Dynamic Arrays: Using malloc() and realloc()

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This had bugging me on table of listening during a small moment, but I appeared it. I will share my knowledge with the rest of the world! If you must take data of the user who could be any length, you could define a really large line as thus :

int array[100000];

There are several problems with this. Anyhow large the line is, the user could still have more entry. If the user does not have that much entry, wasted you the memory.

When you are using malloc(), realloc() and free() you need the following header file:

#include <stdlib.h>

First, I will show you how to allocate memory for a pointer. You can declare a pointer like so:

int *pointer;

The pointer can point to any location at first. You should always make it point to something or you can allocate some memory that your pointer will point to. To do this, you need to use the malloc() function. Use it like so:


malloc() returns a void pointer and takes an argument of how many bytes to allocate. Because pointer points to an integer, we use the 2*sizeof(int). Using malloc like the above is similar to doing this:

int array[2];

Error checking:
If the operating system can't allocate more memory for your program, malloc will fail and return a NULL value. It's always a good idea to make sure malloc is successful:

pointer=malloc(1*sizeof(*pointer));if (pointer==NULL) {printf("Error allocating memory!\n"); //print an error messagereturn 1; //return with failure}

Now I will show you how to use realloc(). You use realloc after you have used malloc to give a pointer more or less memory. Let's say you want to give a pointer 5 integers of memory. The code should look like this:

int *temp = realloc(pointer, 5*sizeof(int));if ( temp != NULL ) //realloc was successful{pointer = temp;}else //there was an error{free(pointer);printf("Error allocating memory!\n");return 1;}

This is just like malloc, except realloc takes two arguments. The first argument is the pointer you want to copy the data from. The above code copies pointer to temp, then copies temp back to pointer if everything goes correctly. You may have noticed a new function though, and that is free().


Free is used to free the memory you have allocated with malloc or realloc. All memory that you allocate should be freed when you are done using it. Free takes a pointer as an argument like so:


Here a program of example which makes use of a dynamic line. 

#include <stdio.h>#include <stdlib.h>
int main()
{int *data,*temp;data=malloc(sizeof(int));int c; 
int i; 
for (i=0;;i++) {c=getchar(); 
if (c==EOF) 
if ( temp != NULL ) {data=temp;} 
else {free(data);
printf("Error allocating memory!\n");return 1;}}
for (i--;i>=0;i--)putchar(data[i]);
Return success */return 0;
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