,
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.Xr socketpair 2 ,
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.Xr wait4 2 ,
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.Xr fclose 3 ,
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.Xr fflush 3 ,
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.Xr fopen 3 ,
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.Xr stdio 3 ,
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.Xr system 3
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.Sh HISTORY
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A
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.Fn popen
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and a
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.Fn pclose
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function appeared in
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.At v7 .
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.Sh BUGS
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Since the standard input of a command opened for reading
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shares its seek offset with the process that called
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.Fn popen ,
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if the original process has done a buffered read,
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the command's input position may not be as expected.
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Similarly, the output from a command opened for writing
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may become intermingled with that of the original process.
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The latter can be avoided by calling
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.Xr fflush 3
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before
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.Fn popen .
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.Pp
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Failure to execute the shell
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is indistinguishable from the shell's failure to execute command,
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or an immediate exit of the command.
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The only hint is an exit status of 127.
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.Pp
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The
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.Fn popen
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argument
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always calls
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.Xr sh 1 ,
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never calls
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.Xr csh 1 .
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