Question

I am trying to read a file “file.raw” and 4 bytes at a time to an array and check if it has the particular 4 byte signature I am looking for. I am having trouble with this. The value of result I get is 0, instead of 4 when using fread.

#include<stdint.h> 
#include<stdio.h>
#include<stdlib.h>
#include<string.h>

typedef uint8_t BYTE; 

int main(void)
{
    size_t result;
    FILE *inptr = fopen("file.raw","r+");
    //Check if file can be opened.
    if (inptr == NULL)
    {
        printf("File Open Error\n");
        return -1;
    }
    long int x = 0;

    while(!feof(inptr))
    {
        // Make space for reading in to an array
        BYTE *array = (BYTE *) malloc(10);
        if(array == NULL)
        {
            printf("Array Initialization Error\n");
            return -1;
        }

        result = fread(array,1,4,inptr);
        //Exit if file not read. ** This is where I can't get past.
        if(result != 4)
        {
            printf("File Read Error\n");
            printf("%d\n",result);
            free(array);
            fclose(inptr);
            return -1;
        }

        //Compare strings

        if(memcmp(array,"0xffd8ffe0",4)==0)
        {
            printf("File Start found\n");
            printf("Exiting...\n");
            printf("%p\n",inptr);
            free(array);
            fclose(inptr);
            return 0;
        }
        x++;
        free(array);
    }

    printf("%p\n",inptr);
    printf("%ld\n",x);
    fclose(inptr);
    return 0;
}

Answer 1

My guess is that it doesn’t fail on the first iteration of the while loop, but rather keeps reading the file until you reach end of the file, at which point fread() returns 0 and your program exits.

The reason it’s not finding the signature is this:

memcmp(array,"0xffd8ffe0",4)==0

That memcmp() call is almost certainly not what you want (it’s looking for the sequence of ASCII characters '0', 'x', 'f' and 'f').

PS As noted by @Mat in the comments, for maximum portability you should open the file in binary mode ("r+b" instead of "r+").