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Full Discussion: C++ application halts at popen system call
Top Forums Programming C++ application halts at popen system call Post 302323368 by achenle on Sunday 7th of June 2009 07:20:07 AM
Old 06-07-2009
From "man attributes":

Quote:
Async-Signal-Safe

Async-Signal-Safe refers to particular library functions
that can be safely called from a signal handler. A
thread that is executing an Async-Signal-Safe function
will not deadlock with itself if interrupted by a sig-
nal. Signals are only a problem for MT-Safe functions
that acquire locks.

Async-Signal-Safe functions are also MT-Safe. Signals
are disabled when locks are acquired in Async-Signal-
Safe functions. These signals prevent a signal handler
that might acquire the same lock from being called.
Your code is doing a popen() in your SEGV signal handler. From "man popen":

Quote:
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:

____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Interface Stability | See below. |
|_____________________________|_____________________________|
| MT-Level | Safe |
|_____________________________|_____________________________|

The F character in the mode argument of popen() is Evolving.
In all other respects this function is Standard. The
pclose() function is Standard.


So, popen() is not async-signal safe. Hence your deadlock.

The fact that it works on other servers does not make it correct.

Of cource, fixing this deadlock isn't going to fix your code. You still have the SEGV in free() to deal with. That's almost certainly caused by application code corrupting the memory heap by overwriting dynamically-allocated memory or passing an improper pointer to a dynamic memory routine such as free().
 

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LEARN ABOUT PLAN9

thr_kill

thr_kill(3C)															      thr_kill(3C)

NAME

       thr_kill - send a signal to a thread

SYNOPSIS

       cc -mt [ flag... ] file... [ library... ]

       #include <signal.h>
       #include <thread.h>

       int thr_kill(thread_t thread, int sig);

       The  thr_kill()	function sends the sig signal to the thread designated by thread. The thread argument must be a member of the same process
       as the calling thread. The sig argument must be one of the signals listed  in  signal.h(3HEAD),	with  the  exception  of  SIGCANCEL  being
       reserved and off limits to thr_kill(). If sig is 0, a validity check is done for the existence of the target thread; no signal is sent.

       Upon successful completion, thr_kill() returns 0. Otherwise,  an error number is returned. In the event of failure, no signal is sent.

       The thr_kill() function will fail if:

       EINVAL	       The sig argument value is not zero and is an invalid or an  unsupported signal number.

       ESRCH	       No thread was found that corresponded to the thread designated by thread ID.

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |Async-Signal-Safe 	   |
       +-----------------------------+-----------------------------+

       kill(2), sigaction(2), raise(3C), signal.h(3HEAD), thr_self(3C), attributes(5), standards(5)

								    23 Mar 2005 						      thr_kill(3C)
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