How to set the character limit on C - c

Question

I want my user to enter the input which I have set only for each and every declaration, but when I tried running the program it still proceeds, for example, name the ic_size the limit character is only 12 character input, but if I put more than 12 it just accepts it as nothing wrong. Here's the coding I tried:
void login_register()
{
const name_size = 80;
char name[name_size];
const ic_size = 12;
char ic[ic_size];
const no_size = 12;
char no[no_size];
const nationality_size = 20;
char nationality[nationality_size];
const email_size = 50;
char email[email_size];
int select;
int Day =0;
int bed_tax =0;
double RM;
double room_price=0;
double service_tax = 0;
double total = 0;
char term,check;
printf("\n Please enter your name : ");
while(gets(name))/* same as scanf ("%s",name) %s mean print the corresponding argument in string*/
{
if(!isalpha && sizeof(name) > name_size) // restrict user input can only be alphabeth and must not exceed array size
{
printf("\n Please enter a valid input");
}
else
{
break;
}
}
printf(" Please enter your IC number : ");
while(gets(ic))
{
if(isdigit && sizeof(ic) < ic_size) // restrict user input can only be numerical and must not excedd array size
{
printf("\n Please enter a valid input");
}
else
{
break;
}
}
printf(" Please enter your phone number : ");
while(gets(no))
{
if(isdigit && sizeof(no) < no_size)
{
printf("\n Please enter a valid input");
}
else
{
break;
}
}
printf(" Please enter your nationality : ");
while(gets(nationality))/* same as scanf ("%s",nationality) %s mean print the corresponding argument in string*/
{
if(!isalpha && sizeof(nationality) > nationality_size) // restrict user input can only be alphabeth and must not exceed array size
{
printf("\n Please enter a valid input");
}
else
{
break;
}
}
printf(" Please enter your email : ");
while(gets(email))/* same as scanf ("%s",email) %s mean print the corresponding argument in string*/
{
if(!isalpha && sizeof(email) > email_size) // restrict user input can only be alphabeth and must not exceed array size
{
printf("\n Please enter a valid input");
}
else
{
break;
}
}

Answer 1

Rather than gets(), which has no limit on user input, consider fgets().
Shift design goal to:
Allow unlimited input per line, but only save up to N characters of input.
Let user know if too much entered. Try again if needed.
I recommend a helper function that prompts, reads input, and handles long lines.
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
// 'size' is the size of the array 'destination' points to.
int get_user_input(const char *prompt, size_t size, char *destination) {
char buf[size + 2]; // size + \n + 1 extra
do {
fputs(prompt, stdout);
if (fgets(buf, sizeof buf, stdin) == NULL) {
*destination = '\0';
return 0; // End of file
}
char *end_of_line = strchr(buf, '\n');
if (end_of_line) {
*end_of_line = '\0'; // Lop off \n
} else {
// get rest of line
scanf("%*[^\n]"); // Read everything up to a \n and toss
scanf("%*1[\n]"); // Read a \n and toss
}
} while (strlen(buf) >= size);
strcpy(destination, buf);
return 1;
}
Now read as needed. Recall that name_size = 12 means up to 11 characters are saved as 1 more is needed to save the null character. Adjust name_size as needed.
int main(void) {
const int name_size = 12;
char name[name_size];
const int nationality_size = 20;
char nationality[nationality_size];
get_user_input("Please enter your name: ", sizeof name, name);
printf("Name <%s>\n", name);
get_user_input("Please enter your nationality: ", sizeof nationality, nationality);
printf("Nationality <%s>\n", nationality);
}