We recommend using Visual Studio 2017
This documentation is archived and is not being maintained.


Checks heaps for minimal consistency and sets the free entries to a specified value.

int _heapset( 
   unsigned int fill 


Fill character.

_heapset returns one of the following integer manifest constants defined in Malloc.h.


Initial header information invalid or not found.


Heap damaged or bad node found.


Heap not initialized.


Heap appears to be consistent.

In addition, if an error occurs, _heapset sets errno to ENOSYS.

The _heapset function shows free memory locations or nodes that have been unintentionally overwritten.

_heapset checks for minimal consistency on the heap and then sets each byte of the heap's free entries to the fill value. This known value shows which memory locations of the heap contain free nodes and which contain data that were unintentionally written to freed memory. If the operating system does not support _heapset(for example, Windows 98), the function returns _HEAPOK and sets errno to ENOSYS.


Required header

Optional header




For more compatibility information, see Compatibility in the Introduction.

// crt_heapset.c
// This program checks the heap and
// fills in free entries with the character 'Z'.

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

int main( void )
   int heapstatus;
   char *buffer;

   if( (buffer = malloc( 1 )) == NULL ) // Make sure heap is 
      exit( 0 );                        //    initialized     
   heapstatus = _heapset( 'Z' );        // Fill in free entries 
   switch( heapstatus )
   case _HEAPOK:
      printf( "OK - heap is fine\n" );
   case _HEAPEMPTY:
      printf( "OK - heap is empty\n" );
      printf( "ERROR - bad start of heap\n" );
      printf( "ERROR - bad node in heap\n" );
   free( buffer );
OK - heap is fine

Not applicable. To call the standard C function, use PInvoke. For more information, see Platform Invoke Examples.