Synopsis

Preserve precision when converting integral values to floating-point type.

Enabled by default

Yes

Severity/Certainty

Low/Low

Full description

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. Conversion from integral types to floating-point types without sufficient precision can lead to loss of precision (loss of least significant bits).

Coding standards

Code examples

The following code example fails the check and will give a warning:

#include <stdio.h>

int main(void) {
    long int big = 1234567890L;
    float approx = big;
    printf("%ld\n", (big - (long int)approx));
    return 0;
}

The following code example passes the check and will not give a warning about this issue:

#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 = 1234567890L;
    double approx = big;
    printf("%ld\n", (big - (long int)approx));
    return 0;
}