hook.cpp

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <mutex>
#include <dlfcn.h>
#include <unistd.h>

extern "C" {

namespace {
    std::once_flag init_flag;

    using malloc_t  = void*(*)(std::size_t);
    using free_t    = void(*)(void*);
    using calloc_t  = void*(*)(std::size_t, std::size_t);
    using realloc_t = void*(*)(void*, std::size_t);

    malloc_t  native_malloc  = nullptr;
    free_t    native_free    = nullptr;
    calloc_t  native_calloc  = nullptr;
    realloc_t native_realloc = nullptr;

    void init_hooks() noexcept {
        native_malloc  = reinterpret_cast<malloc_t>(dlsym(RTLD_NEXT, "malloc"));
        native_free    = reinterpret_cast<free_t>(dlsym(RTLD_NEXT, "free"));
        native_calloc  = reinterpret_cast<calloc_t>(dlsym(RTLD_NEXT, "calloc"));
        native_realloc = reinterpret_cast<realloc_t>(dlsym(RTLD_NEXT, "realloc"));
    }

    inline void log(const char* msg) noexcept {
        write(STDERR_FILENO, msg, std::strlen(msg));
    }
}

void* malloc(std::size_t size) {
    std::call_once(init_flag, init_hooks);
    void* ptr = native_malloc(size);

    char buf[128];
    int len = std::snprintf(buf, sizeof(buf), "malloc(%zu) = %p\n", size, ptr);
    write(STDERR_FILENO, buf, len > 0 ? len : 0);

    return ptr;
}

void free(void* ptr) {
    std::call_once(init_flag, init_hooks);

    char buf[128];
    int len = std::snprintf(buf, sizeof(buf), "free(%p)\n", ptr);
    write(STDERR_FILENO, buf, len > 0 ? len : 0);

    native_free(ptr);
}

void* calloc(std::size_t num, std::size_t size) {
    std::call_once(init_flag, init_hooks);
    void* ptr = native_calloc(num, size);

    char buf[128];
    int len = std::snprintf(buf, sizeof(buf), "calloc(%zu, %zu) = %p\n", num, size, ptr);
    write(STDERR_FILENO, buf, len > 0 ? len : 0);

    return ptr;
}

void* realloc(void* ptr, std::size_t size) {
    std::call_once(init_flag, init_hooks);
    void* new_ptr = native_realloc(ptr, size);

    char buf[128];
    int len = std::snprintf(buf, sizeof(buf), "realloc(%p, %zu) = %p\n", ptr, size, new_ptr);
    write(STDERR_FILENO, buf, len > 0 ? len : 0);

    return new_ptr;
}

} // extern "C"

g++ -shared -fPIC -ldl lib.cpp -o lib.so
LD_PRELOAD=./lib.so ls