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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
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@ -1,7 +1,7 @@
@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@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.
@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
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
with the scsh fork.
@end deffn

8
libguile/posix.c
View file

@ -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
we can do is to allow forking only very early, before any call to
sigaction spawns the signal-handling thread. */
SCM_MISC_ERROR ("attempt to fork while multiple threads are running",
SCM_EOL);
scm_display
(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 ();
if (pid == -1)
SCM_SYSERROR;