#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <dlfcn.h>

#ifdef INTERCEPT_PRELOAD
#define load(name) \
    if (((__real_ ## name) = dlsym(RTLD_NEXT, #name)) == NULL) { \
        fprintf(stderr, "intercept: unable to load symbol '%s': %s", \
            #name, strerror(errno)); \
        return; \
    }
#define sym(name) name
#define func_def(ret, name) static ret (*__real_ ## name)
#else
#define sym(name) __wrap_ ## name
#define func_def(ret, name) extern ret __real_ ## name
#endif

func_def(void *, malloc)(size_t);

static int mode = 0;

static void fin(void) {
    if (mode > 0) fprintf(stderr, "intercept: stopped\n");
    mode = 0;
}

static void init(void) {
    if (mode) return;
    mode = -1;
#ifdef INTERCEPT_PRELOAD
    load(malloc);
#endif
    atexit(fin);
    fprintf(stderr, "intercept: intercepting function calls\n");
    mode = 1;
}

void *sym(malloc)(size_t size) {
    init();
    // before call to malloc
    void *ret = __real_malloc(size);
    // after call to malloc
    return ret;
}