Why does C use 5 bytes to write/fwrite the number 10, but 4 bytes to write 9 or 11?

Question

GCC on Windows

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

struct test {
    int n1;
    int n2;
};

int main() {
    FILE *f;
    
    f = fopen("test.dat", "w");
    struct test test1 = {10, 10};
    fwrite(&test1, sizeof(struct test), 1, f);
    fclose(f);
    
    struct test test2;
    f = fopen("test.dat", "r");
    while(fread(&test2, sizeof(struct test), 1, f))
        printf("n1=%d n2=%d\n", test2.n1, test2.n2);
    fclose(f);
    
    return 0;
}

If I set test1 to 10,10 then fwrite will write 10 bytes to file: 0D 0A 00 00 00 0D 0A 00 00 00

(each 4-byte int will be padded with a 0D carriage return character before it)

If I set test1 to 11,11 then fwrite will write 8 bytes to file: 0B 00 00 00 0B 00 00 00

(as I would expect)

If I set test1 to 9,9 then fwrite will write 8 bytes to file: 09 00 00 00 09 00 00 00

(as I would expect)

If I set test1 to 9,10 then fwrite will write 9 bytes to file: 09 00 00 00 0D 0A 00 00 00

The 9 gets 4 bytes as expected, but the 10 gets padded with an extra 0D byte, resulting in 5 bytes. What is so special about the number 10 that requires this padding? And why do both smaller and larger numbers (8, 9, 11, 12, 13, 14, etc) not get padded? I thought maybe GCC is confusing the number 10 for a new-line character (a new-line is 10 is acsii), but this does not explain how fread is able to get the number 10 back out correctly.

And how do I write a struct to file without getting this extra padding on the number 10?

Answer 1

You opened the file in text mode, so on Windows every '\n' character gets a carriage return prepended in front of it.

You should write (and read) binary data in binary mode instead (fopen(..., "wb")) -- this will be much faster, and avoids surprises (and also requires only 8 bytes, which is what sizeof(struct test) is).

What is so special about the number 10 that requires this padding?

The number 10 just happens to be the ASCII code for a newline ('\n') character.