Has anyone an example for wrapping a function in C++? - c++

Question

I have searched online a lot but I couldn't find an example that works with g+, all examples work with GCC.
The error I keep getting is:
wrap_malloc.o: In function `__wrap_malloc(unsigned int)':
wrap_malloc.cc:(.text+0x20): undefined reference to `__real_malloc(unsigned int)'
wrap_malloc.o: In function `main':
wrap_malloc.cc:(.text+0x37): undefined reference to `__wrap_malloc'
collect2: ld returned 1 exit status
The code that creates this error is the following (this code works if I compile it with GCC and change the headers from cstdio to stdio.h):
#include <cstdio>
#include <cstdlib>
void *__real_malloc(size_t);
void *__wrap_malloc(size_t c) {
printf("My malloc called with %d\n", c);
return __real_malloc(c);
}
int main(void) {
void *ptr = malloc(12);
free(ptr);
return 0;
}
This is how I compile it:
wrap_malloc.o: wrap_malloc.cc
g++ -c wrap_malloc.cc -o wrap_malloc.o
wrap_malloc: wrap_malloc.o
g++ wrap_malloc.o -o wrap_malloc -Wl,--wrap,malloc

Answer 1

When you use a C++ compiler, all names are mangled. What this means becomes clear when you run nm wrap_malloc.o, which should give you something like this:
00000000 b .bss
00000000 d .data
00000000 r .rdata
00000000 t .text
U __Z13__real_mallocj
00000000 T __Z13__wrap_mallocj
U _printf
This means that you use (U) a symbol called __Z13__real_mallocj and that you define a symbol in the text segment (T) called __Z13__wrap_mallocj. But you probably want a symbol called __real_malloc. To achieve this you have to say the compiler that __real_malloc is a C-style function, like this:
extern "C" void *__real_malloc(size_t);
extern "C" void *__wrap_malloc(size_t c) {
printf("My malloc called with %d\n", c);
return __real_malloc(c);
}
Now the output of nm is:
00000000 b .bss
00000000 d .data
00000000 r .rdata
00000000 t .text
U ___real_malloc
00000000 T ___wrap_malloc
U _printf
You can see that the name _printf hasn't changed. This is because in the header files, many functions are declared as extern "C" already.
Note: I did all of the above on Windows in the cygwin environment. That's why there is an additional leading underscore in the external symbols.

Answer 2

If this is the complete code, the problem you are having is that you haven't implemented __real_malloc()!
And by the way, identifiers with double-underscores are reserved by the language. You might want to think about picking different names.