problem with recover.c using malloc and fread

Question

i tried to write a function that sample 4 bytes from card.raw file but i doesn't work because fread doesn't put any data into block buffer so i wrote this simple code and i found out that fgetc works while fread doesn't

FILE *ptr = fopen("card.raw","w+");
unsigned char *block = malloc(sizeof(unsigned char) * 512);
if (block == NULL)
{
    fprintf(stderr, "couldn't allocate memory\n");
    return 5;
}

fread(block, 1, 512, ptr);
unsigned char c = fgetc(ptr);
printf("*block = %i\nc = %i\n", *block ,c);

that's the output of this program

*block = 0
c = 255

and the same thing happens when i try to use array instead of using malloc any idea why is this happening ?

Answer 1

You just opened your file for writing (and reading). This erased your card.raw's content.

The 0 you find in block[0] is the initial value of your allocated memory, which by coincidence is 0.

The 255 you find in c is actually a -1 returned by fgetc (and means EOF - End Of File). When converting the int-type -1 to an unsigned char, only the least significant byte is kept, which in this case corresponds to value 255.

Use r, not w+ for reading (and replace card.raw with a working copy).

Also, in this problem, there is little use for fgetc. You are meant to always read 512 bytes at a time.