Search C function definition using Compilation Error - c

Question

I am working on a C project which has same definition of C function in multiple places. I am trying to get the definition of C function using compilation errors like "error: conflicting types for foo_func()"; which also provides header file declaration as "bar_file.h:< line no >: note: previous declaration of foo_func() was here". I want to rely on compilation error instead of grep like commands because compiler knows for sure which function it is linking with, so I can be sure of the function definition which is being used.
I am able to locate the header file with deliberately applying compilation error. Is there a way to use the similar technique to find the definition of function I am using at compilation time?
Example: Let's find the header file which has the declaration of the function we are using. We can end up into the following compilation error by purposeful introduction of multiple definition with a known parameter mismatch.
$ gcc -Wall -Wextra -c exmpl_01.c -I./
exmpl_01.c:5: error: conflicting types for ‘call_func_other_file’
exmpl_01.h:1: note: previous declaration of ‘call_func_other_file’ was here
$ head -4 exmpl_01.c
#include <stdio.h>
#include "exmpl_01.h"
int call_func(void);
int call_func_other_file(char *);
$ cat exmpl_01.h
int call_func_other_file(void);

Answer 1

You can try deliberately addind a duplicate definition in a separate translation unit. Then link all object files together (compiled with debugging symbols enabled). The linker will tell you in what file and on what line the duplicate is defined.

Answer 2

Maybe you are looking for -E switch ?
// a.h
int a() {
return 1;
}
and then, we have
// main.c
#include "a.h"
int main() {
return a();
}
and, with -E you can get
> gcc -E main.c
# 1 "main.c"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 331 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "main.c" 2
# 1 "./a.h" 1
int a() {
return 1;
}
# 2 "main.c" 2
int main() {
return a();
}
now, let's say our source has another file
// b.h
float a() {
return 1.0;
}
and main.c is slightly different
#include "a.h"
#include "b.h"
int main() {
return a();
}
you will be able to see the last definition of function
gcc -c -E main.c
# 1 "main.c"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 331 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "main.c" 2
# 1 "./a.h" 1
int a() {
return 1;
}
# 2 "main.c" 2
# 1 "./b.h" 1
float a() {
return 1.0;
}
# 3 "main.c" 2
int main() {
return a();
}
But, I am not sure whether this is something you are looking for ;)
Hmm ... if you want to stop at certain point, maybe this one will help you:
#include "a.h"
#include "b.h"
int main() {
a();
#error Quit!
printfun("aaa");
}
and this way, compilation will fail a #error
> gcc -c -Wfatal-errors main.c
In file included from main.c:2:
./b.h:1:7: fatal error: conflicting types for 'a'
float a() {
^
./a.h:1:5: note: previous definition is here
int a() {
^
1 error generated.
Or, you can simply introduce some sort of fake type
#include "a.h"
struct fake_type {
int fake_component;
};
struct fake_type *a();
int main() {
a();
}
and enforce the error
> gcc -c -Wfatal-errors main.c
main.c:7:19: fatal error: conflicting types for 'a'
struct fake_type *a();
^
./a.h:1:5: note: previous definition is here
int a() {
^
1 error generated.