I want to call my custom malloc/new calls instead of standard library functions. 1. for malloc, I have this short program

    #include <stdio.h>
    #include <stdlib.h>

    void *__real_malloc(size_t size);

    void *__wrap_malloc(size_t size)
    {
        void *ptr = __real_malloc(size);
        printf("malloc(%ld) = %p\n", size, ptr);
        return ptr;
    }

    int main(void) {
      int* ptr = (int*) malloc(sizeof(int));
      return 0;
    }

I use the following command to compile g -Wl,--wrap,malloc main.cpp But getting below error

/usr/bin/ld: /tmp/ccnB04KY.o: in function `__wrap_malloc(unsigned long)':
b3.cpp:(.text 0x18): undefined reference to `__real_malloc(unsigned long)'
/usr/bin/ld: /tmp/ccnB04KY.o: in function `main':
b3.cpp:(.text 0x54): undefined reference to `__wrap_malloc'
collect2: error: ld returned 1 exit status

This works with gcc for .c files but not with g and .cpp files. What's the issue?

2. Also I cant figure out how to overide new calls?

CodePudding user response：

Why does g not work with -Wl,--wrap,malloc?

g is for C , and C ABI is different from C ABI. So you need to add extern "C" around __real_malloc and __wrap_malloc:

extern "C" {
void *__real_malloc(size_t size);

void *__wrap_malloc(size_t size) {
  void *ptr = __real_malloc(size);
  printf("malloc(%ld) = %p\n", size, ptr);
  return ptr;
}
}

C rewrites names to some kind of random (at least it seems so) strings to avoid conflicts, while C simply inserts _ before the "C" name to create a name in assembly. (This naming convention is called _cdecl. It's default for C and is used most commonly.) extern "C" forces C compiler to generate names according to C's convention. See https://en.wikipedia.org/wiki/Name_mangling for more information.

How to override new?

You can override new specifically for a class, or provide a global new. See https://en.cppreference.com/w/cpp/memory/new/operator_new.

CodePudding user response：

If you are using glibc then you can use the GNU extensions for malloc: man malloc_hook

   #include <malloc.h>

   void *(*__malloc_hook)(size_t size, const void *caller);

   void *(*__realloc_hook)(void *ptr, size_t size, const void *caller);

   void *(*__memalign_hook)(size_t alignment, size_t size,
                            const void *caller);

   void (*__free_hook)(void *ptr, const void *caller);

   void (*__malloc_initialize_hook)(void);

   void (*__after_morecore_hook)(void);

For new you can provide a global new. See https://en.cppreference.com/w/cpp/memory/new/operator_new.