Pointer of Function

Discussion created by David LECOMTE on Nov 12, 2008
Latest reply on Nov 12, 2008 by CompilerGuru

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I have some compilator errors due to pointer of function

void MyFunct(void)

{

.....

}

void main(void)

{

void (*MyPtrfunc) ();

MyPtrFunct = &MyFunct;

MyPtrFunct = MyPtrFunct - OffSetSW //Compilator error desciption : Error C1829 : + - incompatible Type

MyPtrFunct();

}

I already use this kind of declaration with another compilator, and i have no issue!!!

Do you have any about idea about my issue?

For you comprehension : Before run this command line, i rewrite Myfunction in spectific section in flash.

In fact, i try run a function place @ specific adress. This specific adress is Default adress (cofigured by emulator) - 0x1800.

Thanks

David

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CrasyCat Nov 12, 2008 1:44 PM
Hello

This is an ANSI C limitation you cannot use pointer arithmetic on function pointer.

Keep in mind that if you have a pointer to an int, ptr - 2 is in fact subtracting 4 (2 * sizeof(int)) from the pointer.
You cannot use sizeof on a function pointer, so you will not be able to use pointer arithmetic on such a pointer either

So you need to cast the function pointer to a pointer to char and do the arithmetic.

CrasyCat
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- Daniel Lundin @ CrasyCat on Nov 12, 2008 2:06 PM
  : This is an ANSI C limitation you cannot use pointer arithmetic on function pointer.
  
  I very much doubt this is true. Could you please cite the ISO standard?
  
  It is however indeed true that you can't use sizeof on function pointers.
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  - CompilerGuru @ Daniel Lundin on Nov 12, 2008 10:24 PM
    I would not call it a limitation, for me its just the way the language is defined.
    
    ANSI-C does not support pointer arithmetic for function pointers, more concretely ANSI-C defines pointer arithmetic for object type pointers only, here the definition for the addition:
    
    6.3.6 Additive operators
    ....
          [#2] For addition, either both   operands   shall   have
           arithmetic type, or one operand shall be a pointer to an
           object type and the other shall have   integer   type.
           (Incrementing is equivalent to adding 1.)
    
    And object types are distinct types form function types:
    
    6.1.2.5 Types...       Types are partitioned       into object types (types that describe objects), function       types (types that describe functions), and incomplete types       (types that describe objects but lack information needed to       determine their sizes).
    
    So in other words, ANSI-C does not support pointer arithmetic on function pointers.
    
    Daniel
    
    PS: Snippets above copied from some C standard draft found on the web.
    
    Message Edited by CompilerGuru on 2008-11-12 03:24 PM
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David LECOMTE Nov 12, 2008 2:36 PM
so my line programmation becomes

//OffsetSW = 0x1800 may be i must cast with unsigned int???
MyPtrFunct = (char*)MyPtrFunct - OffSetSW

Now i don't have any compilator error but a following warning appears :
Non stardart conversion used. Is it important??

David
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- Daniel Lundin @ David LECOMTE on Nov 12, 2008 2:57 PM
  Yes. Substracting one pointer from another when they aren't pointing at the same array is undefined behavior in C. Typecasting from a function pointer to any other pointer type is also undefined behavior, you can only cast to integer.
  
  The warning means that the code isn't legal C but that the compiler has made a non-standard extension to allow it. So the code will be non-portable.
  
  The quick & dirty way around it is to cast both pointers to integer, then substract, then cast the result back to a function pointer.
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David LECOMTE Nov 12, 2008 5:10 PM
OK thanks much
david
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