Question

I’m trying to write a simple shared library that would log malloc calls to stderr (a sort of ‘mtrace’ if you will).

However, this is not working.
Here’s what I do:

/* mtrace.c */
#include <dlfcn.h>
#include <stdio.h>

static void* (*real_malloc)(size_t);

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

static void __mtrace_init(void) __attribute__((constructor));
static void __mtrace_init(void)
{
    void *handle = NULL;
    handle = dlopen("libc.so.6", RTLD_LAZY);
    if (NULL == handle) {
        fprintf(stderr, "Error in `dlopen`: %s\n", dlerror());
        return;
    }
    real_malloc = dlsym(handle, "malloc");
    if (NULL == real_malloc) {
        fprintf(stderr, "Error in `dlsym`: %s\n", dlerror());
        return;
    }
}

I compile this with:

gcc -shared -fPIC -o mtrace.so mtrace.c

And then when I try to execute ls:

$ LD_PRELOAD=./mtrace.so ls
malloc(352) = Segmentation fault

Now, I suspect that dlopen needs malloc, and as I am redefining it within the shared library, it uses that version with the still unassigned real_malloc.

The question is…how do I make it work?

P.S. sorry for the paucity in tags, I couldn’t find appropriate tags, and I still don’t have enough reputation to create new ones.

Answer 1

I always do it this way:

#define _GNU_SOURCE

#include <stdio.h>
#include <dlfcn.h>

static void* (*real_malloc)(size_t)=NULL;

static void mtrace_init(void)
{
    real_malloc = dlsym(RTLD_NEXT, "malloc");
    if (NULL == real_malloc) {
        fprintf(stderr, "Error in `dlsym`: %s\n", dlerror());
    }
}

void *malloc(size_t size)
{
    if(real_malloc==NULL) {
        mtrace_init();
    }

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

Don’t use constructors, just initialize at first call to malloc. Use RTLD_NEXT to avoid dlopen. You can also try malloc hooks. Be aware that all those are GNU extensions, and probably wont work elsewhere.