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#include <limits.h>

void func(void) {
  signed int si = INT_MIN;
  unsigned int ui;
  if (si < 0) {
    /* Handle error */
  } else {
    ui = (unsigned int)si;  /* Cast eliminates warning */
  }
  /* ... */
}

Subclause 6.2.5 paragraph 9 of the C Standard [ISO/IEC 9899:2011] provides the necessary guarantees to ensure this solution works on an conforming implementation:

The range of nonnegative values of a signed integer type is a subrange of the corresponding unsigned integer type, and the representation of the same value in each type is the same.

Noncompliant Code Example (Signed, Loss of Precision)

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