How to set fgets() lenth at runtime (in C)

I am new to C language.

I want to determine the fgets() lenght at runtime, something like:

int i;
char str[100];
scanf("%d",&i);    
fgets(str, i, stdin);

At execution, the program just skips my fgets() line, there is no error, I don’t see anything in dbg.

If I set a concrete value for the input length, it works fine.

fgets(str, 10, stdin);

Could somebody please help me understand what is happening? According to this C Reference the second argument is a simple int.

This is the full example:

#include <stdlib.h>
#include <stdio.h>

int main() {
   int i;
   char str[100];

   puts("Enter a number: ");
   scanf("%d", &i);
   fgets(str, i, stdin);
   printf("String contents: %s", str);
   return 0;
}

Vs. hard-coded value which is working fine:
#include <stdlib.h>
#include <stdio.h>

int main() {
 
   char str[100];

   fgets(str, 10, stdin);
   printf("String contents: %s", str);
   return 0;
}

>Solution :

By calling scanf, you solicit the user to enter a line of text. scanf("%d", &i); consumes the numeral the user enters in that line but leaves the new-line character in the buffer. The later fgets reads that new-line character, and that causes it not to read any further, so you get an empty line from fgets. Use getchar() after your call to scanf() to consume the new-line character:

int i;
char str[100];
scanf("%d",&i);
getchar();

fgets(str, i, stdin);

Or, more thoroughly, after scanf, consume the entire rest of the line up to the new-line character, in case the user entered other text:

int i;
char str[100];
scanf("%d",&i);
while (getchar() != '\n')
    ;

fgets(str, i, stdin);