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 ?


1 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.

  • i almost forgot about "w" erasing all file's contents , one last thing , when i edited the code fread worked fine but i always get 0 when i try to print any element of "block" pointer , unlike fgetc i get number 99 - and if used it again i get 115 - unlike block[0] and block[1] Jun 11, 2019 at 17:26
  • ok i tried other block and "block" elements have values now but i still can't understand why fgetc has get values in first block although it's all zeros, i'm sorry for my dumb question Jun 11, 2019 at 18:08
  • Sorry, I can't follow you. I don't know how you changed your code, I don't know which values you refer to. Maybe make a new question, since it's another problem. Also, both fread and fgetc move the position within the file.
    – Blauelf
    Jun 11, 2019 at 18:43
  • ok i will do that and thank you for your help really appreciate it Jun 11, 2019 at 18:51

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