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warning instead of error on multithreaded fork

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* libguile/posix.c (scm_fork): Issue a warning instead of an error on a
  multithreaded fork.
* doc/ref/posix.texi (Processes): Add note about multithreaded fork.
This commit is contained in:
Andy Wingo 2013-01-17 12:18:22 +01:00
parent 94027fe6ec
commit 22cdf986db
2 changed files with 21 additions and 3 deletions
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16
doc/ref/posix.texi
View file

@ -1,7 +1,7 @@
@c -*-texinfo-*- @c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual. @c This is part of the GNU Guile Reference Manual.
@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2006, 2007, @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2006, 2007,
@c 2008, 2009, 2010, 2011, 2012 Free Software Foundation, Inc. @c 2008, 2009, 2010, 2011, 2012, 2013 Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions. @c See the file guile.texi for copying conditions.
@node POSIX @node POSIX
@ -1785,6 +1785,20 @@ Creates a new ``child'' process by duplicating the current ``parent'' process.
In the child the return value is 0. In the parent the return value is In the child the return value is 0. In the parent the return value is
the integer process ID of the child. the integer process ID of the child.
Note that it is unsafe to fork a process that has multiple threads
running, as only the thread that calls @code{primitive-fork} will
persist in the child. Any resources that other threads held, such as
locked mutexes or open file descriptors, are lost. Indeed,
@acronym{POSIX} specifies that only async-signal-safe procedures are
safe to call after a multithreaded fork, which is a very limited set.
Guile issues a warning if it detects a fork from a multi-threaded
program.
If you are going to @code{exec} soon after forking, the procedures in
@code{(ice-9 popen)} may be useful to you, as they fork and exec within
an async-signal-safe function carefully written to ensure robust program
behavior, even in the presence of threads. @xref{Pipes}, for more.
This procedure has been renamed from @code{fork} to avoid a naming conflict This procedure has been renamed from @code{fork} to avoid a naming conflict
with the scsh fork. with the scsh fork.
@end deffn @end deffn

8
libguile/posix.c
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@ -1260,8 +1260,12 @@ SCM_DEFINE (scm_fork, "primitive-fork", 0, 0, 0,
async-signal-safe. We can't guarantee that in general. The best async-signal-safe. We can't guarantee that in general. The best
we can do is to allow forking only very early, before any call to we can do is to allow forking only very early, before any call to
sigaction spawns the signal-handling thread. */ sigaction spawns the signal-handling thread. */
SCM_MISC_ERROR ("attempt to fork while multiple threads are running", scm_display
SCM_EOL); (scm_from_latin1_string
("warning: call to primitive-fork while multiple threads are running;\n"
" further behavior unspecified. See \"Processes\" in the\n"
" manual, for more information.\n"),
scm_current_warning_port ());
pid = fork (); pid = fork ();
if (pid == -1) if (pid == -1)
SCM_SYSERROR; SCM_SYSERROR;