c - warning: left shift count >= width of type

Question

I'm very new to dealing with bits and have got stuck on the following warning when compiling:

 7: warning: left shift count >= width of type

My line 7 looks like this

unsigned long int x = 1 << 32;

This would make sense if the size of long on my system was 32 bits. However, sizeof(long) returns 8 and CHAR_BIT is defined as 8 suggesting that long should be 8x8 = 64 bits long.

What am I missing here? Are sizeof and CHAR_BIT inaccurate or have I misunderstood something fundamental?

Answer 1

long may be a 64-bit type, but 1 is still an int. You need to make 1 a long int using the L suffix:

unsigned long x = 1UL << 32;

(You should also make it unsigned using the U suffix as I've shown, to avoid the issues of left shifting a signed integer. There's no problem when a long is 64 bits wide and you shift by 32 bits, but it would be a problem if you shifted 63 bits)