C: unsigned long int not behaving as expected

Question

This code is not behaving as expected. It simply tries to set bit 31 in an unsigned long int.

int main() {
    printf("sizeof(unsigned long int) is %ld bytes\n", sizeof(unsigned long int));
    unsigned long int v = 1 << 30;
    printf("v is (%lx)\n", v);
    v = 1 << 31;
    printf("v is (%lx)\n", v);
}

Here is the output:

sizeof(unsigned long int) is 8 bytes
v is (40000000)
v is (ffffffff80000000)

Can anyone explain this? Maybe a problem with the printf formatting?

Yep. That was it. Thanks. I never would have figured that out. — JB_User, Jun 28 '19 at 14:34

Eric Postpischil · Accepted Answer · 2019-06-28T14:38:13.263

3

In v = 1 << 31;, 1 is not an unsigned long int. It is an int. Shifting it by 31 bits overflows the int type (in your C implementation).

To get an unsigned long int with a 1 in bit 31, you should shift an unsigned long int by 31 bits: v = (unsigned long int) 1 << 31; or v = 1ul << 31.

edited Jun 28 '19 at 14:38

answered Jun 28 '19 at 14:34

Eric Postpischil

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Thanks. That solves it. – JB_User Jun 28 '19 at 14:36

C: unsigned long int not behaving as expected

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