Question

I have succesfully using dyld -macosx- to interpose standard C functions to a third party application, getting important information about the workarounds it does. But what I really need is to replace a certain function of a certain class.

The function I want to override is QString::append(…, …, …), so each time a string is appended to another -the whole application uses qstring-, i find out.

Is there a way? Here’s the code I already have.

// libinterposers.c
#include <stdio.h>
#include <stdint.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdarg.h>
#include <dlfcn.h>
#include <stdlib.h>

typedef struct interpose_s {
    void *new_func;
    void *orig_func;
} interpose_t;

int my_open(const char *, int, mode_t);
int my_close(int);
void* my_malloc(size_t);

static const interpose_t interposers[] \
    __attribute__ ((section("__DATA, __interpose"))) = {
        { (void *)my_open,  (void *)open  },
        { (void *)my_close, (void *)close },
        { (void *)my_malloc, (void *)malloc },
    };

int
my_open(const char *path, int flags, mode_t mode)
{
    int ret = open(path, flags, mode);
    printf("--> %d = open(%s, %x, %x)\n", ret, path, flags, mode);
    return ret;
}

int
my_close(int d)
{
    int ret = close(d);
    printf("--> %d = close(%d)\n", ret, d);
    return ret;
}

void*
my_malloc(size_t size)
{
    void *ret = malloc(size);
    //fprintf(stderr, "Reserva de memoria");
    return ret;
}

Thank you very much

Answer 1

C++ does name mangling. This means member function QString::mid() looks something like __ZNK7QString3midEii to the linker. Run the nm(1) command on the library you are interposing on to see the symbols.