Warning C4013: undefined; assuming extern returning int - c

Question

I got another warning which is
C4047: 'initializing' : 'char *' differs in levels of indirection from 'int'.
Why do I get those warnings? I have read on MSDN that it generated if I wrote a different function name to that I wrote in the header file or it should be extern function.
Could someone shed some light on it?
My relevant code is:
GetSignature.c
#include "Parse_Database.h"
#include "Alloc_Mem.h"
#include "Get_Signature.h"
int Get_Signature(const char *filename) {
char *database = AllocMem(filename);
int error=0;
//ParseDatabase(database);
return 1;
}
AllocMem
#include "Alloc_Mem.h"
char *AllocMem(const char *filename) {
FILE *fp = NULL;
int ch = EOF;
char *buf = NULL, *tmp = NULL;
unsigned int size=0, index=0;
/*
Mallicous use of fopen(anyone could replace filename with a malicous modified file)
*/
fp = fopen(filename, "r");
if(fp == NULL) {
fprintf(stderr, "Can't open the signature database");
exit(1);
}
while(ch) {
fread(&ch, 1, 1, fp);
if(ch == EOF) {
ch = 0;
}
if(size <= index) {
size+=chunk;
tmp = realloc(buf, size);
if(!tmp) {
free(buf);
buf = NULL;
break;
}
buf = tmp;
}
buf[index++] = ch;
}
fclose(fp);
return buf;
}
Alloc_Mem.h
#ifndef HEADERFILE_H
#define HEADERFILE_H
#ifdef _MSC_VER
#define _CRT_SECURE_NO_WARNINGS
#endif
#define chunk 255
#include <stdio.h>
#include <stdlib.h>
char *AllocMem(const char *);
#endif

Answer 1

I guess this shall resolve your problem: replace these lines:
#ifndef HEADERFILE_H
#define HEADERFILE_H
with
#ifndef _ALLOC_MEM_H
#define _ALLOC_MEM_H
Reason: header guards shall be unique for each header file, or there would be corruption when including multiple header files. (And I guess you're using the same name in all your header files.)
EDITED: Also in other of your headers (Parse_Database.h and Get_Signature.h) you shall replace the header guards with different names.

Answer 2

It would be helpful to know the exact line where the error is happening. I am assuming it is
char *database = AllocMem(filename);
in GetSignature.c.
Generally, this error comes about because the C compiler does not have the declaration of a function when it is called (what function that is depends upon the line mentioned in the error message). When this occurs, the C compiler assumes that the function returns int.
Do you have the include guard:
#ifndef HEADERFILE_H
#define HEADERFILE_H
In Parse_Database.h as well? If so, it is blocking the declaration of AllocMem from being included because HEADERFILE_H is already defined.
The usual custom is to name the defined macro the same as the header file. For example:
#ifndef ALLOC_MEM_H
#define ALLOC_MEM_H