DAtum: Function pointers: number of parameters mismatch

Function pointers: number of parameters mismatch

Here is a strange quirk of gcc - It ignores number-of-parameter mismatch if we use function pointers, if and only if we pass more arguments than necessary.
Consider the following piece of code



#include <stdio.h> 

typedef int (*myFunc)(int a, int b);
int func1(int a){
        printf("In func1 \n");
}

int func2(int a, int b){
        printf("In func2 \n");
}

void executeFunc(myFunc someFunc ){
//void executeFunc(void* someFunc ){
        myFunc actualFunc=someFunc;
        actualFunc(1,2);
        //actualFunc(1,2);

}

main(){
  executeFunc((myFunc)func1);

}

As you can see, I am actually calling "func1", which takes one argument. However,I am actually passing two arguments. The code works fine.
Strangely enough, the other way does'nt work - i.e.



#include <stdio.h> 

typedef int (*myFunc)(int a, int b);
int func1(int a){
        printf("In func1 \n");
}

int func2(int a, int b){
        printf("In func2 \n");
}

void executeFunc(myFunc someFunc ){
//void executeFunc(void* someFunc ){
        myFunc actualFunc=someFunc;
        actualFunc(1,2);
        //actualFunc(1);

}

main(){
  executeFunc((myFunc)func2);

}

The above piece of code calls "func2", which actually takes two arguments with only one parameter. Compilation fails in this case.
What is more puzzling is that, in case I had called "func1" directly (without going through all the function-pointers thing), it gives a compilation error of (too many arguments). I can deduce that gcc is doing checking w.r.t the "typedef" in the beginning, but given that ordinarily a function cannot be called with too many arguments, should'nt this be disallowed?
6.3.2.2 of C89 (which correspondsto 6.5.2.2 of C99) states that:

If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters

Which brings me to another "feature" of C vs C++. Consider the following function:



#include <stdio.h> 

int func3(int a){
return 1;

}

main(){
 func3(1,2,3);
}

This gives a compilation error with both "gcc" and "g++" about too many arguments.
However, consider the following piece of code:



#include <stdio.h> 

int func3(){
return 1;

}

main(){
 func3(1,2,3);
}

This gives no error with "gcc", but an error with "g++". Apparently, in c++, a function declared with no arguments is taken as void, whereas in c, it is simply unknown.

Does gcc have a soft-spot for function parameters with too many arguments?

Only time or maybe Raymond Chen can tell.

This entry was posted on 6/20/2005 at 6/20/2005 10:09:00 PM.

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