Reading Binary PE Files in C - c

Question

I'm attempting to read a PE file in C. After reading the relevant Microsoft documentation and other articles on OSDev and on SO, I'm still confused as to how to deal with offsets in the file.
The MS-DOS stub at the head of the file contains a flag at 0x3c which provides the offset to the start of the actual PE header (the PE signature, which is literally a 4-byte "PE\0\0").
What I've tried to do is to use pointer arithmetic to interpret the address given in the PE file as exactly that - an address, but I'm afraid I don't quite know how to do it.
Also, I am aware this is possible via the Win32 API; however, I want to learn from this and learn how to do it myself.
Thank you very much for any help in advance.
#include <stdio.h>
#include <stdlib.h>
#define PE_SIG_OFFSET_LOCATION 0x3c
int main(void)
{
const char* filepath = "sample.exe";
FILE* file = fopen(filepath, "rb");
int i, n = 0;
if(!file)
{
perror("Failed to open file");
return EXIT_FAILURE;
}
fpos_t start = fgetpos(file, &start);
int c;
while((c = fgetc(file)) != EOF)
{
n++;
// putchar(c);
}
if(ferror(file))
{
puts("I/O error when reading.");
}
else if(feof(file))
{
fsetpos(file, &start);
int c = 0;
char* contents = (char*)malloc(n * sizeof(char));
for(i=0;i<=n;i++)
{
if(c == EOF)
{
break;
}
c = fgetc(file);
contents[i] = c;
}
fclose(file);
i = 0;
char offset[4 * sizeof(char)] = {contents[PE_SIG_OFFSET_LOCATION * sizeof(char)], contents[(PE_SIG_OFFSET_LOCATION * sizeof(char))+ (1 * sizeof(char))], contents[(PE_SIG_OFFSET_LOCATION * sizeof(char))+ (2 * sizeof(char))], contents[(PE_SIG_OFFSET_LOCATION * sizeof(char))+ (3 * sizeof(char))]};
char* sig = &contents[PE_SIG_OFFSET_LOCATION * sizeof(char)];
printf("sig = %c \n", sig);
// somehow interpret the character the sig currently points to as a hex memory and get sig to point to that (i.e. the start of the PE header)
free(contents);
return EXIT_SUCCESS;
}
}
EDIT: After technosaurus advised I union a packed struct representing the DOS header with a char array of the actual contents, the code still doesn't appear to be functioning correctly (it reads and prints the file okay, but right at the end when it needs to print the signature it prints garbage instead of MZ).
#include <stdio.h>
#include <stdlib.h>
#define PE_SIG_OFFSET_LOCATION 0x3c
int main(void)
{
const char* filepath = "sample.exe";
FILE* file = fopen(filepath, "rb");
int i, n = 0;
if(!file)
{
perror("Failed to open file");
return EXIT_FAILURE;
}
fpos_t start = fgetpos(file, &start);
int c;
while((c = fgetc(file)) != EOF)
{
n++;
// putchar(c);
}
if(ferror(file))
{
puts("I/O error when reading.");
}
else if(feof(file))
{
fsetpos(file, &start);
int c = 0;
typedef struct DOS_Header
{
char signature[2];
short lastsize;
short nblocks;
short nreloc;
short hdrsize;
short minalloc;
short maxalloc;
void *ss;
void *sp;
short checksum;
void *ip;
void *cs;
short relocpos;
short noverlay;
short reserved1[4];
short oem_id;
short oem_info;
short reserved2[10];
long e_lfanew;
}DOS_Header;
union
{
DOS_Header header;
char* contents;
}u;
u.contents = (char*)malloc(n * sizeof(char));
for(i=0;i<=n;i++)
{
if(c == EOF)
{
break;
}
c = fgetc(file);
u.contents[i] = c;
}
fclose(file);
i = 0;
for(i=0;i<=n;i++)
{
printf("%c", u.contents[i]);
}
printf("\n DOS sig: %c%c \n", u.header.signature[0], u.header.signature[1]);
free(u.contents);
return EXIT_SUCCESS;
}
}