proper use of scanf in a while loop to validate input

Question

I made this code:

/*here is the main function*/
int x , y=0, returned_value;
int *p = &x;
while (y<5){
    printf("Please Insert X value\n");
    returned_value = scanf ("%d" , p);
    validate_input(returned_value, p);
    y++;
}

the function:

void validate_input(int returned_value, int *p){
    getchar();
    while (returned_value!=1){
        printf("invalid input, Insert Integers Only\n");
        getchar();
        returned_value = scanf("%d", p);
    }
}

Although it is generally working very well but when I insert for example "1f1" , it accepts the "1" and does not report any error and when insert "f1f1f" it reads it twice and ruins the second read/scan and so on (i.e. first read print out "invalid input, Insert Integers Only" and instead for waiting again to re-read first read from the user, it continues to the second read and prints out again "invalid input, Insert Integers Only" again...

It needs a final touch and I read many answers but could not find it.

Answer 1

If you don't want to accept 1f1 as valid input then scanf is the wrong function to use as scanf returns as soon as it finds a match.

Instead read the whole line and then check if it only contains digits. After that you can call sscanf

Something like:

#include <stdio.h>

int validateLine(char* line)
{
    int ret=0;
    
    // Allow negative numbers
    if (*line && *line == '-') line++;
    
    // Check that remaining chars are digits
    while (*line && *line != '\n')
    {
        if (!isdigit(*line)) return 0; // Illegal char found

        ret = 1;  // Remember that at least one legal digit was found
        ++line;
    }
    return ret;
}

int main(void) {
    char line[256];
    int i;

    int x , y=0;
    while (y<5)
    {
        printf("Please Insert X value\n");
        if (fgets(line, sizeof(line), stdin)) // Read the whole line
        {
            if (validateLine(line))  // Check that the line is a valid number
            {
                // Now it should be safe to call sscanf - it shouldn't fail
                // but check the return value in any case
                if (1 != sscanf(line, "%d", &x)) 
                {
                    printf("should never happen");
                    exit(1);
                }

                // Legal number found - break out of the "while (y<5)" loop
                break;
            }
            else
            {
                printf("Illegal input %s", line);
            }
        }
        y++;
    }
    
    if (y<5)
        printf("x=%d\n", x);
    else
        printf("no more retries\n");

    return 0;
}

Input

1f1
f1f1

-3

Output

Please Insert X value
Illegal input 1f1
Please Insert X value
Illegal input f1f1
Please Insert X value
Illegal input 
Please Insert X value
x=-3

Another approach - avoid scanf

You could let your function calculate the number and thereby bypass scanf completely. It could look like:

#include <stdio.h>

int line2Int(char* line, int* x)
{
    int negative = 0;
    int ret=0;
    int temp = 0;
    
    if (*line && *line == '-') 
    {
        line++;
        negative = 1;
    }
    else if (*line && *line == '+')  // If a + is to be accepted
        line++;                      // If a + is to be accepted
       
    while (*line && *line != '\n')
    {
        if (!isdigit(*line)) return 0; // Illegal char found
        ret = 1;

            // Update the number
        temp = 10 * temp;
        temp = temp + (*line - '0');

        ++line;
    }
    
    if (ret)
    {
        if (negative) temp = -temp;
        *x = temp;
    }
    return ret;
}

int main(void) {
    char line[256];
    int i;

    int x , y=0;
    while (y<5)
    {
        printf("Please Insert X value\n");
        if (fgets(line, sizeof(line), stdin)) 
        {
            if (line2Int(line, &x)) break;  // Legal number - break out

            printf("Illegal input %s", line);
        }
        y++;
    }
    
    if (y<5)
        printf("x=%d\n", x);
    else
        printf("no more retries\n");

    return 0;
}