dealing with string input in C - c

Question

I am trying to input string into fixed size char array.
I have a questions:
when I input a string which is bigger than the char array, the array become bigger without any additional declaration. I want to make the code only take the string that 'equal or smaller than the char array'.
Thank You.
CODE:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
/***************************Function****************************/
int string_length(char s[]) {
int c = 0;
while (s[c] != '\0')
c++;
return c;
}
/**************************************************************/
char *str;
int arrSize;
void opt1()
{
printf("Enter array size: ");
scanf("%d", &arrSize);
arrSize=arrSize+1;
str = malloc(arrSize);
return;
}
void opt2()
{
printf("Enter characters: ");
scanf("%s", str);
length = string_length(str);
printf("your input is '%s'\n", str);
printf("your input length is '%d'\n", length);
return;
}
int main()
{
int input = 0;
while(input != 3) {
printf("\n NAME \n");
printf("\n");
printf("--------------------------------------\n");
printf("1) Making Array \n");
printf("2) Check Array \n");
printf("3) Quit\n");
printf("\nEnter selection: ");
scanf("%d", &input);
if( input == 1 ) {
/* */
opt1();
}
else if(input == 2) {
opt2();
}
}
return 1;
}

Answer 1

OP wants to read data, yet if larger that the target array, then do not change the target array.
// 1: success
// -1 EOF
// 0: Input too long
int read_array(char *buffer, size_t size) {
char tmp[size];
size_t i = 0;
int ch;
while ((ch = fgetc(stdin)) != EOF && ch != '\n') {
if (i < size) {
tmp[i++] = ch;
}
}
if (ch == EOF && i == 0) return EOF;
if (i >= size) return 0; // too many
memcpy(buffer, tmp, i);
buffer[i] = '\0';
return 1;
}
Normally code could use fgets(), but there are corner cases that fail to meet OP goals.

Answer 2

To read in a whole line, you can use fgets:
char line[80];
if (fgets(line, sizeof line, stdin) != NULL) {
// use the input
}
Now you won't need to check if the user entered more than the limit, since fgets will only get the first 79 (-1 for null terminator) characters; the remainder, if any, will be ignored.