check if String representing float/unsigned int is too big

I have a file containing Strings representing float and uint64_t values.

I know exactly which string contains float values and which contains uint64_t values- that is not the problem I’m facing.

Here is how I convert them to their respective data-type:

char* t, v; 
uint64_t cn; 


cn = strtoull(t, &v, 10);

char* tt, vv; 
float cn2; 

cn2 = strtof(tt, vv); 

But the problem arises at the following edge-case I want to catch:

let’s say the String for the uint64_t is "99999999999999999999999999999999999999999999999999"

This can’t be represented within 8 Bytes and therefore causes an overflow resulting in cn = 18446744073709551615.

Same problem with the float cn2.

How can catch this behavior? Thank’s!

>Solution :

strtoull provides an indication that the value is out of range but does not provide an unambiguous indication that it is too big, per C 2018 7.22.1.4 8. Consider this code:

#include <errno.h>
…
errno = 0; // Set error code to zero before call.
unsigned long long x = strtoull(t, &v, 10);
if (errno == ERANGE)
{
    // Handle out-of-range error.
}

This will reach the error case if the numeral in t is too large. But it will also reach the error case if the numeral in t is negative. If you want to distinguish the too-large case particularly, you must examine the string to see if it contains a minus sign, possibly after initial spaces.

In contrast, strtof does provide sufficient information to distinguish cases, per C 2018 7.22.1.3 10:

• If the value is positive and too large, HUGE_VALF is returned and errno is set to ERANGE.
• If the value is negative and too large, -HUGE_VALF is returned and errno is set to ERANGE.
• If the value underflows the normal range, a small value is returned and errno is set to ERANGE.