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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 .

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