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Cast function pointer to unsigned int and back.

Silver Diamond
Genius 4010 points

In C, you can cast a function pointer to void, can the TI C/C++ compiler do that?  For example:

int increase(int abc){
     abc++;
     return abc;
}
void main(){
     unsigned int FnPtr[50];
     fnPtr[5] = (unsigned int)increase;   // this does not work, bu this is what I need to work
 // then I need to cast back to function pointer
     int b;
     b = ((int (*)(int))(fnPtr+5)(6);
}

I need to place function pointers and data into unsigned int array[], how can I make this work in TI C/C++ compilers.  It works on MinGW C++ (size of unsigned int and pointer on my MinGW system is the same)

  • 0
    TI__Guru* 84225 points

    Exactly how do you know that it "does not work?"  Do you get a compilation error?  Does it fail at run time?  Does it produce incorrect results?   What command-line options are you using?

    Your example cannot compile because it has errors in it; for instance, FnPtr has inconsistent case.  When posting examples, please cut-and-paste the actual text rather than typing it in again, as that is prone to this sort of error.   Fixing the compilation problems, I think that your cast in the final line is incorrect.  It should be:

    b = ((int (*)(int))*(fnPtr+5))(6);

    Notice the dereference and the missing parenthesis.  (A function call has higher precedence than a cast, so it must go there.)  With those changes, the test case works as expected.

    This is a complicated expression; you should generally avoid complicated expressions.  Consider the following:

    int (*p)(int) = (int (*)(int))fnPtr[5];
b = p(6);
  • 0
    Guru 26430 points

    On the MSP420, pointers can be 20-bits. You could try avoiding casts entirely:

    typedef int (*FNPTR)(int);

    int increase(int abc){
         abc++;
         return abc;
    }

    void main(){
         FNPTR fnPtr[50];
         fnPtr[5] = increase;
         int b;
         b = fnPtr[5](6);
    }

  • 0 in reply to Norman Wong
    Prodigy 160 points

    Norman Wong said:

    On the MSP420, pointers can be 20-bits. You could try avoiding casts entirely [...]

    This is the right way. Casting between integer and function pointer types results in undefined behavior and thus is unsafe in general.

  • 0 in reply to Archaeologist
    Genius 4010 points

    Archaeologist,

    You are right about my typos, I should have copied and pasted.  But my main problem was due to compiler settings, once I changed the settings in red (memory and data model), it worked great.

    Thanks...

  • 0 in reply to Norman Wong
    Genius 4010 points

    Norman,

    You pointing out the 20bit pointer size did help me to find solution.  And in 99% of the cases I agree with solution, it is a unique case where I really need that ability.  I agree that I have to be very careful with this technique and it is better to avoid it.

    Thanks...