Binary file Reading, add extra characters in c?

Question

I try to read binary(executeable) files, butsome problem as it prints extra character(0xffffff) after every few bytes. But this code work perfectly for text file(Non-binary). The code is:

int main(int argc, char **argv) {
  uint64_t i=0;
  FILE * fp=fopen(argv[1],"rb");
  uint8_t a;
  long int size=0;
  char *msg;
  size=findSize(argv[1]);
  msg=(char *)malloc(size+1);

  while((a=fgetc(fp))!=EOF)
  {
    printf("%02x",a);
      msg[i]=(char)a;
    i++;
    if(i==size)
      break;
  }
  i=0;
  for(i=0;i<size;i++)
  {
    printf("%x",msg[i]);

  }
    return 0;
}

When i try to print the value(s) of a, It works perfectly, while printing msg[i], it prints extra bytes i,e:

Orginal bytes: 0xa31c4b1ab66b900

Output with extra bytes: 0xa31c4ffffffb1ffffffafffffffb66ffffffb900

`fgetc()` returns an int value. So, `a` should be defined as an int. — wildplasser, Aug 07 '19 at 10:27
... which is especially important when reading binary data, to distinguish `EOF` from `0xFF`. — Weather Vane, Aug 07 '19 at 10:28
Default parameter promotion converts a `char` to an `int` when passing to `printf`. This include sign extensions for negative values. — Gerhardh, Aug 07 '19 at 10:30
and the `fgetc` thing **is** relevant when you're working with binary files — Antti Haapala -- Слава Україні, Aug 07 '19 at 10:33
also `long` might not be sufficiently large to contain every file size, but it perhaps wouldn't be a problem. — Antti Haapala -- Слава Україні, Aug 07 '19 at 10:34

score 2 · Accepted Answer · answered Aug 07 '19 at 10:29

char *msg;

Your msg variable has datatype char, which is probably signed on your platform. That means, it represents values from -128 to +127.

printf("%x",msg[i]);

Any arguments to printf() that have an integer datatype smaller than int will be expanded to int. That conversion will keep the arithmetic value, so -128 which is 0x80 in char, will expand to 0xFFFFFF80 for int.

To avoid this, you need to store or cast the values to unsigned char first. The uint8_t type is same as unsigned char and easier to read, so:

printf("%x", (uint8_t)msg[i]);

That way the numeric value will range from 0 to 255, like you would expect, and print as 0x00 to 0xFF even after the implicit conversion to int.

Binary file Reading, add extra characters in c?

1 Answers1