C fgets and sscanf in loop : prevent useless looping

I have a problem with looping and fgets and sscanf to get the input.

I know to problem is from the size of the malloc with the input. If the user enter number larger than the malloc i want to ask again to enter a new number. But in this code, if an user enter a too large number, it's looping lot of time (size of the word / 8) i think.

How to ask again to the user to enter new number without looping 4 times for example. See the example i made with big number.

The idea was to free the input after the loop but it's doesn't works. Any ideas ?

There is my code :

#include <stdio.h>
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <stdbool.h>
#include <string.h>


int main(void) {
    char x[8];
    char y[8];
    int result = 0;
    char *input=malloc(sizeof(char)*8);
    bool answer = 0;
    char *pos;

    while(!answer) {

        fgets(input, sizeof(input)-1, stdin);
        //remove the /n from fgets
        if ((pos=strchr(input, '\n')) != NULL)
            *pos = '\0';

        result = sscanf (input, "%s %s", x, y);
        printf("%d\n", result);
        if(result < 2) {
            fprintf(stderr, "There is an error with the number you give, try again\n");
        } else {
            printf("%s\n", x);
            printf("%s\n", y);
        }

    }
    return 0;
}

And the output for : "01 01"

Output for : 000000005 000000005

0000000000005 000000000000005
1
There is an error with the number you give, try again
1
There is an error with the number you give, try again
2
5
0000
1
There is an error with the number you give, try again
1
There is an error with the number you give, try again

Solution

fgets() doesn't throw away the rest of the line when it's longer than its buffer. You have to do it yourself.

If you look at this code I frequently use with fgets, you'll see the two tasks separated, and in which circumstances which one is done:

/*Returns 0 if OK, a negative value if EOF.*/
int fpurge(FILE *f)
{
    int c;
    while((c=fgetc(f))!=EOF && c!='\n')
    { }
    return (c==EOF ? -1 : 0);
}

/* Returns a nonzero value if found, zero if not. */
int truncate_newline(char *str)
{
    int bRet=0;
    if(str!=NULL)
    {
        char *pNewLine = strchr(str, '\n');
        if(pNewLine!=NULL)
        {
            bRet = 1;
            *pNewLine = '\0';
        }
    }
    return bRet;
}

/* Returns 0 if buffer is full, a positive value if line is complete,
   a negative value if EOF (implies buffer full). */
int fclean(char *str, FILE *f)
{
    int ret = 1;
    if(!truncate_newline(str))
        ret = fpurge(f);
    return ret;
}

You can see that your own code does the truncate_newline part, but not the "throw away the rest of the line" (here in the function fpurge) part.

If you change your code thusly, it should work:

#include <stdio.h>
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <stdbool.h>
#include <string.h>

#define BUFFER_SIZE 8

int main(void) {
    char x[BUFFER_SIZE];
    char y[BUFFER_SIZE];
    int result = 0;
    char *input=calloc(BUFFER_SIZE, sizeof(char));
    bool answer = 0;
    char *pos;

    while(!answer) {

        fgets(input, BUFFER_SIZE, stdin);
        //remove the /n from fgets
        if ((pos=strchr(input, '\n')) != NULL)
            *pos = '\0';
        else
        {
            int c;
            while((c=getchar())!='\n' && c!=EOF) {}
        }

        result = sscanf (input, "%s %s", x, y);
        printf("%d\n", result);
        if(result < 2) {
            fprintf(stderr, "There is an error with the number you give, try again\n");
        } else {
            printf("%s\n", x);
            printf("%s\n", y);
        }

    }
    return 0;
}

Or simply replace the whole if() with fclean(input, stdin);