NetBSD/lib/libc/gen/popen.3

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.\"     @(#)popen.3	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt POPEN 3
.Os
.Sh NAME
.Nm popen ,
.Nm pclose
.Nd process
.Tn I/O
.Sh SYNOPSIS
.Fd #include <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 a pipe,
forking,
and invoking the shell.
Since a pipe is by definition unidirectional, the
.Fa type
argument may specify only reading or writing, not both;
the resulting stream is correspondingly read-only or write-only.
.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.
The
.Fa mode
argument is a pointer to a null-terminated string
which must be either
.Ql r
for reading
or
.Ql w
for writing.
.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 VALUE
The
.Fn popen
function returns
.Dv NULL
if the
.Xr fork 2
or
.Xr pipe 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
already
.Dq pclosed ,
or if
.Xr wait4
returns an error.
.Sh ERRORS
The
.Fn popen
function does not reliably set
.Va errno .
.Sh SEE ALSO
.Xr fork 2 ,
.Xr sh 1 ,
.Xr pipe 2 ,
.Xr wait4 2 ,
.Xr fflush 3 ,
.Xr fclose 3 ,
.Xr fopen 3 ,
.Xr stdio 3 ,
.Xr system 3
.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 ,
never calls
.Xr csh .
.Sh HISTORY
A
.Fn popen
and a
.Fn pclose
function appeared in
.At v7 .