libc/gen/popen.3

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.Dd May 3, 1995
.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 a bidirectional pipe
forking,
and invoking the shell.
Any streams opened by previous
.Fn popen
calls in the parent process are closed in the new child process.
Historically,
.Fn popen
was implemented with a unidirectional pipe;
hence many implementations of
.Fn popen
only allow the
.Fa type
argument to specify reading or writing, not both.
Since
.Fn popen
is now implemented using a bidirectional pipe, the
.Fa type
argument 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.
.Pp
The
.Fa type
argument may be augmented by appending an
.Ql e
to set the descriptor's close-on-exec flag.
For example,
.Ql re
for reading,
.Ql we
for writing, or
.Ql r+e
for reading and writing.
Use of this flag is important when operating in threaded environments.
.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
.Fn popen
automatically interlocks and closes descriptors associated with other
active
.Fn popen
files in any sub-process it creates, preventing file descriptor leakage
between
.Fn popen
calls in a thread-safe manner.
However,
.Fn popen
has no control over fork or fork/exec sequences run by other threads which
do not use the popen mechanism and in this situation it is likely that
popen descriptors will leak into those sub-processes.
It is recommended that the
.Ql e
flag be used to prevent descriptor leakages into miscellaneous fork/exec
sequences that might be executed by other threads in a multi-threaded
program.
.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
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 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 wait4 2 ,
.Xr fclose 3 ,
.Xr fflush 3 ,
.Xr fopen 3 ,
.Xr stdio 3 ,
.Xr system 3
.Sh HISTORY
A
.Fn popen
and a
.Fn pclose
function appeared in
.At v7 .
.Pp
Bidirectional functionality was added in
.Fx 2.2.6 .
.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
function
always calls
.Xr sh 1 ,
never calls
.Xr csh 1 .