Difference between realloc() and free()?  

The free subroutine frees a block of memory previously allocated by the malloc subroutine. Undefined results occur if the Pointer parameter is not a valid pointer. If the Pointer parameter is a null value, no action will occur. The realloc subroutine changes the size of the block of memory pointed to by the Pointer parameter to the number of bytes specified by the Size parameter and returns a new pointer to the block. The pointer specified by the Pointer parameter must have been created with the malloc, calloc, or realloc subroutines and not been deallocated with the free or realloc subroutines. Undefined results occur if the Pointer parameter is not a valid pointer  
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sangappa

  • Sep 22nd, 2005
 

realloc function takes 2 parameters.

example:

tpedef struct test

{

int i;

float j;

};

test *ptest = (test*)malloc(sizeof(test));

realloc(ptest,sizeof(test));

After realloc ptest contains new address and memory allocated will double the size of prvious memory.

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Raju Tewari

  • Mar 28th, 2007
 

Realloc () function used to expands the existing allocated memory ,if it finds enough continus memory
location that are required,if enough continus memory are not available new memory of block is allocated.The existing data is copied and the origibnal memory block is freed. for example::

int *p=new int[5];
p[1]=3;
p=(int*) realloc (p,sizeof(int) *5);
cout<<p[1];

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kazam0076

  • Apr 24th, 2007
 

Hi Sangappa,

I think the example mentioned in your answer wont double the size of memory pointed by the ptest. Instead it creates equal sized one and copies all the content to new memory. To get the expected result i believe the code should look like this.


>>      realloc(ptest,sizeof(test)*2); // to double the size of the mem.

mitra.s

  • Apr 27th, 2007
 

free() deallocates memoru allocated by maaloc,calloc or realloc. Whenevr there is a need to allocate extra memory we can use realloc.
realloc means free+malloc once more. following example may explain this behaviour.

void main()
{
char *ptr;
ptr= (char*)malloc(7);
strcpy(ptr,"STRING");
cout<<"Content is "<<ptr;

realloc(ptr,10);
cout<<"nContent after Realloc is "<<ptr;
}

output
Content is STRING
Content after Realloc is //some garbage-> STRING is lost

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Roopesh Majeti

  • May 27th, 2007
 

Mitra,
as you mentioned that realloc will delete the existing memory and allocate the new memory . I tried it on solaris and following is found :

1. If the new size allocated is less than the existing memory size, then realloc will allocate existing memory size. In other words, will leave it.
2. If the new size allocated is more than the existing memory size, then realloc will allocate new memory size.

In either of these two cases, the realloc will not delete the contents.

What you said is partially correct. Realloc when trying to allocate required memory size and if it could not find a continuous memory then it will allocate new memory, which will automatically give garbage, as it now, points to new memory.


Hope this clarifies many things.

-Roopesh.

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SomGollakota

  • Jun 13th, 2007
 

free() - releases the memory of the pointer passed as parameter, to the OS/application consumption. Using the pointer after free() will result in undefinded results

realloc() - used to resize the memory held by the pointer to the number of bytes specificed. If the new size is larger than current size, new memory is allocated. If it is less, the remaining (additional) bytes are released to general OS/application consumption.

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