Problem - Kevin Tafuro

#endif
}
static int signal_was_caught(void)
{
if (sigint_received) printf("SIGINT received!\n");
if (sigterm_received) printf("SIGTERM received!\n");
if (sigquit_received) printf("SIGQUIT received!\n");
return (sigint_received || sigterm_received || sigquit_received);
}
int main(int argc, char *argv[ ]) {
char buffer[80];
setup_signal_handler(SIGINT, handle_sigint);
setup_signal_handler(SIGTERM, handle_sigterm);
setup_signal_handler(SIGQUIT, handle_sigquit);
/* The main loop of this program simply reads input from stdin, and
* throws it away. It's useless functionality, but the point is to
* illustrate signal handling, and fread is a system call that will
* be interrupted by signals, so it works well for example purposes
*/
while (!feof(stdin)) {
fread(buffer, 1, sizeof(buffer), stdin);
if (signal_was_caught( )) break;
}
return (sigint_received || sigterm_received || sigquit_received);
}
This code clearly illustrates both points made in the “Solution” section. Separate signal
handlers are used for each signal that we want to handle: SIGINT, SIGTERM, and
SIGQUIT. For each signal handler, a global flag is set to nonzero to indicate that the
signal was caught. Later, when the system call—fread( ) in this case—returns, the
flags are checked and fully handled. (It is true that fread( ) itself is not really a system
call, but it is a wrapper around the read( ) system call.)
In the function setup_signal_handler( ), weusesigaction( ) to set up our signal
handlers, rather than signal( ), if it is available. On most modern Unix systems,
sigaction( ) is available and should be used. One problem with signal( ) is that on
some platforms it is subject to race conditions because it is implemented as a wrapper
around sigaction( ). Another problem is that on some systems—most notably
those that are BSD-derived—some system calls are restarted when interrupted by a
signal, which is typically not the behavior we want. In this particular example, it certainly
is not because we won’t get the opportunity to check our flags until after the
call to fread( ) completes, which could be a long time. Using sigaction( ) without
the nonportable SA_RESTART flag will disable this behavior and cause fread( ) to
return immediately with the global errno set to EINTR.
Performing Proper Signal Handling | 715
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