Function hijacking in c

client.c

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

int main() {
        int *arr = malloc(sizeof(int)*10);
        int i = 0;
        for(; i < 10; ++i) {
                arr[i] = i;
        }
        for(i = 0; i < 10; ++i) {
                printf("%d \n", arr[i]);
        }
        free(arr);
        return 0;
}

hijack.c

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

void *memory[1000];

void *curr_head = memory;

void * cust_malloc(size_t size) {
        printf("malloc called\n");
        void *head_to_send = curr_head;
        curr_head+=size;
        return head_to_send;
}

void cust_free(void *ptr) {
        printf("free called\n");
}

Makefile

CHECK := $(shell which clang)

ifeq ($(CHECK),)
$(warning no clang found, consider apt-get install clang, using gcc now)
CC = gcc
else
$(info using clang over gcc)
CC = clang
endif

CFLAGS=-Dmalloc=cust_malloc -Dfree=cust_free

client: client.o hijack.o
        $(CC) client.o hijack.o -o client

client.o: client.c
        $(CC) $(CFLAGS) -c client.c

hijack.o: hijack.c
        $(CC) -c hijack.c

clean:
        rm -rf *.o

val: client
        valgrind --leak-check=full --show-leak-kinds=all --track-origins=yes --verbose ./client