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Narrower primitive types can be cast to wider types without any effect on the magnitude of numeric values. However, whereas integers represent exact values, floating-point numbers have limited precision. Subclause 6.3.1.4 paragraph 2 of the C Standard [ISO/IEC 9899:2011] states:

When a value of integer type is converted to a real floating type, if the value being converted can be represented exactly in the new type, it is unchanged. If the value being converted is in the range of values that can be represented but cannot be represented exactly, the result is either the nearest higher or nearest lower representable value, chosen in an implementation-defined manner. If the value being converted is outside the range of values that can be represented, the behavior is undefined. Results of some implicit conversions may be represented in greater range and precision than that required by the new type (see 6.3.1.8 and 6.8.6.4).

Conversion from integral types to floating-point types without sufficient precision can lead to loss of precision (loss of least significant bits). No runtime exception occurs despite the loss.

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#include <assert.h>
#include <stdio.h>
#include <float.h>
 
/* 8 = bits / char */
#include <limits.h>
 
static_assert(sizeof(int) * 8CHAR_BIT <= DBL_MANT_DIG);

int main() {
  int big = 1234567890;
  double approx = big;
  printf("%d\n", (big - (int)approx));
  return 0;
}

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