getopt error handling for integer values and negative values

Question

How to verify if the user has passed only integer value to a getopt option from the command line ? User should pass only positive integer values .

Isdigit() function does not work properly here .

                case 's' :
                    flags=1;

                    start = atoi(optarg);

How to check if start contains only integer value ?

                ./prog -scf

By default its taking s value as 0 when any character is entered , since i am using atoi function but here actually it should give error as cf is passed .

Even tried strtol() , but no use

            start = (int) strtol(optarg, &ptr, 10);

even if i use strtol() function , it works only if both number and strings are passed .

            eg ./prog -s5abc        --> this works

            eg  ./prog -sabc         -->does not work

here again start takes 0 value , as only charters is passed ! but if user itself passes 0 then how can i handle error ?

           eg ./prog -s0        --> s takes 0 value, valid

           eg  ./prog -sabc     --> s takes 0 value , but invalid

Answer 1

As you are working with signed numbers, this should do:

if (optarg[0] != '-' && (optarg[0] < '0' || optarg[0] > '9'))
    *error handling*
for (size_t i = 1; optarg[i] != 0; i++)
    if (optarg[i] < '0' || optarg[i] > '9')
        *error handling*