I'm trying to simulate the malloc function in C by using mmap and having 2 linked list to act as memory blocks. The program works by reserving a region of memory and then keeps track of the free and allocated memory address with size in the linked lists. I have tested the program with normal cases like (free memory > needed to be allocate, free memory < needed to be allocate, free memory == needed to be allocate) but couldn't think of special cases, is there anyway cases that the program fails ?

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>

struct block {
void *addr; /*start address of memory for this block */
int size;
struct block *next;
};




void *mem;
struct block *freelist;
struct block *allocated_list;



void *smalloc(unsigned int nbytes) {
struct block *cur = freelist;

 while(cur!=NULL){
        
        if (cur ->size >= nbytes){
            struct block *tempBlock;
            tempBlock= malloc(sizeof(struct block));
            tempBlock->addr = cur ->addr;
            tempBlock->size = nbytes;
            tempBlock->next = NULL;


            cur->addr = cur->addr + nbytes;
            cur ->size = cur ->size - nbytes;

                if(allocated_list == NULL){
                    allocated_list = tempBlock;
                }
                else{
                    cur = allocated_list;
                    while(cur->next!=NULL){
                        cur =cur->next;
                    }
                    cur->next=tempBlock;
                }
            

            return (tempBlock->addr);

        }
        cur = cur->next;

}

return NULL;
}


int sfree(void *addr) {
struct block *cur = allocated_list;
struct block *tempBlock;
while(cur!=NULL){
        if (cur ->next->addr == addr){
            tempBlock=cur->next;        //disjoint the matched block from the allocated_list
            cur->next = cur->next->next;
            tempBlock->next = NULL;     // set the matched block pointing to null
            
            cur = freelist;
            while(cur->next!=NULL){
                cur =cur->next;
                }
            cur->next=tempBlock;
            
            return(0);
        }
        cur= cur->next;

}
return -1;
}


void mem_init(int size) {
mem = mmap(NULL, size,  PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1, 0);
if(mem == MAP_FAILED) {
     perror("mmap");
     exit(1);
}
else {
    freelist = malloc(sizeof(struct block));
    freelist ->size = size;
    freelist ->next = NULL;
    freelist ->addr = mem;


    allocated_list= NULL;

}

/* NOTE: this function is incomplete */

}

void mem_clean(){
struct block *cur = freelist;
struct block *tempBlock;
while(cur!=NULL){
    tempBlock = cur->next;
    munmap(cur->addr,cur->size);
    free(cur);
    cur = tempBlock;
}
cur = allocated_list;
while(cur!=NULL){
    tempBlock = cur->next;
    munmap(cur->addr,cur->size);
    free(cur);
    cur = tempBlock;
}

}