Question about C behaviour for unsigned integer underflow

-1, when expressed as a 2's complement number, amounts to 0xFF...F for how ever many bits your number is. In an unsigned number space that value is the maximum value possible (i.e. all the bits are set). Therefore yes, x == UINT_MAX. The following code emits "1" on a C99 strict compiler:

#include <stdio.h>
#include <stdint.h>
#include <limits.h>

int main(int argc, char **argv){
  uint32_t x = -1;      
  printf("%d", x == UINT_MAX ? 1 : 0);
  return 0;
}

§6.2.5, paragraph 9:

A computation involving unsigned operands can never overflow, because a result that cannot be represented by the resulting unsigned integer type is reduced modulo the number that is one greater than the largest value that can be represented by the resulting type.

Edit:

Sorry, wrong reference, but the result is still pinned down. The correct reference is §6.3.1.3 (signed and unsigned integer conversion):

if the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type.

So yes, x == UINT_MAX.