c - custom memory allocator -

i wrote custom memory allocator. has 2 restrictions want remove works malloc/free.

1.) mem_free call requires cast unsigned char * input parameter. take pointer of type. how can done?

2.) memory allocator wrote allocates block of memory front of buffer , writes size. free function removes last block of allocated memory in buffer. order of malloc/free calls matter or not work. how can remove restriction?

want able this:

char* ptr1 = mem_alloc(10); char* ptr2 = mem_alloc(4);  mem_free(ptr1); mem_free(ptr2); 

have now:

char* ptr1 = mem_alloc(10); char* ptr2 = mem_alloc(4);  mem_free(ptr2); mem_free(ptr1); 

code:

unsigned char* mem_alloc(unsigned int size) {     unsigned int s;     if( (size + mem_header_size)  > (mem_max_size - mem_current_size_bytes) ) {         return null;     }     if(is_big_endian() == 0) {         s = (mem_buff[3] << 24) + (mem_buff[2] << 16) + (mem_buff[1] << 8) + mem_buff[0];     } else {         s = (mem_buff[0] << 24) + (mem_buff[1] << 16) + (mem_buff[2] << 8) + mem_buff[3];     }     memcpy(mem_buff + mem_current_size_bytes, &size, sizeof(unsigned int));     unsigned char* result = mem_buff + (mem_current_size_bytes + mem_header_size);     mem_current_size_bytes += mem_header_size + size;     return result; }  void mem_free(unsigned char* ptr) {     unsigned int i,s;     for(i=0; i<mem_current_size_bytes; i++) {         if( (char*)ptr == (char*)(mem_buff + i) ) {             if(is_big_endian() == 0) {                 s = (*(ptr - 1) << 24) + (*(ptr - 2) << 16) + (*(ptr - 3) << 8) + *(ptr - 4);             } else {                 s = (*(ptr - 4) << 24) + (*(ptr - 3) << 16) + (*(ptr - 2) << 8) + *(ptr - 1);             }             mem_current_size_bytes-=s;             mem_current_size_bytes-=mem_header_size;             break;         }     } } 

1) use void* instead.
2) store map of addresses allocated blocks , seperate map of unallocated blocks. can block being freed in allocated map, remove , add block unallocated map (making sure merge free blocks either side of it). of course, can , lead memory fragmentation rather unavoidable really.