Function prototype in header file doesn't match definition, how to catch this?

Question

(I found this question which is similar but not a duplicate: How to check validity of header file in C programming language )

I have a function implementation, and a non-matching prototype (same name, different types) which is in a header file. The header file is included by a C file that uses the function, but is not included in the file that defines the function.

Here is a minimal test case :

header.h:

void foo(int bar);

File1.c:

#include "header.h"
int main (int argc, char * argv[])
{
    int x = 1;
    foo(x);
    return 0;
}

File 2.c:

#include <stdio.h>

typedef struct {
    int x;
    int y;
} t_struct;

void foo (t_struct *p_bar)
{
    printf("%x %x\n", p_bar->x, p_bar->y);
}

I can compile this with VS 2010 with no errors or warnings, but unsurprisingly it segfaults when I run it.

• The compiler is fine with it (this I understand)
• The linker did not catch it (this I was slightly surprised by)
• The static analysis tool (Coverity) did not catch it (this I was very surprised by).

How can I catch these kinds of errors?

[Edit: I realise if I #include "header.h" in file2.c as well, the compiler will complain. But I have an enormous code base and it is not always possible or appropriate to guarantee that all headers where a function is prototyped are included in the implementation files.]

Answer 1

Have the same header file included in both file1.c and file2.c. This will pretty much prevent a conflicting prototype.

Otherwise, such a mistake cannot be detected by the compiler because the source code of the function is not visible to the compiler when it compiles file1.c. Rather, it can only trust the signature that has been given.

At least theoretically, the linker could be able to detect such a mismatch if additional metadata is stored in the object files, but I am not aware if this is practically possible.

Answer 2

-Werror-implicit-function-declaration, -Wmissing-prototypes or equivalent on one of your supported compilers. then it will either error or complain if the declaration does not precede the definition of a global.

Compiling the programs in some form of strict C99 mode should also generate these messages. GCC, ICC, and Clang all support this feature (not sure about MS's C compiler and its current status, as VS 2005 or 2008 was the latest I've used for C).