GCC Makefile Define

Question

I have some C code that I want to do some tests on. It uses malloc, calloc, and free through out the code. I want to change those functions to a custom function that internally calls the original function. For example:

emxArray->size = (int *)malloc((unsigned int)(sizeof(int) * numDimensions));

would become:

emxArray->size = (int *)myMalloc((unsigned int)(sizeof(int) * numDimensions));

where myMalloc is:

void* myMalloc(unsigned size)
{
    if (size < 8)
    {
        //printf("*** Bumped from %d....\n", size);
        size = 8;
    }
    allocated += size;
    return malloc(size);
}

As you can see, myMalloc internally calls malloc. It just does some extra stuff. I wanted to replace the usage of malloc through out the code with myMalloc. I have done this successfully by going through all the code and replacing malloc with myMalloc manually, but this is far from ideal. I will be replacing this code on a test only basis, thus the production code should contain only malloc calls. I realize I could also do this with a script, but wanted to just use a define statement in the Makefile:

-Dmalloc=myMalloc

But that also replaces malloc in the myMalloc function, which causes an infinite recursive situation. I tried changing the malloc call in the myMalloc function to malloc_d, and added a second define to the Makefile.

-Dmalloc=myMalloc -Dmalloc_d=malloc

I was thinking that the first define would not replace the malloc_d (which it didn't) and that the second define would only change the malloc_d (which it didn't). I got the same recursive situation. Is there anyway to do this with Makefile defines? Or are multipass pre-compile situations going to always mess this up?

UPDATE:

Ok, so I have started looking at the LD_PRELOAD option that has been pointed out. I thought I had a workable solution, however, I am still having trouble! Here is what I did...

1. I moved myMalloc() and myFree() out of the main file and into its own file. I then compiled it into a shared library using: gcc -shared -o libMyMalloc.so -fPIC myMalloc.c

2. I then added the following 'dummy functions' to the main file:

   void* myMalloc(unsigned size) { void* ptr; return ptr; }

   void myFree(void* ptr) { }

As you can see, they do nothing.

3. I added the following defines to the make file: -Dmalloc=myMalloc \\ -Dfree=myFree

4. I compiled the code and ran it against the libMyMalloc.so library I created:

   LD_PRELOAD=/home/rad/Desktop/myMalloc/libMyMalloc.so ./testRegress

However, I am still not getting it to run with the myMalloc functions that are defined in the libMyMalloc.so file.

Answer 1

The simplest solution is to not call malloc directly in your code: If you choose a different name (say MALLOC ), it's trivial to switch to a custom allocator.

Example code:

#ifndef MALLOC
#define MALLOC malloc
#endif

For test builds, you'd do -DMALLOC=myMalloc .

It gets more complicated if for some reason you want keep the calls to malloc . Then you'd have to add something like the following after all standard library headers have been included :

#ifdef USE_MY_MALLOC
#undef malloc
#define malloc(SIZE) myMalloc(SIZE)
#endif

You can call the standard library function by using parens, ie

void* myMalloc(unsigned size)
{
    ...
    return (malloc)(size);
}

and enable it via -DUSE_MY_MALLOC .

---

Considering the additional requirements given in the comments, two approaches come to mind:

• pre-process the generated source, textually replacing calls to malloc
• intercept inclusion of stdlib.h (assuming that's where MATLAB gets its malloc declaration from)

Your own version of stdlib.h would look something like this:

#ifndef MY_STDLIB_H_
#define MY_STDLIB_H_

#include "/usr/include/stdlib.h"

#undef malloc
#define malloc(SIZE) myMalloc(SIZE)

#endif

Then, you can conditionally add the directory where you've placed that file to the include path. Also note that this is not a particularly robust solution, but it might work for you anyway.

Answer 2

You can use a pointer to a function. In the normal case, make it point to malloc. In debugging mode, let it point to you function.

In some h file:

extern void *(*myMalloc)(size_t size);

In one of you c files:

#ifdef DEBUG
void *(*myMalloc)(size_t size) = dMalloc;
#else
void *(*myMalloc)(size_t size) = malloc; // derived from libc
#endif

Answer 3

I found my solution and wanted to share. Thank you to everyone that contributed and pointed me in the right direction.

I ended up creating my custom library code and compiling it into a shared library:

gcc -shared -o libtmalloc.so -fPIC tmalloc.c

I then modified the makefile to use the shared library and globally define 'malloc' to my custom function name (which internally calls malloc()) to malloc_t, as well as calloc() and free():

gcc -L/home/path/to/mallocTrace -Wall -o test test.c lib/coder/*.c -lm -ltmalloc \
-Dmalloc=malloc_t \
-Dcalloc=calloc_t \
-Dfree=free_t

The defines changed all the function calls for me which were linked to the implementation in the shared library. Because I am using a shared library (which is already compiled), I didn't have to worry about my makefile defines causing a recursive call situation in my custom functions. With this usage, I can take any pre-generated C code from my other tools and observe the memory usage with these simple makefile changes and using my custom malloc trace library.