C程序，将参数传递给信号处理程序函数？

Question

I have a possible " It Can't Be Done! " problem, which I'd like to bounce off the community.

I'm working the libIPFIX program, written in C. At the start of the code, the program sets up a Signal Handler function:

void exit_func ( int signo )
{
    if ( verbose_level && signo )
        fprintf( stderr, "\n[%s] got signo %d, bye.\n\n", progname, signo );

    ...clean up global variables...

    exit( 1 );
}

Later in main(), the signal handler is wired into the larger code:

int main (int argc, char *argv[])
{
    ...

    // signal handler
    signal( SIGKILL, exit_func );
    signal( SIGTERM, exit_func );
    signal( SIGINT,  exit_func );

    ...

    exit(1);
}

Pretty boilerplate, from what I can tell.

Here's my issue: I'm tracking a lot of extra data in my modified version of the program, using malloc() and linked lists and whatnot. When the code detects a signal, it would be great if exit_func() could call my cleanup() functions to eliminate memory leaks, blah blah blah. What I'd love is this:

void exit_func ( int signo, LLNode* myData )
{
    ...same...

    cleanUp( myData );

    exit( 1 );
}

But a reading of sigaction plus posts like these plus my own experimenting strongly suggest that there isn't a way to pass in an additional argument to exit_func() . And that the only way to do what I want is to make my data into a global variable. I'd really hate to do that for other design reasons.

So I thought I'd go for broke and just ask: Is there was way to pass an argument into exit_func() ? Thanks in advance.

Answer 1

You cannot send arguments to a signal handler. Anything that needs to be accessed from the handler needs to be put into some kind of global struct.

Answer 2

Your original code is already wrong (even if it appears to work; it still is already undefined behavior ). Read first the signal (IPC) wikipage.

The original signal handler exit_func is already wrong. Read signal(7) and signal-safety(7) . Neither exit , nor fprintf , can be called, even indirectly from a signal handler, because they are not async-signal-safe (and neither is malloc ).

You'll better set some volatile sig_atomic_t variable in your signal handler, and test it in your event loops (see this and POSIX documentation on signal.h ). This is usual practice (and once you do that, which you have to do to be compliant with the signal(7) and signal-safety(7) rules, your question becomes irrelevant).

You might be interested by the Linux-specific signalfd(2) (it has some minor flaws. STFW for them).

The Qt documentation has a nice chapter for Calling Qt functions from Unix signal handlers . Some of the wisdom there (eg the pipe(7) to self trick) may apply to your program, even if it does not use Qt.