Why is 017 == 15 in C?

问题

I've done a little bit of reading on endianness and its role in C, but nothing has really managed to clarify this for me. I'm just starting out with C and I saw this example:

#include <stdio.h>

int main(void) {
    int x = 017;
    int y = 12;
    int diff = x - y;
    printf("diff is %d\n", diff);
    return 0;
}

and it asks what will print. I compiled and ran the example and got that diff is 3, so x is 15. I sort of see why this is, but would really appreciate if somebody really clarified it for me.

[1] I've looked for similar questions but haven't found any that explained the issue thoroughly. If someone could link me to one that would be good also.

回答1:

Prefixing a number with 0 will tell the compiler to mark it as a number in octal (base 8)

Just like prefixing it with 0x will tell it to use hex (base 16)

For example:

int x = 05;  // 5 in octal
int y = 5;   // 5 in decimal
int z = 0x5; // 5 in hex

回答2:

017 is an octal constant if we look at the C99 draft standard section 6.4.4.1 Integer constants the grammar for octal constant is as follows:

octal-constant:
  0
  octal-constant octal-digit
octal-digit: one of
   0 1 2 3 4 5 6 7

So any integer constant that starts 0 is in octal(base 8), this includes 0 itself.

来源：https://stackoverflow.com/questions/19485548/why-is-017-15-in-c