IDL signal handler functions are defined as:

typedef void (* IDL_SignalHandler_t)(int signo);

When a signal is delivered to the process, all registered signal handlers are called. signo is the integer number of the signal delivered, as defined by the C language header file signal.h (found in /usr/include/signal.h on most UNIX systems). signo can be used by a signal handler registered for more than one signal to tell which signal called it.