2024-06-01

Avoid char with getc

Using a char to represent the output of getc (get char) might sound like the right way to do it, but it is not.

Let's show an example. First, create a binary file with the following content:

10101010 00000000 11111111 01010101 00001111 11000011
# or in hex:
AA00FF550FC3

This can be done in bash:

echo "AA00FF550FC3" | xxd -r -p > /tmp/file.bin

You can confirm the content by using xxd:

xxd -b /tmp/file.bin

And a dumb code that will:

• Read one byte using getc.
• Print the byte.

#include <stdio.h>

int main() {
  FILE* fp = fopen("/tmp/file.bin", "rb");

  char byte;
  while ((byte = getc(fp)) != EOF) {
    printf("byte: %x\n", byte);
  };

  fclose(fp);
}

The expected result should have been:

byte: aa
byte: 00
byte: ff
byte: 55
byte: 0f
byte: c3

But instead you'll get:

byte: aa
byte: 00

This happens because the byte 11111111 equals EOF which is a macro that returns -1. This evaluation breaks the while loop.

Instead, one should use either int or uint8_t to get the correct result.

Note: The same is valid for ungetc!