C fgets and sscanf in loop : prevent useless looping - c

Question

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"
01 01
2
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

Answer 1

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);