anything to do with pointer to functions maybe?

anything to do with pointer to functions maybe?

[soraberri]

Hi all!
I'm having troubles understanding code:

what type is __sighandler_t if it is defined this way:?
typedef void (*__sighandler_t) (int);

and then, what does this definition  mean:
  #define SIG_IGN  ((__sighandler_t) 1)	

thanks very much if you know and you tell me ;·)

regards

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Re: anything to do with pointer to functions maybe?

[Jan-Benedict Glaw]

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On Thu, 2004-12-23 11:24:39 +0100, soraberri <421246@posta.unizar.es>
wrote in message <cqe6ed$ldf$1@sea.gmane.org>:
> Hi all!
> I'm having troubles understanding code:
> 
> what type is __sighandler_t if it is defined this way:?
> typedef void (*__sighandler_t) (int);

We define that any signal handling function doesn't return anything
useful (void, that is), and all these functions get a single argument
(of type int) supplied.

> and then, what does this definition  mean:
>  #define SIG_IGN  ((__sighandler_t) 1)	

Also, we don't want *all* signals to actually be handled. Some are to be
ignored. Since we can only use function pointers (now of type
__sighandler_t), we need to have a special magic value (1 in this case)
that we can use.

However, "1" alone doesn't make a good function pointer, nor is it a
function at all or does accept arguments. To "look" like a function
pointer of correct type, "1" is casted to be of __sighandler_t.

There would have been another approach: we could have written a small
function that does nothing and could be used instead. However, it's
address could change, so it doesn't work well as an API. This is why
instead a distenct value (1) is used instead.

MfG, JBG

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Re: anything to do with pointer to functions maybe?

[soraberri]

Jan-Benedict Glaw wrote:
> On Thu, 2004-12-23 11:24:39 +0100, soraberri <421246@posta.unizar.es>
> wrote in message <cqe6ed$ldf$1@sea.gmane.org>:
> 
>>Hi all!
>>I'm having troubles understanding code:
>>
>>what type is __sighandler_t if it is defined this way:?
>>typedef void (*__sighandler_t) (int);
> 
> 
> We define that any signal handling function doesn't return anything
> useful (void, that is), and all these functions get a single argument
> (of type int) supplied.
> 
> 
>>and then, what does this definition  mean:
>> #define SIG_IGN  ((__sighandler_t) 1)	
> 
> 
> Also, we don't want *all* signals to actually be handled. Some are to be
> ignored. Since we can only use function pointers (now of type
> __sighandler_t), we need to have a special magic value (1 in this case)
> that we can use.
> 
> However, "1" alone doesn't make a good function pointer, nor is it a
> function at all or does accept arguments. To "look" like a function
> pointer of correct type, "1" is casted to be of __sighandler_t.
> 
> There would have been another approach: we could have written a small
> function that does nothing and could be used instead. However, it's
> address could change, so it doesn't work well as an API. This is why
> instead a distenct value (1) is used instead.
> 
> MfG, JBG
> 

Thanks Jan, I think I have understood...

regards and have happy days

Re: anything to do with pointer to functions maybe?

[Glynn Clements]

soraberri wrote:

> I'm having troubles understanding code:
> 
> what type is __sighandler_t if it is defined this way:?
> typedef void (*__sighandler_t) (int);

Pointer to function taking an int argument and returning void.

C's type syntax uses the same grammatical structure as expressions, so
the above is interpreted as saying that the expression:

	(*__sighandler_t) (int)

has type "void".

The syntax <expr>(<args>) indicates a function call where the function
is <expr> and the arguments are <args>, so:

	(*__sighandler_t) (int)

is a function call where (*__sighandler_t) is the function and the
arguments consist of a single integer. Coupled with the knowledge that
the overall type is "void", the expression:

	(*__sighandler_t)

must be a function taking an integer argument and returning void.

If an expression *P has type T then the type of P is "pointer to T",
so __sighandler_t must be a pointer to a function taking an integer
argument and returning void.

-- 
Glynn Clements <glynn@gclements.plus.com>