Be confused with one function prototype

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

Code:

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

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I'm confused with the first line

Code:

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int (*XSetErrorHandler(handler))()

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Most people use the function like that

Code:

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

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Others direct invoke the function

Code:

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XSetErrorHandler(old);

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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:

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handler = XSetErrorHandler(old);

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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.

Quote:

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

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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:

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

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

Code:

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XSetErrorHandler(handler);

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Quote:

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Originally Posted by chings

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

Code:

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

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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.

Quote:

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Originally Posted by chings

I'm confused with the first line

Code:

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int (*XSetErrorHandler(handler))()

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

Quote:

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Originally Posted by chings

Most people use the function like that

Code:

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

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

Quote:

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Originally Posted by chings

Others direct invoke the function

Code:

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XSetErrorHandler(old);

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I have no idea how the hell XSetErrorHandler works.

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

Quote:

---

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.

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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.