Question

I made my own implementation of _init , malloc , free ( and others ).

Inside these functions I use the dlfcn.h (dlopen , dlsym etc) library to call the actual standard versions. I put then in a single file and compile them as a shared library ( memory.so ). When I wish to run an executable and make it call my versions of these functions I simply set LD_PRELOAD=memory.so .

The problem is that I have a number of other modules which memory.c depends on. These include a file containing functions to scan elf files ( symbols.c ) and my own implementation of a hash table ( hashtable.c ) which I use to keep track of memory leaks among others.

My question is if there is a way to separately compile hashtable.c & symbols.c so any malloc references are resolved with the standard library and not with the ones included on memory.c. I could of course use the dlfcn.h libraries on everything that memory.c depends on but I would prefer it if there was a way to avoid that.

I still haven’t completely figured out how linking works so any help would be appreciated.

Thank you

Answer 1

If you are working with glibc you can use alternative non-overriden function names:

[max@truth ~]$ nm --defined-only --dynamic /lib64/libc.so.6 | egrep "malloc\b"
0000003e56079540 T __libc_malloc
0000003e56079540 T malloc

Note the same function address in the above. In other words, malloc() function is given two names, so that the original malloc() version is available under __libc_malloc() name in case malloc() has been interposed.

A quick grep on glibc sources reveals the only caller of __libc_malloc() is mcheck. These function aliases are a glibc implementation detail and there is no header for them. malloc/mcheck.c declares the internal functions as below:

extern __typeof (malloc) __libc_malloc;
extern __typeof (free) __libc_free;
extern __typeof (realloc) __libc_realloc;

Other C libraries may have differently named aliases, so using dlsym() to get the original malloc() address is more portable.