Question

First my context is that of a compiler writer who needs to convert floating point literals (strings) into float/double values. I haven’t done any floating point programming the last 15 years so i’m pretty sure this is a total stupid newbie question.

double res; 
errno=0; 
*res = strtod((const char*)literal,NULL); 
if (errno==ERANGE) throw_a_literal_out_of_range_exception();
return res;

but there is no “strtof” function in the c runtime library?

EDIT: To clarify. I already know that the string ‘literal’ is a valid pattern for a C floating point. It already passed a regular expression test. I just want to check if there is a range/precision problem.

The reason is in Eiffel source code a user can write

a := {REAL_32} 3.1415
b := {REAL_32} 3.0E200

To cast a written floating point number explit to a 32bit floating point. I think in the second case the compiler should detect that the value is out of range and raise an error or at least a warning.

Answer 1

Since you have your value in a double, you can just check if it’s outside of the range of float:

#include <stdlib.h>
#include <stdio.h>
#include <float.h>

int main(int argc, char *argv[])
{
    double d = FLT_MAX;
    if (argc > 1) {
        d = strtod(argv[1], NULL);
    }
    if ((d > 0 && (d > FLT_MAX || d < FLT_MIN))
                || (d < 0 && (d < -FLT_MAX || d > -FLT_MIN)))
        printf("Invalid float: %g\n", d);
    else
        printf("Valid: %g\n", d);

    return EXIT_SUCCESS;
}

Running the program:

$ ./a.out
Valid: 3.40282e+38
$ ./a.out 3.5e38
Invalid float: 3.5e+38
$ ./a.out -1e40
Invalid float: -1e+40

You may or may not need to add a test for correct strtod() return, depending upon whether there’s a possibility of an overflow in double type as well.