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.\"     @(#)popen.3	8.2 (Berkeley) 5/3/95
.\"
.Dd September 11, 2021
.Dt POPEN 3
.Os
.Sh NAME
.Nm popen ,
.Nm popenve ,
.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 FILE *
.Fn popenve "const char *path" "char * const *argv" "char * const *envp" "const char *type"
.Ft int
.Fn pclose "FILE *stream"
.Sh DESCRIPTION
The
.Fn popen
function
.Dq opens
a process by creating an IPC connection,
forking,
and invoking the shell.
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 sockets, the
.Fa type
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.
In addition if the character
.Ql e
is present in the
.Fa type
string, the file descriptor used internally is set to be closed on
.Xr exec 3 .
.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
.Fn popenve
function is similar to
.Fn popen
but the first three arguments are passed
to
.Xr execve 2
and there is no shell involved in the command invocation.
.Pp
The return value from
.Fn popen
and
.Fn popenve
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
.Xr wait4 2 .
.Sh RETURN VALUES
The
.Fn popen
function returns
.Dv NULL
if the
.Xr vfork 2 ,
.Xr pipe 2 ,
or
.Xr socketpair 2
calls fail,
or if it cannot allocate memory, preserving
the errno from those functions.
.Pp
The
.Fn pclose
function
returns \-1 if
.Fa stream
is not associated with a
.Dq popened
command, if
.Fa stream
has already been
.Dq pclosed ,
setting errno to
.Er ESRCH ,
or if
.Xr wait4 2
returns an error, preserving the errno returned by
.Xr wait4 2 .
.Sh SEE ALSO
.Xr sh 1 ,
.Xr execve 2 ,
.Xr fork 2 ,
.Xr pipe 2 ,
.Xr socketpair 2 ,
.Xr vfork 2 ,
.Xr wait4 2 ,
.Xr fclose 3 ,
.Xr fflush 3 ,
.Xr fopen 3 ,
.Xr shquote 3 ,
.Xr stdio 3 ,
.Xr system 3
.Sh STANDARDS
The
.Fn popen
and
.Fn pclose
functions conform to
.St -p1003.2-92 .
.Sh HISTORY
A
.Fn popen
and a
.Fn pclose
function appeared in
.At v7 .
.Pp
The
.Fn popenve
function first appeared in
.Nx 8 .
.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 the 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 1 ,
never calls
.Xr csh 1 .