C: switch case with hex value ignore the 0 in the front

Issue

I init a uint32 variable by shifting two uint8 variables.

uint32_t test = ((first_uint8 << 16) | second_uint8);

In my test, the value of test should be 0x00170001 (0x17<<16|0x01)

Then it runs into a switch case

switch(test) {
   case 0x00170001:
        //do sth
        break;
   default:
        //printf invalid
        break;
}

It should go to the first case, however, it always run into default.

Then I printf("%x", test), the result is 0x170001, which is equal to 0x00170001.

At last I try to modify it like this:

switch(test) {
   case 0x170001:
        //do sth
        break;
   default:
        //printf invalid
        break;
}

Then it works well.

So I'm curious about the result.

1. for a uint32_t variable, why 0x170001 does not equal to 0x00170001?
2. if it is caused by I didn't memset test by 0, then test should also not be equal to 0x170001, it should be 0x11170001 or something with a garbage first byte.?
3. is it caused by the compiler ignores the 0 in the front of hex value? I'm using Android NDK to compile my c code.

Solution

1. for a uint32_t variable, why 0x170001 does not equal to 0x00170001?

It does. 0x170001, 0x0170001 and 0x00170001 are all equal.

2.if it is caused by I didn't memset test by 0, then test should also not be equal to 0x170001, it should be 0x11170001 or something with a garbage first byte.?

It is not caused by a missing memset. Any assignment test = X will set all bits in test.

3.is it caused by the compiler ignores the 0 in the front of hex value? I'm using Android NDK to compile my c code.

That could explain it but I doubt it. I think it is more likely that your being tricked by something else. Perhaps you are not actually running the code, you think. If it really turns out that it makes a difference whether you write 0x170001 or 0x00170001, you should report it as a compiler bug - but be really sure before doing that.

See here for a working example compiled with gcc : http://ideone.com/UR566Q

Answered By - 4386427