Segmentation fault in reading a .dat file - c

Question

Right now, I'm working on an example code that I wish to integrate into my program later. What essentially I'm trying to do is read a .dat file byte by byte and interpret the data (ie. interpret boot sector to output the sector size, reserved sectors etc.)
To do this, I am reading the data byte by byte and, using the descriptions in fat12 of https://www.win.tue.nl/~aeb/linux/fs/fat/fat-1.html#ss1.3 , I translate the data into the information that I want. Right now, I can pull individual bytes from the file (Is it right to assume that the data pulled is in hex?). However, I need two bytes to have something meaningful. So, I need to combine two bytes into one, convert the hex data into decimal and output the information. Unfortunately, right now, I'm getting a seg fault and for the life of me, I can't figure out what's wrong. Thanks in advance!
int main (int argc, char **argv){
FILE *fp ,*fptest;
long lSize;
char *buffer;
//Open file
fptest= open("fat_volume.dat", "rb");
//Read file into buffer
fread(buffer,1,512,fptest);
//Parse the boot sector
char tmpA, tmpB;
tmpA = buffer[10]; //First byte
tmpB = buffer[11]; //Second byte
//Combine the two bytes into one
char combinedBytes[3];
strcpy (combinedBytes, tmpA);
strcat (combinedBytes, tmpB);
//Hex to decimal converter
long int li1;
li1 = strtol (combinedBytes,NULL,16);
printf ("The sector size is: %ld.\n", li1);
return 0;
}

Answer 1

You must allocate buffer; e.g.
char buffer[512];
or
char *buffer = malloc(512);
EDIT:
The string operations
strcpy (combinedBytes, tmpA);
strcat (combinedBytes, tmpB);
do not make sense either and access/copy too much data (the compiler will warn you about this!).
I suggest do read values as
unsigned char tmpA = buffer[10];
unsigned char tmpB = buffer[11];
unsigned int tmp = (tmpA << 8) | (tmpB << 0); /* or revert in in case of
little-endian */
To make things more efficient, I would write it as
struct fat_header {
uint8_t pad0[10];
uint16_t my_val;
uint8_t pad1[500];
} __attribute__((__packed__)); /* this is not portable and for gcc! */
...
struct fat_header hdr;
fread(&hdr, 1, sizeof hdr, f);
uint16_t val = be16toh(hdr.my_val); /* or le16toh() in case of le */

Answer 2

You are reading into a buffer you never allocated memory for.
What you're trying now is to read from some junk value in memory, who knows, which almost always leads to a segmentation fault.
Use:
char *buffer = malloc(512 * sizeof(char)); // this allocates 512 times the size of a single char of memory
If you don't specify the number inside malloc to be of a specific size (e.g. malloc(512) the number is in bytes, though I think it's better to always include it.
This specific error is called dereferencing a null pointer
EDIT:
I've managed to run this code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main (int argc, char **argv)
{
FILE *fp ,*fptest;
long lSize;
char *buffer;
//Open file
fptest = fopen("input.txt", "rb");
if (fptest == NULL)
{
printf("Error occurred when opening file");
return 1;
}
buffer = malloc(sizeof(char) * 512);
//Read file into buffer
fread(buffer,1,512,fptest);
//Parse the boot sector
char tmpA, tmpB;
tmpA = buffer[10]; //First byte
tmpB = buffer[11]; //Second byte
//Combine the two bytes into one
char combinedBytes[3];
strcpy (combinedBytes, &tmpA);
strcat (combinedBytes, &tmpB);
//Hex to decimal converter
long int li1;
li1 = strtol (combinedBytes,NULL,16);
printf ("The sector size is: %ld.\n", li1);
return 0;
}
You also used a function open() which must be fopen(), and you need to pass the address of tmpA and tmpB to strcpy and strcat.
This is why I don't understand why your compiler doesn't give any errors or warnings..