minix/lib/libc/gen/popen.3
Ben Gras 2fe8fb192f Full switch to clang/ELF. Drop ack. Simplify.
There is important information about booting non-ack images in
docs/UPDATING. ack/aout-format images can't be built any more, and
booting clang/ELF-format ones is a little different. Updating to the
new boot monitor is recommended.

Changes in this commit:

	. drop boot monitor -> allowing dropping ack support
	. facility to copy ELF boot files to /boot so that old boot monitor
	  can still boot fairly easily, see UPDATING
	. no more ack-format libraries -> single-case libraries
	. some cleanup of OBJECT_FMT, COMPILER_TYPE, etc cases
	. drop several ack toolchain commands, but not all support
	  commands (e.g. aal is gone but acksize is not yet).
	. a few libc files moved to netbsd libc dir
	. new /bin/date as minix date used code in libc/
	. test compile fix
	. harmonize includes
	. /usr/lib is no longer special: without ack, /usr/lib plays no
	  kind of special bootstrapping role any more and bootstrapping
	  is done exclusively through packages, so releases depend even
	  less on the state of the machine making them now.
	. rename nbsd_lib* to lib*
	. reduce mtree
2012-02-14 14:52:02 +01:00

202 lines
4.8 KiB
Groff

.\" $NetBSD: popen.3,v 1.16 2007/08/02 23:45:10 wiz Exp $
.\"
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.\"
.\" @(#)popen.3 8.2 (Berkeley) 5/3/95
.\"
.Dd August 2, 2007
.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 an IPC connection,
forking,
and invoking the shell.
Historically,
.Nm popen
was implemented with a unidirectional pipe;
hence many implementations of
.Nm popen
only allow the
.Fa type
argument to specify reading or writing, not both.
Since
.Nm 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.
.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
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 ,
.Xr pipe 2 ,
or
.Xr socketpair 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
has already been
.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 socketpair 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 .
.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 1 ,
never calls
.Xr csh 1 .