SIGCHLD interrupts its own handler Post: 302405364

Sponsored Content

Top Forums Programming SIGCHLD interrupts its own handler Post 302405364 by Loic Domaigne on Thursday 18th of March 2010 04:05:51 PM

03-18-2010

Registered User

Quote:

Originally Posted by jrichemont

Hi Corona688. You are right. I already use WNOHANG with waitpid in a loop but only until that child has been dealt with. This is still liable to be interrupted though.

However you gave me an idea: I am using threads, I don't really need signals anyway and they mess with my head. What I did was start a thread and get it to sit in a loop with a blocking call to waitpid. As children die it gets unblocked, deals with it and loops back to blocking again. All in a nice serial manner.

I can simply ignore SIGCHLD now, that thread will pick up any children needing attention via waitpid.

Now, try as I might, I cannot break the system and I have it running in production today.

Cheers;

Jeremy

Good job, Jeremy!

You just discovered the way we usually deal with asynchronous signal in multi-threaded application. We spend a thread that waits (and consumes) synchronously the signal.

The function to wait synchronously for a signal is called sigwait(). You need in addition to ensure that the signal shall be delivered to the right thread, that is the one blocked in sigwait(). Pthreads offers such possibility with pthread_sigmask().

In your particular application, you luckier. First you can use waitpid(). And second, you can ignore SIGCHLD "signal redirection", since the default signal action for SIGCHLD is IGN.

Cheers,
Lo�c.

Loic Domaigne

View Public Profile for Loic Domaigne

Find all posts by Loic Domaigne

10 More Discussions You Might Find Interesting

1. Programming

Need help with SIGCHLD

Hello everybody, this is my first post on this forum. I have a program that has a child process that sleeps for 5 second and exit. I'm suppose to modify this program so that when the child exits, the parent reports the exit status of the child, so I also have to deal with SIGINT and SIGQUIT. Can...

2. UNIX for Advanced & Expert Users

catch SIGCHLD signal in parent process

I want to catch SIGCHLD signal in parent process. I can't use wait() system call to catch SIGCHLD according to project requirment. Operating system linux 3.1 can any one have a solution for this. Thanking you, ranjan

3. Programming

signal handler for SIGCHLD

Hi, I have an c++ application which uses the function fork and execvp(). The parent does not wait until the child ends. The parents just creates children and let them do their stuff. You can see the parent program as a batch-manager. I have added a SIGCHLD handler to the program: void...

4. Programming

When is SIGCHLD is raised.

Hi, I have 2 processes X and Y. Y is exec() from X. In Y i have an exit handler, which is called when i return from main. With in exit handler i delete and object which in turn calls the destructor of the object, which terminates all the threads of Y. I believe that SIGCHLD is raised by Y as...

5. Programming

adv reqd on SIGCHLD on accept call

Hi, I have small problem. In my (concurrent)server programm, I am handling accept problem from client like this. sample of server code. /*******************/ end = 0; while (! end ) { sockfd = accept(...) if(sockfd == -1) { if (errno == EINTR) /* this is bcoz...

6. UNIX for Dummies Questions & Answers

About SIGCHLD

When the SIGCHLD is sent? SIGCHLD is sent either a child exits spontaneously (e.g. exit(0)) or it is killed ? thank you in advance

7. UNIX for Dummies Questions & Answers

Who sent the process SIGCHLD ?

I want to know whicj process send the signal SIGCHLD to the parent's child. Thank you in advance,

8. Programming

SIGCHLD trace problem

Hello, I'd like to know whether it is possible to let the parent know who kills its child process. The case is likely as below: if there are four processes, we call them A, B, C and D. B is the child of A, and can be killed by both C and D. if B is killed, then A will receive SIGCHLD from B....

9. UNIX for Dummies Questions & Answers

about concept of Interrupts.

Hi all, I am new here ,i want to know about interrupts in detail.What r Interrupts .how they r handeled. Thanx in adavnce.

10. UNIX for Dummies Questions & Answers

using SIGCHLD

I'm testing out how to use SIGCHLD and I had a question about intercepting the signal and executing an action in the signal handler. signal(SIGCHLD,countdown); What I'm trying to achieve is be able to printf(Hello) every second that child is set to sleep. I'm setting sleep = 3; so...

LEARN ABOUT FREEBSD

sigwait

SIGWAIT(2)						      BSD System Calls Manual							SIGWAIT(2)

NAME

     sigwait -- select a set of signals

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <signal.h>

     int
     sigwait(const sigset_t * restrict set, int * restrict sig);

DESCRIPTION

     The sigwait() system call selects a set of signals, specified by set.  If none of the selected signals are pending, sigwait() waits until one
     or more of the selected signals has been generated.  Then sigwait() atomically clears one of the selected signals from the set of pending
     signals (for the process or for the current thread) and sets the location pointed to by sig to the signal number that was cleared.

     The signals specified by set should be blocked at the time of the call to sigwait().

     If more than one thread is using sigwait() to wait for the same signal, no more than one of these threads will return from sigwait() with the
     signal number.  If more than a single thread is blocked in sigwait() for a signal when that signal is generated for the process, it is
     unspecified which of the waiting threads returns from sigwait().  If the signal is generated for a specific thread, as by pthread_kill(),
     only that thread will return.

     Should any of the multiple pending signals in the range SIGRTMIN to SIGRTMAX be selected, it will be the lowest numbered one.  The selection
     order between realtime and non-realtime signals, or between multiple pending non-realtime signals, is unspecified.

IMPLEMENTATION NOTES

     The sigwait() function is implemented as a wrapper around the __sys_sigwait() system call, which retries the call on EINTR error.

RETURN VALUES

     If successful, sigwait() returns 0 and sets the location pointed to by sig to the cleared signal number.  Otherwise, an error number is
     returned.

ERRORS

     The sigwait() system call will fail if:

     [EINVAL]		The set argument specifies one or more invalid signal numbers.

SEE ALSO

     sigaction(2), sigpending(2), sigqueue(2), sigsuspend(2), sigtimedwait(2), sigwaitinfo(2), pause(3), pthread_sigmask(3)

STANDARDS

     The sigwait() function conforms to ISO/IEC 9945-1:1996 (``POSIX.1'').

BSD
								 September 6, 2013							       BSD