How to write a shared library that:
- wraps a system function (say malloc),
- internally uses the real version of wrapped functions (e.g., malloc defined in libc), AND
- can be linked from client code without giving --wrap=malloc every time it is used?
I learned from several posts that I can wrap system functions with --wrap option of ld; something like this:
void * __wrap_malloc(size_t sz) {
return __real_malloc(sz);
}
and get a shared library with:
gcc -O0 -g -Wl,--wrap=malloc -shared -fPIC m.c -o libwrapmalloc.so
But when a client code links this library, it needs to pass --wrap=malloc every time. I want to hide this from the client code, as the library I am working on actually wraps tons of system functions.
An approach I was using was to define malloc and find the real malloc in libc using dlopen and dlsym. This was nearly what I needed, but just as someone posted before Function interposition in Linux without dlsym, dlsym and dlopen internally call mem-alloc functions (calloc, as I witnessed it) so we cannot easily override calloc/malloc functions with this approach.
I recently learned --wrap and thought it was neat, but I just do not want to ask clients to give tons of --wrap=xxxx arguments every time they get executables...
I want to have a situation in which malloc in the client code calls malloc defined in my shared library whereas malloc in my shared library calls malloc in libc.
If this is impossible, I would like to reduce the burden of the clients to give lots of --wrap=... arguments correctly.
