Compiling errors in Malloc

Question

I'm following this tutorial to write my own version of malloc and free. The full source code is here which I'm trying to run on my laptop.

I have 3 files:

mymalloc.h
mymalloc.c
memgrind.c // Where my main method is to test my malloc function

I try to compile it by executing the following on the command line:

gcc -c mymalloc.c
gcc -o memgrind.c memgrind

But I get the following errors:

warning: implicit declaration of function MyMalloc [-Wimplicit-function-declaration]
 int *p=(int)MyMalloc(100*sizeof(int));
             ^~~~~~~~
memgrind.c:5:8: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
 int *p=(int)MyMalloc(100*sizeof(int));

Am I compiling my files wrong? Or something wrong with the code? (The code I'm executing & compiling is no different to the source code provided in the link)

Thank you.

EDIT:

Now it is compiling correctly but I get this error:

/tmp/ccKyTOaY.o:(.data.rel.local+0x0): multiple definition of `freeList'
/tmp/cc0jpDeq.o:(.data.rel.local+0x0): first defined here
collect2: error: ld returned 1 exit status

But I'm only declaring it once inside of my mymalloc.h?

For reference, here is the code:

mymalloc.c

#include<stdio.h>
#include<stddef.h>
#include "mymalloc.h"


void initialize(){
 freeList->size=20000-sizeof(struct block);
 freeList->free=1;
 freeList->next=NULL;
}

void split(struct block *fitting_slot,size_t size){
 struct block *new=(void*)((void*)fitting_slot+size+sizeof(struct block));
 new->size=(fitting_slot->size)-size-sizeof(struct block);
 new->free=1;
 new->next=fitting_slot->next;
 fitting_slot->size=size;
 fitting_slot->free=0;
 fitting_slot->next=new;
}


void *MyMalloc(size_t noOfBytes){
 struct block *curr,*prev;
 void *result;
 if(!(freeList->size)){
  initialize();
  printf("Memory initialized\n");
 }
 curr=freeList;
 while((((curr->size)<noOfBytes)||((curr->free)==0))&&(curr->next!=NULL)){
  prev=curr;
  curr=curr->next;
  printf("One block checked\n");
 }
 if((curr->size)==noOfBytes){
  curr->free=0;
  result=(void*)(++curr);
  printf("Exact fitting block allocated\n");
  return result;
 }
 else if((curr->size)>(noOfBytes+sizeof(struct block))){
  split(curr,noOfBytes);
  result=(void*)(++curr);
  printf("Fitting block allocated with a split\n");
  return result;
 }
 else{
  result=NULL;
  printf("Sorry. No sufficient memory to allocate\n");
  return result;
 }
}

void merge(){
 struct block *curr,*prev;
 curr=freeList;
 while((curr->next)!=NULL){
  if((curr->free) && (curr->next->free)){
   curr->size+=(curr->next->size)+sizeof(struct block);
   curr->next=curr->next->next;
  }
  prev=curr;
  curr=curr->next;
 }
}

void MyFree(void* ptr){
 if(((void*)memory<=ptr)&&(ptr<=(void*)(memory+20000))){
  struct block* curr=ptr;
  --curr;
  curr->free=1;
  merge();
 }
 else printf("Please provide a valid pointer allocated by MyMalloc\n");
}

mymalloc.h

#include<stdio.h>
#include<stddef.h>

char memory[20000];

struct block{
 size_t size;
 int free;
 struct block *next; 
};

struct block *freeList=(void*)memory;

void initialize();
void split(struct block *fitting_slot,size_t size);
void *MyMalloc(size_t noOfBytes);
void merge();
void MyFree(void* ptr);

memgrind.c

#include<stdio.h>
#include "mymalloc.h"

int main(){
 
 int *p=(int*)MyMalloc(100*sizeof(int));
 char *q=(char*)MyMalloc(250*sizeof(char));
 int *r=(int*)MyMalloc(1000*sizeof(int));
 MyFree(p);
 char *w=(char*)MyMalloc(700);
 MyFree(r);
 int *k=(int*)MyMalloc(500*sizeof(int));
 printf("Allocation and deallocation is done successfully!");
}

Answer 1

the warining says that you try to put int into int * so try this

int *p=(int*)MyMalloc(100*sizeof(int));