CODEWITHSUNDEEP
c
Nikky
Nikky

Reputation: 518

C signal code explanation needed

We came across this snippet of code in our textbook that was lacking explanation except the fact that it handles signals.

#include <signal.h> 
void (*signal(int signr,
            void(*sighandler)(int)
        )
    )(int)

I know that sighandler is a pointer to a function, but I do not understand if it is actually executed or just returned?

and what does the call with (int) do? It looks almost like a reversed cast.

Upvotes: 2

Views: 114

Answers (1)

dbush
dbush

Reputation: 224062

This is the signature of the signal function.

The first argument signr is of type int and is the signal whose handler you want to change.

The second argument sighandler is a function pointer of type void (*)(int), i.e. a function that takes an int and returns void. This parameter is the name of a function which will handle the signal.

The function returns a function pointer of type void (*)(int) (same type as argument 2) which points to the previous signal handling function.

Breaking it down:

 signal                                 // signal
 signal()                               // is a function
 signal(int)                            // taking a int
 signal(int, void (*)(int))             // and a function pointer
                                        // which takes an int and return void
 (*signal)(int, void (*)(int))(int)     // and returns a function pointer
                                        // which takes an int
 void (*signal)(int, void (*)(int))(int)   // and returns void

The man page includes a typedef which makes this a bit more clear:

typedef void (*sighandler_t)(int);

sighandler_t signal(int signum, sighandler_t handler);

Upvotes: 6

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