Can't get rid of "this decimal constant is unsigned only in ISO C90" warning

c warnings constants unsigned c89
rfgamaral picture rfgamaral · Feb 27, 2010 · Viewed 22.4k times · Source

I'm using the FNV hash as a hashing algorithm on my Hash Table implementation but I'm getting the warning in the question title on this line:

unsigned hash = 2166136261;

I don't understand why this is happening because when I do this:

printf("%u\n", UINT_MAX);
printf("2166136261\n");

I get this:

4294967295
2166136261

Which seems to be under the limits of my machine...

Why do I get the warning and what are my options to get rid of it?

Answer

kennytm picture kennytm · Feb 27, 2010 
unsigned hash = 2166136261u; // note the u.

You need a suffix u to signify this is an unsigned number. Without the u suffix it will be a signed number. Since

2166136261 > 2³¹ - 1 = INT_MAX,

this integer literal will be problematic.

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