Be confused with one function prototype

Here the function prototype manual goes,
Xlib Programming Manual: XSetErrorHandler

Code:

int (*XSetErrorHandler(handler))()
      int (*handler)(Display *, XErrorEvent *)

I'm confused with the first line

Code:

int (*XSetErrorHandler(handler))()

Most people use the function like that

Code:

int (*handler)(Display *, XErrorEvent *) = NULL;
int old(Display *d, XErrorEvent *e){...}
handler = XSetErrorHandler(old);
XSetErrorHandler(handler);

Others direct invoke the function

Code:

XSetErrorHandler(old);

I have no idea how the hell XSetErrorHandler works.

As I understood for XSetErrorHandler()
At first it in the bracket returns one function which returned pf integer value,and then invoke the function.
But it conflicts with

Code:

handler = XSetErrorHandler(old);

which obvisouly return function pointer.


what's the theory of the function?
how hell does function works?



I'm looking forware to the explanation.

Ching.

okay i get it from someone's help.

typedef int (*HANDLER)(Display*, XErrorEvent*);
HANDLER XSetErrorHandler( HANDLER handler );

It's just a function which takes a pointer to a function as a parameter, and returns a pointer to a function (the previous handler).

Sometimes you want to temporarily use your own handler

Code:

old = XSetErrorHandler(handler);
/// code
XSetErrorHandler(old);

Other times, you want a permanent replacement, so you ignore the return result.

Code:

XSetErrorHandler(handler);

Originally Posted by chings

Here the function prototype manual goes,
Xlib Programming Manual: XSetErrorHandler

Code:

int (*XSetErrorHandler(handler))()
      int (*handler)(Display *, XErrorEvent *)

First line is the prototype for XSetErrorHandler() while the next line clarifies that the single argument to XSetErrorHandler() ie handler is a pointer to a function that returns an int.

Originally Posted by chings

I'm confused with the first line

Code:

int (*XSetErrorHandler(handler))()

It means that XSetErrorHandler is a function that takes a single argument and returns a pointer to a function that returns an int.

Originally Posted by chings

Most people use the function like that

Code:

int (*handler)(Display *, XErrorEvent *) = NULL;
int old(Display *d, XErrorEvent *e){...}
handler = XSetErrorHandler(old);
XSetErrorHandler(handler);

That is correct since the return value of XSetErrorHandler() is being assigned to handler.

Originally Posted by chings

Others direct invoke the function

Code:

XSetErrorHandler(old);

I have no idea how the hell XSetErrorHandler works.

Also correct except the return value of XSetErrorHandler() is lost.

Originally Posted by chings

As I understood for XSetErrorHandler()
At first it in the bracket returns one function which returned pf integer value,and then invoke the function.
But it conflicts with

Code:

handler = XSetErrorHandler(old);

which obvisouly return function pointer.


what's the theory of the function?
how hell does function works?



I'm looking forware to the explanation.

Ching.

The above assignment statement is correct because XSetErrorHandler() takes old as its argument - a function that takes two arguments and returns an int.
old is the name of a function and used by itself becomes a pointer to that function which is exactly the type of the argument that XSetErrorHandler() expects.
XSetErrorHandler() returns a pointer to a function returning int, which agrees with the declaration of handler as it is a pointer to a function that returns an int.