dynamic library interposition

libint.c

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

void* malloc(size_t size)
{
        static void* (*rmalloc)(size_t) = NULL;
        void* p = NULL;

        // resolve next malloc
        if(!rmalloc) rmalloc = dlsym(RTLD_NEXT, "malloc");

        // do actual malloc
        p = rmalloc(size);

        // show statistic
        fprintf(stderr, "[MEM | malloc] Allocated: %lu bytes\n", size);

        return p;
}

Makefile

libint.so: libint.c
        clang -shared -fPIC libint.c -o libint.so

str: str.c
        clang -o str str.c

clean:
        rm -f str libint.so

run: str libint.so
        @echo "========= direct exec ========="
        @./str
        @echo
        @echo "========= With lib interposition ========="
        @LD_PRELOAD=./libint.so ./str

str.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define STR_LEN 128

int main(int argc, const char *argv[])
{
        char *c;
        char *str1 = "Hello ";
        char *str2 = "World";

        //allocate an empty string
        c = malloc(STR_LEN * sizeof(char));
        c[0] = 0x0;

        //and concatenate str{1,2}
        strcat(c, str1);
        strcat(c, str2);

        printf("New str: %s\n", c);


        return 0;
}