.TH POPEN 3S
.CT 2 comm_proc
.SH NAME
popen, ppopen, vepopen, pclose \(mi open a pipe to/from a process
.SH SYNOPSIS
.nf
.B #include <stdio.h>
.PP
.B FILE *popen(command, type)
.B char *command, *type;
.PP
.B FILE *ppopen(command, type)
.B char *command, *type;
.PP
.B FILE *vepopen(command, type, args, env)
.B char *command, *type, **args, **env;
.PP
.B int pclose(stream)
.B FILE *stream;
.fi
.SH DESCRIPTION
The first argument to
.I popen
is a pointer to a null-terminated string
containing a command line for
.IR sh (1).
.I Type
is as in
.IR fopen (3).
.I Popen
creates a pipe between
the calling process and
the command and returns
a stream pointer that
can be used to write to the standard input
of the command or to read from the standard output.
.PP
.I Ppopen
uses the
.B -p
shell flag to restrict the environment of the shell.
Both
.I popen
and
.I ppopen
set the effective userid to the real userid
before calling the shell.
.PP
.I Vepopen
has arguments akin to those of
.I execve 
(see
.IR exec (2)):
a file to be executed,
a mode as above,
a null-terminated vector of argument strings,
and a null-terminated vector of environment strings.
The shell is not called, and the effective userid is preserved.
.PP
A stream opened by these routines
should be closed by
.I pclose,
which waits for the associated process to terminate
and returns the exit status of the command.
.PP
Because the 
.I command
inherits open files, in particular standard input and output, a type
.L
"r"
call may be used to insert a filter in the input, and type
.L
"w"
in the output.
.SH "SEE ALSO"
.IR exec (2),
.IR pipe (2),
.IR fopen (3),
.IR stdio (3),
.IR system (3)
.SH DIAGNOSTICS
.I Popen
returns a null pointer
if files or processes cannot be created, or the Shell
cannot be accessed.
.PP
.I Pclose
returns \-1
if there is
no process to wait for.
.SH BUGS
Buffered reading before opening an input filter
may leave the standard input of that filter mispositioned.
Similar problems with an output filter may be
forestalled by calling
.IR fflush ;
see
.IR fopen (3).
.br
The resetting of the userid
is probably gratuitous;
it is there as a defense against incautious use
of the routine by set-uid programs.
.br
I/O type
.L
"r+w"
exists but is not useful.