Define a function pointer to be32toh() function

Question

I'm newly learned about function pointers here but I couldn't define a function pointer to be32toh() or other functions of endian.h header.

First of all, I can do what is told in the given thread:


#include <endian.h>
int addInt(int n, int m) {
    return n+m;
}
int main(){
    int (*functionPtr)(int,int);
    functionPtr = addInt;    
    return 0;
}

But when I try to do the same for a function like be32toh(), I'll get a compilation error:

#include <stdint.h>
#include <endian.h>
int addInt(int n, int m) {
    return n+m;
}
int main(){
    int (*functionPtr)(int,int);
    functionPtr = addInt;
    uint32_t (*newptr)(uint32_t);
    newptr = &be32toh;
    return 0;
}

Compile as:

$ gcc test.c

Result is as below:

test.c: In function ‘main’:
test.c:15:15: error: ‘be32toh’ undeclared (first use in this function)
   15 |     newptr = &be32toh;
      |               ^~~~~~~
test.c:15:15: note: each undeclared identifier is reported only once for each function it appears in

What's the problem and how to fix it?

Thank you all, So there is no way to do it? I mean, sometimes I need to call `be32toh()` and other times `le32toh()`, without pointer I should use `if(){...}else{}` style which has a lot of redundancy. — milad, Nov 24 '21 at 07:32
@milad You could use wrappers, that is, you could define your own functions, called something like be32toh_func, and pass a pointer to them. Then, each of this function calls the corresponding macro. — Fabio says Reinstate Monica, Nov 24 '21 at 07:50
`sometimes I need to call be32toh() and other times le32toh(), without pointer I should use if(){...}else{}` Could you elaborate? It's rarely that you take pointers to these functions, you just know which one you want to use cause you know the input format, and you _want_ the compiler to optimize it. — KamilCuk, Nov 24 '21 at 08:31
@KamilCuk I'm developing an ELF parser, the input file could be either Little-endian or Big-endian. — milad, Nov 24 '21 at 08:53

score 2 · Accepted Answer · answered Nov 24 '21 at 08:29

2

What's the problem

be32toh is a macro.

how to fix it?

Just write the function yourself.

uint32_t be32toh_func(uint32_t a) {
     return be32toh(a);
}

....
     newptr = &be32toh_func;

answered Nov 24 '21 at 08:29

KamilCuk

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Define a function pointer to be32toh() function

1 Answers1