malloc and realloc

[C-Struggler]

Hi everyone,

I was wondering if someone could go over my code to check to see if I did this right. Basically what this program is supposed to do is allocate enough memory for 10 random characters and assigns them to memory. Next it reallocates the memory to hold 15 elements and adds 5 more random characters. It then writes all of the characters to disk. To my knowledge I covered all of this, but my gut tells me I went about it the wrong way.

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

#define Max 10

int i, randNum, NewMax;


int main(void)
{
char array[] = {"abcdefghijklmnopqrstuvwxyz0123456789"};
char *ptr_array = array;
char RandChar;

ptr_array = malloc(10); //Allocates 150 bytes of memory
ptr_array = realloc ( ptr_array, 15 );
NewMax = Max + 5;

srand(time(NULL));

for (i=0; i<NewMax; i++)
{

RandChar = array[rand() % (sizeof(array)-1)];

printf("%c\n", RandChar);

}
free(ptr_array);
}

[Prelude]

>To my knowledge I covered all of this
More or less. I believe the requirement was to allocate 10 spaces and fill them, then reallocate to 15 spaces without losing the first 10. You should also be more careful with return values and error checking when it comes to memory allocation:

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

#define Max 10

int main(void)
{
  int i, NewMax;
  char array[] = "abcdefghijklmnopqrstuvwxyz0123456789";
  char *ptr_array;
  
  /* Allocates 10 bytes of memory */
  ptr_array = malloc(Max * sizeof *ptr_array);
  
  /* Check that it worked */
  if (ptr_array != NULL)
  {
    char *hold;
    
    srand((unsigned)time(NULL));
    
    /* Fill the 10 spots */
    for (i=0; i<Max; i++)
      ptr_array[i] = array[rand() % (sizeof(array)-1)];

    /* Print all 10 characters */
    for (i=0; i<Max; i++)
      printf("%c", ptr_array[i]);

    printf("\n");
    
    /* Reallocated to 15, save in a second pointer in case of failure */
    NewMax = Max + 5;
    hold = realloc ( ptr_array, NewMax * sizeof *ptr_array );
    
    /* Check that it worked */
    if (hold != NULL)
    {
      /* Save the realloc'd memory to the original pointer */
      ptr_array = hold;
      
      /* Fill the next 5 spots */
      for (i=Max; i<NewMax; i++)
        ptr_array[i] = array[rand() % (sizeof(array)-1)];
      
      /* Print all 15 characters */
      for (i=0; i<NewMax; i++)
        printf("%c", ptr_array[i]);

      printf("\n");
    }
    
    free(ptr_array);
  }
  
  return 0;
}

-Prelude

[C-Struggler]

Thanks Prelude. Very much appreciated.