is there a way to use fgets size dynamically

Question

So im learning about pointers and dynamic memory and how am experimenting with fgets.

So i want to take in a string input into a pointer using fgets, but i want fgets size to change dynamically with malloc function that i used for the pointer is there a way to do it ? e.g.

int main(){
    char *text;
    text =(char *)malloc(200 * sizeof(char));
    fgets(text, n, stdin);
    return 0;
}

Explanation

1. I created a char pointer called 'text' to eventually store the string.
2. I then use malloc planning to store 200 characters
3. Next i want to use fgets to take in the string input by the user Where 'n' is the size of text that malloc allocated for the pointer?

i have tried fgets(text, sizeof(text), stdin); but it does not work ?

Answer 1

This will not work since sizeof(ptr) where ptr is a dynamic pointer to a char array will always be size of pointer (8 for 64bits machines), not size of array. You would have to:

• either store 200 in some kind of variable; or
• statically allocate memory for text, like char text[200];

Answer 2

A common approach is to allocate some upper bounded size buffer and then right-size it.

char *fgets_alloc(FILE *f, size_t max_size) {
  char *buf = malloc(max_size);
  if (buf) {
    if (fgets(buf, max_size, f)) {
      size_t sz = strlen(buf) + 1;
      void *ptr = realloc(buf, sz);  // Reduce allocation
      if (ptr) {
        buf = ptr;
      }
      return buf;
    }
    free(buf);
  }
  return NULL;
}   
  

To Do: code to handle excessively long lines and max_size outside int range.