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Narrower arithmetic types can be cast to wider types without any effect on the magnitude of numeric values. However, whereas integer types represent exact values, floating-point types have limited precision. The C Standard, 6.3.1.4 paragraph 2 [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).

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This compliant solution replaces the type float with a double. Furthermore, it uses an assertion to guarantee that the double type can represent any long int without loss of precision. (see See INT35-C. Use correct integer precisions and MSC11-C. Incorporate diagnostic tests using assertions.):

#include <assert.h>
#include <float.h>
#include <limits.h>
#include <math.h>
#include <stdint.h>
#include <stdio.h>

extern size_t popcount(uintmax_t); /* See INT35-C */
#define PRECISION(umax_value) popcount(umax_value) 

int main(void) {
  assert(PRECISION(LONG_MAX) <= DBL_MANT_DIG * log2(FLT_RADIX));
  long int big = 1234567890;
  double approx = big;
  printf("%ld\n", (big - (long int)approx));
  return 0;
}

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