So can someone explain what this is supposed to do:
void (*signal(int sig, void (*func)(int)) ) (int);
It is a definition taken from the standard signal library.
Start with the name:
signal
Go right as far as you can:
signal(int sig, void (*func)(int))
You have a parenthesized list of parameters, so it's a function taking 2 parameters: an int
named sig
and a function pointer named func
(you can analyze it in the same way later).
Then you hit another rightparen, so you go left:
*signal(int sig, void (*func)(int))
So the function signal
returns a pointer to... something. Let's take down the parenthesis and go right again, since we can:
(*signal(int sig, void (*func)(int)) ) (int)
We have again a parenthesized list of arguments, so signal
returns a pointer to function which takes an int
as an only argument. Then go left again:
void (*signal(int sig, void (*func)(int)) ) (int)
Thus the function signal
returns the pointer to function taking int
and returning void
.
Yes, this language is weird, but at least it's consistent. :)
The function signal
takes as arguments:
int sig - a signal value
void (*func)(int) - a pointer to a function that takes an int and returns void
and returns:
void (*)(int) - a function that takes an int and returns void
signal
registers a function to be called when the signal occurs and returns the previous function handler.
Basically it allows to decide how to handle a specific signal (identified by argument int sig
) sent to your program.
The void (*func)(int)
is a pointer to the function that will handle the signal (you can provide a custom one or use SIG_DFL
SIG_IGN
which are default actions to manage it normally or ignore it).
The function signal
then returns the pointer to the handler present BEFORE the call of this function or SIG_ERR
is an error occurred. This can be used to restore the default handler lately when you've done with custom behaviour.
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