On some platforms such as Windows where the fork() system call is not
available, Perl can be built to emulate fork() at the interpreter level.
While the emulation is designed to be as compatible as possible with the
-real fork() at the the level of the Perl program, there are certain
+real fork() at the level of the Perl program, there are certain
important differences that stem from the fact that all the pseudo child
"processes" created this way live in the same real process as far as the
operating system is concerned.
=item %ENV
-Each pseudo-process maintains its own virtual enviroment. Modifications
+Each pseudo-process maintains its own virtual environment. Modifications
to %ENV affect the virtual environment, and are only visible within that
pseudo-process, and in any processes (or pseudo-processes) launched from
it.
One can avoid this by opening files that need distinct seek pointers
separately in the child.
+=item Forking pipe open() not yet implemented
+
+The C<open(FOO, "|-")> and C<open(BAR, "-|")> constructs are not yet
+implemented. This limitation can be easily worked around in new code
+by creating a pipe explicitly. The following example shows how to
+write to a forked child:
+
+ # simulate open(FOO, "|-")
+ sub pipe_to_fork ($) {
+ my $parent = shift;
+ pipe my $child, $parent or die;
+ my $pid = fork();
+ die "fork() failed: $!" unless defined $pid;
+ if ($pid) {
+ close $child;
+ }
+ else {
+ close $parent;
+ open(STDIN, "<&=" . fileno($child)) or die;
+ }
+ $pid;
+ }
+
+ if (pipe_to_fork('FOO')) {
+ # parent
+ print FOO "pipe_to_fork\n";
+ close FOO;
+ }
+ else {
+ # child
+ while (<STDIN>) { print; }
+ close STDIN;
+ exit(0);
+ }
+
+And this one reads from the child:
+
+ # simulate open(FOO, "-|")
+ sub pipe_from_fork ($) {
+ my $parent = shift;
+ pipe $parent, my $child or die;
+ my $pid = fork();
+ die "fork() failed: $!" unless defined $pid;
+ if ($pid) {
+ close $child;
+ }
+ else {
+ close $parent;
+ open(STDOUT, ">&=" . fileno($child)) or die;
+ }
+ $pid;
+ }
+
+ if (pipe_from_fork('BAR')) {
+ # parent
+ while (<BAR>) { print; }
+ close BAR;
+ }
+ else {
+ # child
+ print "pipe_from_fork\n";
+ close STDOUT;
+ exit(0);
+ }
+
+Forking pipe open() constructs will be supported in future.
+
=item Global state maintained by XSUBs
External subroutines (XSUBs) that maintain their own global state may
knows nothing about the containing application's state. For example, any
state carried on the application's own call stack is out of reach.
+=item Thread-safety of extensions
+
+Since the fork() emulation runs code in multiple threads, extensions
+calling into non-thread-safe libraries may not work reliably when
+calling fork(). As Perl's threading support gradually becomes more
+widely adopted even on platforms with a native fork(), such extensions
+are expected to be fixed for thread-safety.
+
=back
=head1 BUGS
This document may be incomplete in some respects.
+=back
+
=head1 AUTHOR
-Support for the fork() emulation was implemented by ActiveState, supported
-by funding from Microsoft Corporation.
+Support for concurrent interpreters and the fork() emulation was implemented
+by ActiveState, with funding from Microsoft Corporation.
This document is authored and maintained by Gurusamy Sarathy
E<lt>gsar@activestate.comE<gt>.