Thread: five questions about dynamic memory allocation

Q0: What is the real purpose of malloc(), realloc() and free() functions?

I have read that they enable to allocate memory in cases when the amount of memory to be allocated is unknown. But in that case, ( Q1: ) why does the function malloc() has an argument then? (if the amounf of memory is unknown)

Here is a code:

Code:

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

int main() {

   char name[100];
   char *description;

   strcpy(name, "Zara Ali");

   /* allocate memory dynamically */
   description = malloc( 30 * sizeof(char) );
	
   if( description == NULL ) {
      fprintf(stderr, "Error - unable to allocate required memory\n");
   }
   else {
      strcpy( description, "Zara ali a DPS student.");
   }
	
   /* suppose you want to store bigger description */
   description = realloc( description, 100 * sizeof(char) );
	
   if( description == NULL ) {
      fprintf(stderr, "Error - unable to allocate required memory\n");
   }
   else {
      strcat( description, "She is in class 10th");
   }
   
   printf("Name = %s\n", name );
   printf("Description: %s\n", description );

   /* release memory using free() function */
   free(description);
}

from C Memory Management .

Q2: Can I also have a variable of different type than just chars (eg double, int)?

Q3: What is the following code? Is it allocating a memory for more variables of an array? Why do not they use the calloc() function then?

Code:

int array[10];
int * array = malloc(10 * sizeof(int));

Q4: What is the maximum number I can write inside the argument of the malloc() function?

eg can I write

Code:

   char *a;
   a=malloc(100000*sizeof(char));

?

Could you please help me in getting to know the dynamic memory allocation in C?

Thank you.

Q4: What is the maximum number I can write inside the argument of the malloc() function?

You can find out by executing something like:

Code:

 printf("%zu\n", (size_t)-1);

No assumptions are being made - size_t doesn't have to be the same as unsigned long.