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neil
Joined: 08 Sep 2003
Posts: 128
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| Function pointer assignment back to front?? |
 Posted: Tue Nov 14, 2006 11:12 am |
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Hi again,
I am writing a menu for an LCD (please don't groan and stop reading yet!)
You know the story...
I couldn't get a pointer to function to work (using 3.249 or V4) so I wrote a test program. It won't compile in CCS but it works fine on a DEC terminal!
| Code: | void (*funcptr) (void); // Declare a pointer to func with no parameters
void function1(){
printf("Function 1 selected\n\r");
}
void function2(){
printf("Function 2 selected\n\r");
}
main(){
while(TRUE){
funcptr = &function1; // Select function 1
funcptr(); // call it via the pointer.
delay_ms(1000);
funcptr = &function2;
funcptr();
delay_ms(1000);
}
}
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The line "funcptr = &function1;" gives the error "Expecting an identifier" but on the DEC terminal it does exactly what is required.
More strangely, I have played about with different variations in syntax and found that the following code compiles (in CCS) and works as expected of the first code!
| Code: | void (*funcptr) (void); // Declare a pointer to func with no parameters
void function1(){
printf("Function 1 selected\n\r");
}
void function2(){
printf("Function 2 selected\n\r");
}
main(){
while(TRUE){
*funcptr = function1; // What is going on here?!
(*funcptr)(); // call it via the pointer.
delay_ms(1000);
*funcptr = function2;
(*funcptr)();
delay_ms(1000);
}
}
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Surely "*funcptr = function1;" is completely the wrong way to assign a function to the function pointer and should simply not work?
Neil. |
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Ttelmah
Guest
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| Posted: Tue Nov 14, 2006 11:33 am |
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The compiler is just ignoring the extra level of indirection.
| Code: |
funcptr = function1; // Select function 1
funcptr(); // call it via the pointer.
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Is normal syntax, and works.
Remember a function name, _is_ it's address. Normally, compilers are smart enough, to ignore the use of &, but CCS isn't at present, so your first syntax is asking for the address of an address, without allocating any storage for it (hence the error).
Best Wishes |
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neil
Joined: 08 Sep 2003
Posts: 128
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| Posted: Wed Nov 15, 2006 5:15 am |
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Hi, I understand that the & may be omitted and that CCS may still require it to be used. I actually prefer to use the & as it explains more when you glance at the code.
What I don't understand is how in real C a function pointer is assigned like this:
| Code: | | funcptr = &function1; |
and called like this:
or:
but CCS will only work when it is assigned like this:
| Code: | | *funcptr = function1; |
the first (correct) way assigns a function name (the address) to the pointer, but the second way is assigning a value 'function1' to the location that the pointer points to, which also happens to be void!!!!
One of the (proper) programmers I work with has looked at this and suggests that it is a compiler bug.
What do you guys think?
Neil. |
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Ttelmah
Guest
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| Posted: Wed Nov 15, 2006 6:21 am |
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Read what I said.
CCS, _doesn't_ 'only work when it is assigned like this'. It works without the need for the * on the variable.
Now the point is that adding '&', _should_ be legal (it is exactly the same as on an array, where though the name is the address, you can add the &, and this is then ignored by the compiler). However on functions, this 'ignoring' doesn't work for CCS. Yes this is a bug.
Now on the use of the *, this in fact represents exactly the same 'ignoring', but working beyond the 'call of duty'!...
Best Wishes |
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