How to avoid using printf in a signal handler?

Question

Since printf is not reentrant, it\'s not supposed to be safe to use it in a signal handler. But I\'ve seen lots of example codes that uses printf t

Answer 1

One technique which is especially useful in programs which have a select loop is to write a single byte down a pipe on receipt of a signal and then handle the signal in the select loop. Something along these lines (error handling and other details omitted for brevity):

static int sigPipe[2];

static void gotSig ( int num ) { write(sigPipe[1], "!", 1); }

int main ( void ) {
    pipe(sigPipe);
    /* use sigaction to point signal(s) at gotSig() */

    FD_SET(sigPipe[0], &readFDs);

    for (;;) {
        n = select(nFDs, &readFDs, ...);
        if (FD_ISSET(sigPipe[0], &readFDs)) {
            read(sigPipe[0], ch, 1);
            /* do something about the signal here */
        }
        /* ... the rest of your select loop */
    }
}

If you care which signal it was, then the byte down the pipe can be the signal number.