Validating unsigned integers with scanf()

Code:

---

#include <stdio.h>

int main() {
        unsigned int x = 0;
        int scanf_value = scanf("%u", &x);
        if (scanf_value != 1) {
                printf("The user has not entered a valid unsigned integer\n");
        }
        else {
                printf("The user has entered a valid unsigned integer\n");
        }
        return 0;
}

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Why is it that if I input the value -1 it says the user has entered a valid unsigned integer? Isn't scanf() supposed to return the number of successful conversions?

Thank you.

How would a negative unsigned value be valid?

Yes, it's probably a bit dependant on the implementation of scanf() whether it checks if "sign given" for unsigned numbers. In the glibc (Gnu C library) it doesn't seem to check specifically for this.

Perhaps you want to check it yourself?

--
Mats

Yeah, I assumed scanf() would fail once it encountered a minus sign. It seems not to be the case. I solved the problem using the following code:

Assuming the user won't enter more than 512 characters:

Code:

---


char input [512];
int scanf_value = scanf("%[0-9]", input);

if (scanf_value != 1) {
        printf("The user has not entered a valid unsigned integer\n");
}

---

My preferred method is to read user input as a string and use strtol or strtoul to attempt conversion.

Quote:

---

Originally Posted by Dave_Sinkula

My preferred method is to read user input as a string and use strtol or strtoul to attempt conversion.

---

You mean like this?

Code:

---

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

int main() {
        char input [512];
        unsigned int x = 0;
        fgets(input, 512, stdin);
        char* input_ptr = (char*)input;
       
        x = strtoul(input, &input_ptr, 10);
        if (*input_ptr != '\n') {
                printf("Not an unsigned integer!\n");
        }
        return 0;
}

---

More or less. Some additional doo-dads might help. Maybe add a check for a leading - as well.

I think the scanf("%[0-9]", string) would accept input like 1abc as "one entry", which means that something that isn't technically a number could still be "accepted input". And of course, if you don't clear the input buffer, the NEXT input will fail.

fgets() and strto[u]l is much better in that respect. I'm not sure if strtoul actually checks for negative numbers - it may still accept those [I seem to remember that all strto* functions use a common function, and it may actually not do any checking specifically to ensure negative input is unaccepted by unsiged input].

--
Mats