gno/usr.man/man3/popen.3

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.\"     @(#)popen.3	8.1 (Berkeley) 6/4/93
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.TH POPEN 3 "23 February 1997" GNO "Library Routines"
.SH NAME
.BR popen ,
.BR pclose
\- process I/O
.SH SYNOPSIS
#include <stdio.h>
.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.