How can I check if a string can be converted to a

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

I was going to say, simply use the code you already have, but assign the result of strod() to a float instead of a double. But your code is wrong in several ways.

Firstly, you cannot test errno unless an error has ocurred. Secondly, strtod() will not set errno except for things like range errors. If you pass it an invalid number, like "XYZ", it will not be set.

More correct use of strtod is:

char *p;
double d = strtod( "123.4xyz", & p );
if ( * p != 0 ) {
   // not a number - in this case will point at 'x'
}

Using strtod() to read a float, you may lose some precision, but that's the price you pay for using floats - in general, unless you have a very good reason not to, you should always prefer the use of double to float.