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The _Exit() function called from within the int_handler() signal handler causes immediate program termination, and is asynchronous-safe, whereas exit() may call cleanup routines first, and is consequently not asynchronous-safe.

Implementation Details

OpenBSD

The OpenBSD signal() man page identifies functions that are asynchronous-signal safe. Applications may consequently invoke them, without restriction, from signal-catching functions.

POSIX

The following table from the the Open Group Base Specifications \[[Open Group 04|AA. C References#Open Group 04]\] defines a set of functions that are asynchronous-signal-safe. Applications may consequently invoke them, without restriction, from signal-catching functions:

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Note that while raise() is on the list of asynchronous-safe functions, it is specifically covered by SIG33-C. Do not recursively invoke the raise() function.

OpenBSD

The OpenBSD signal() man page identifies functions that are asynchronous-signal safe. Applications may consequently invoke them, without restriction, from signal-catching functions.

The OpenBSD signal() man page also says:

     A few other functions are signal race safe in OpenBSD but
     probably not on other systems:

           snprintf()    Safe.
           vsnprintf()   Safe.
           syslog_r()    Safe if the syslog_data struct is initialized
                         as a local variable.

Note that, in general, I/O functions are not safe to invoke inside signal handlers. Check your system's asynchronous-safe functions before using them in signal handlers.

Compliant Solution

Signal handlers should be as concise as possible, ideally unconditionally setting a flag and returning. They may also call the _Exit() function.

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