Be confused with one function prototype

This is a discussion on Be confused with one function prototype within the C Programming forums, part of the General Programming Boards category; Here the function prototype manual goes, Xlib Programming Manual: XSetErrorHandler Code: int (*XSetErrorHandler(handler))() int (*handler)(Display *, XErrorEvent *) I'm confused ...

  1. #1
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    Unhappy 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.

  2. #2
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    okay i get it from someone's help.
    typedef int (*HANDLER)(Display*, XErrorEvent*);
    HANDLER XSetErrorHandler( HANDLER handler );

  3. #3
    and the hat of wrongness Salem's Avatar
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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:
    old = XSetErrorHandler(handler);
    /// code
    XSetErrorHandler(old);
    Other times, you want a permanent replacement, so you ignore the return result.
    Code:
    XSetErrorHandler(handler);
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    Quote Originally Posted by chings View Post
    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.
    Quote Originally Posted by chings View Post
    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.
    Quote Originally Posted by chings View Post
    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.
    Quote Originally Posted by chings View Post
    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.
    Quote Originally Posted by chings View Post
    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.

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