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POsafe to access shared data structure from signal handler
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copied!<p>I'm trying to decide wether it's safe to access a common (read: shared between handler-code and rest of the programm) data structure from a signal handler in perl (v5.14.2) built for <code>x86_64-linux-thread-multi</code>, but target platform is solaris11).</p> <p><a href="http://perldoc.perl.org/perlipc.html" rel="nofollow">perlipc</a> has the following sample code:</p> <pre><code>use POSIX ":sys_wait_h"; # for nonblocking read my %children; $SIG{CHLD} = sub { # don't change $! and $? outside handler local ($!, $?); my $pid = waitpid(-1, WNOHANG); return if $pid == -1; return unless defined $children{$pid}; delete $children{$pid}; cleanup_child($pid, $?); }; while (1) { my $pid = fork(); die "cannot fork" unless defined $pid; if ($pid == 0) { # ... exit 0; } else { $children{$pid}=1; # ... system($command); # ... } } </code></pre> <p>So, <code>%children</code> is accessed from the while-loop and the handler. This seems to be no problem as:</p> <ol> <li>There won't be two processes having the same <code>pid</code></li> <li>Access is keyed by <code>pid</code> (I am not sure if <code>$childer{pid}=1</code> is atomic and interruptible without causing corruption, though.)</li> </ol> <p>Now, i'm trying to do even more in my handler:</p> <pre><code>my %categoryForPid; my %childrenPerCategory; $SIG{CHLD} = sub { # ... acquire pid like above my $category = $categoryForPid{$pid}; $childrenPerCategory{$category}--; delete $categoryForPid{$pid}; } while (1) { # ... same as above } else { $children{$pid}=1; my $category = # ... chose some how $childrenPerCategory{$category}++; $categoryForPid{$pid} = $category; # ... } } </code></pre> <p>The idea here is: every child belongs to a certain category (N to 1). I want to keep track of how many children per category exist. That information could be derived from <code>$categoryForPid</code>, but i think that might be problematic also (e.g., when the subroutine doing the computation gets interrupted while summing up).</p> <p>So my question is:</p> <ul> <li>Do I need to synchronize here somehow?</li> </ul> <p>And on a side note:</p> <ul> <li>Are nested invocations of the signal handler possible in perl 5.12, or are they linearized by the interpreter?</li> </ul> <h2>Update</h2> <p>In addition to the problem spotted by @goldilocks and his proposed solution I block signals now while updating the data structures to ensure "atomicity":</p> <pre><code>my $sigset = POSIX::SigSet->new(SIGCHLD); sub ublk { unless (defined sigprocmask(SIG_UNBLOCK, $sigset)) { die "Could not unblock SIGCHLD\n"; } } sub blk { unless (defined sigprocmask(SIG_BLOCK, $sigset)) { die "Could not block SIGCHLD\n"; } } while (1) { # ... same as above } else { blk; $children{$pid}=1; my $category = # ... chose some how $childrenPerCategory{$category}++; $categoryForPid{$pid} = $category; ublk; # ... } } </code></pre>

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