Create a wrapper function for malloc and free in C

Question

I am trying to create wrapper functions for free and malloc in C to help notify me of memory leaks. Does anyone know how to declare these functions so when I call malloc() and free() it will call my custom functions and not the standards lib functions?

Answer 1

You have a few options:

1. GLIBC-specific solution (mostly Linux). If your compilation environment is glibc with gcc, the preferred way is to use malloc hooks. Not only it lets you specify custom malloc and free, but will also identify the caller by the return address on the stack.

2. POSIX-specific solution. Define malloc and free as wrappers to the original allocation routines in your executable, which will "override" the version from libc. Inside the wrapper you can call into the original malloc implementation, which you can look up using dlsym with RTLD_NEXT handle. Your application or library that defines wrapper functions needs to link with -ldl.

   #define _GNU_SOURCE
   #include <dlfcn.h>
   #include <stdio.h>
   
   void* malloc(size_t sz)
   {
       void *(*libc_malloc)(size_t) = dlsym(RTLD_NEXT, "malloc");
       printf("malloc\n");
       return libc_malloc(sz);
   }
   
   void free(void *p)
   {
       void (*libc_free)(void*) = dlsym(RTLD_NEXT, "free");
       printf("free\n");
       libc_free(p);
   }
   
   int main()
   {
       free(malloc(10));
       return 0;
   }
   

3. Linux specific. You can override functions from dynamic libraries non-invasively by specifying them in the LD_PRELOAD environment variable.

   LD_PRELOAD=mymalloc.so ./exe
   

4. Mac OSX specific.

   Same as Linux, except you will be using DYLD_INSERT_LIBRARIES environment variable.

Answer 2

You can do wrapper and "overwrite" function with LD_PRELOAD - similarly to example shown earlier.

LD_PRELOAD=/path.../lib_fake_malloc.so ./app

But I recommend to do this "slightly" smarter, I mean calling dlsym once.

#define _GNU_SOURCE
#include <stdio.h>
#include <stdint.h>
#include <dlfcn.h>

void* malloc(size_t size)
{
    static void* (*real_malloc)(size_t) = NULL;
    if (!real_malloc)
        real_malloc = dlsym(RTLD_NEXT, "malloc");

    void *p = real_malloc(size);
    fprintf(stderr, "malloc(%d) = %p\n", size, p);
    return p;
}

example I've found here: http://www.jayconrod.com/cgi/view_post.py?23 post by Jay Conrod.

But what I've found really cool at this page is that: GNU linker provides a useful option, --wrap . When I check "man ld" there is following example:

void *
__wrap_malloc (size_t c)
{
    printf ("malloc called with %zu\n", c);
    return __real_malloc (c);
}

I agree with them that's "trivial example" :). Even dlsym is not needed.

Let, me cite one more part of my "man ld" page:

--wrap=symbol
       Use a wrapper function for symbol.
       Any undefined reference to symbol will be resolved to "__wrap_symbol".
       Any undefined reference to "__real_symbol" will be resolved to symbol.

I hope, description is complete and shows how to use those things.

Answer 3

In my case I needed to wrap memalign/aligned_malloc under malloc. After trying other solutions I ended up implementing the one listed below. It seems to be working fine.

mymalloc.c.

/* 
 * Link-time interposition of malloc and free using the static
 * linker's (ld) "--wrap symbol" flag.
 * 
 * Compile the executable using "-Wl,--wrap,malloc -Wl,--wrap,free".
 * This tells the linker to resolve references to malloc as
 * __wrap_malloc, free as __wrap_free, __real_malloc as malloc, and
 * __real_free as free.
 */
#include <stdio.h>

void *__real_malloc(size_t size);
void __real_free(void *ptr);


/* 
 * __wrap_malloc - malloc wrapper function 
 */
void *__wrap_malloc(size_t size)
{
    void *ptr = __real_malloc(size);
    printf("malloc(%d) = %p\n", size, ptr);
    return ptr;
}

/* 
 * __wrap_free - free wrapper function 
 */
void __wrap_free(void *ptr)
{
    __real_free(ptr);
    printf("free(%p)\n", ptr);
}
 

Answer 4

In C, the method I used was similar to:

#define malloc(x) _my_malloc(x, __FILE__, __LINE__)
#define free(x) _my_free(x)

This allowed me to detect the line and file of where the memory was allocated without too much difficulty. It should be cross-platform, but will encounter problems if the macro is already defined (which should only be the case if you are using another memory leak detector.)

If you want to implement the same in C++, the procedure is a bit more complex but uses the same trick.

Answer 5

Here's a set of wrapper functions I used for years (and still do when I dip into C) to detect unfree'd memory, memory free'd multiple times, references to free'd memory, buffer overflows/underflows, and freeing memory that was not allocated.

ftp://ftp.digitalmars.com/ctools.zip

They've been around for 25 years and have proven themselves.

You could use the macro preprocessor to redefine malloc and free to use the mem package ones, but I recommend against it, because it won't redirect library calls to malloc like what strdup does.

Answer 6

If your goal is to eliminate memory leaks, an easier, less intrusive way is to use a tool like Valgrind (free) or Purify (costly).

Answer 7

If you define your own functions for malloc() and free() and explicitly link that with your applications, your functions should be used in preference to those in the library.

However, your function called 'malloc' cannot then call the library malloc function, because in 'c' there's no concept of separate namespaces. In other words, you'd have to implement the internals of malloc and free yourself.

Another approach would be to write functions my_malloc() and my_free(), which call the standard library ones. This would mean that any code calling malloc would have to be changed to call your my_xxx functions.

Answer 8

If you are the only client of the custom malloc and free (i.e. you're not trying to monkey patch those methods for code in some other library), then you can use dependency injection.

#ifndef ALLOCATOR_H
#define ALLOCATOR_H

#include <stddef.h>

struct Allocator;

typedef struct {
    void *(*allocate)(struct Allocator *allocator, size_t size);

    void (*free)(struct Allocator *allocator, void *object);
} AllocatorVTable;

typedef struct Allocator {
    const AllocatorVTable *vptr;
} Allocator;

typedef struct {
    Allocator super;
    char *buffer;
    size_t offset;
    size_t capacity;
} BufferedAllocator;

void BufferedAllocator_init(BufferedAllocator *allocator, char *buffer, size_t capacity);

typedef Allocator MallocAllocator;

void MallocAllocator_init(MallocAllocator *allocator);

void *Allocator_allocate(Allocator *allocator, size_t size);

void Allocator_free(Allocator *allocator, void *object);

#endif

#include "allocator.h"
#include "malloc.h"

void *Allocator_allocate(Allocator *allocator, size_t size) {
    return allocator->vptr->allocate(allocator, size);
}

void Allocator_free(Allocator *allocator, void *object) {
    allocator->vptr->free(allocator, object);
}

void *BufferedAllocator_allocate(Allocator *allocator, size_t size) {
    BufferedAllocator *bufferedAllocator = (BufferedAllocator *) allocator;
    if (bufferedAllocator->offset + size > bufferedAllocator->capacity) {
        fprintf(stderr, "buffer overflow: %ld + %ld > %ld\n",
                bufferedAllocator->offset, size, bufferedAllocator->capacity);
        return NULL;
    }
    bufferedAllocator->offset += size;
    return bufferedAllocator->buffer + bufferedAllocator->offset - size;
}

void BufferedAllocator_free(Allocator *allocator, void *object) {

}

const AllocatorVTable bufferedAllocatorVTable = {
        .allocate = BufferedAllocator_allocate,
        .free = BufferedAllocator_free,
};

void BufferedAllocator_init(BufferedAllocator *allocator, char *buffer,
                            size_t capacity) {
    allocator->super.vptr = &bufferedAllocatorVTable;
    allocator->buffer = buffer;
    allocator->offset = 0;
    allocator->capacity = capacity;
}

void *MallocAllocator_allocate(Allocator *allocator, size_t size) {
    return malloc(size);
}

void MallocAllocator_free(Allocator *allocator, void *object) {
    free(object);
}

const AllocatorVTable mallocAllocatorVTable = {
        .allocate = MallocAllocator_allocate,
        .free = MallocAllocator_free,
};

void MallocAllocator_init(MallocAllocator *allocator) {
    allocator->vptr = &mallocAllocatorVTable;
}

#include <assert.h>
#include "allocator_test.h"
#include "allocator.h"

void testAllocator() {
    {
        BufferedAllocator bufferedAllocator;
        char buffer[4];
        size_t capacity = sizeof(buffer);
        BufferedAllocator_init(&bufferedAllocator, buffer, capacity);
        Allocator *allocator = &bufferedAllocator.super;

        void *chill = Allocator_allocate(allocator, capacity);
        assert(chill == buffer);
        void *oops = Allocator_allocate(allocator, 1);
        assert(oops == NULL);
    }

    {
        MallocAllocator allocator;
        MallocAllocator_init(&allocator);

        void *chill = Allocator_allocate(&allocator, 100);
        assert(chill != NULL);
        void *alsoChill = Allocator_allocate(&allocator, 100);
        assert(alsoChill != NULL);
    }
}

So you would pass around an Allocator * to whichever piece of code you write that wants to allocate stuff (beyond something like char buf[n] on the stack). You can use a MallocAllocator to just use the system malloc/free, or you could use a BufferedAllocator at the very top of your program. A BufferedAllocator is just an example of a really simple malloc/free. It works well in my use-case because I pretty much know how much memory my program will use in advance, and I don't delete any object until the entire program is done. Using this interface, you could write a more complicated algorithm like one of the ones described in this lecture. There are a lot of different strategies for preventing fragmentation and many trade-offs, so rolling your own malloc/free could be really useful.

Answer 9

If you are using Linux, you can use malloc_hook() (with GNU glibc). This function allows you to have malloc call your function prior to calling the actual malloc. The man page has an example on how to use it.

Answer 10

If you are only talk about memory that you have under control, i.e. that you malloc and free on your own, you can take a look on rmdebug. Probably it is what you are going to write anyway, so you can save sometime. It has a very liberal licence, if that should be important for you.

I personally use it in a project, to look for memory leaks, the nice things is that it is much faster then valgrind, however it isn't that powerful so you don't get the full calling stack.