`_heapwalk`

Article
10/26/2022

Traverses the heap and returns information about the next entry.

Important

This API cannot be used in applications that execute in the Windows Runtime except in Debug builds. For more information, see CRT functions not supported in Universal Windows Platform apps.

Syntax

int _heapwalk( _HEAPINFO *entryinfo );

Parameters

entryinfo
Buffer to contain heap information.

Return value

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

Return value	Meaning
`_HEAPBADBEGIN`	Initial header information invalid or not found.
`_HEAPBADNODE`	Heap damaged or bad node found.
`_HEAPBADPTR`	The `_pentry` field of the `_HEAPINFO` structure doesn't contain a valid pointer into the heap or `entryinfo` is a null pointer.
`_HEAPEND`	End of the heap reached successfully.
`_HEAPEMPTY`	Heap not initialized.
`_HEAPOK`	No errors so far; `entryinfo` is updated with information about the next heap entry.

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

Remarks

The _heapwalk function helps debug heap-related problems in programs. The function walks through the heap, traversing one entry per call, and returns a pointer to a structure of type _HEAPINFO that contains information about the next heap entry. The _HEAPINFO type, defined in Malloc.h, contains the following elements.

Field	Meaning
`int *_pentry`	Heap entry pointer.
`size_t _size`	Size of the heap entry.
`int _useflag`	Flag that indicates whether the heap entry is in use.

A call to _heapwalk that returns _HEAPOK stores the size of the entry in the _size field and sets the _useflag field to either _FREEENTRY or _USEDENTRY (both are constants defined in Malloc.h). To obtain this information about the first entry in the heap, pass _heapwalk a pointer to a _HEAPINFO structure whose _pentry member is NULL. If the operating system doesn't support _heapwalk, the function returns _HEAPEND and sets errno to ENOSYS.

This function validates its parameter. If entryinfo is a null pointer, the invalid parameter handler is invoked, as described in Parameter validation. If execution is allowed to continue, errno is set to EINVAL and the function returns _HEAPBADPTR.

Requirements

Routine	Required header	Optional header
`_heapwalk`	<malloc.h>	<errno.h>

For more compatibility information, see Compatibility.

Example

// crt_heapwalk.c

// This program "walks" the heap, starting
// at the beginning (_pentry = NULL). It prints out each
// heap entry's use, location, and size. It also prints
// out information about the overall state of the heap as
// soon as _heapwalk returns a value other than _HEAPOK
// or if the loop has iterated 100 times.

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

void heapdump(void);

int main(void)
{
    char *buffer;

    heapdump();
    if((buffer = (char *)malloc(59)) != NULL)
    {
        heapdump();
        free(buffer);
    }
    heapdump();
}

void heapdump(void)
{
    _HEAPINFO hinfo;
    int heapstatus;
    int numLoops;
    hinfo._pentry = NULL;
    numLoops = 0;
    while((heapstatus = _heapwalk(&hinfo)) == _HEAPOK &&
          numLoops < 100)
    {
        printf("%8s block at %Fp of size %4.4X\n",
               (hinfo._useflag == _USEDENTRY ? "USED" : "FREE"),
               hinfo._pentry, hinfo._size);
        numLoops++;
    }

    switch(heapstatus)
    {
    case _HEAPEMPTY:
        printf("OK - empty heap\n");
        break;
    case _HEAPEND:
        printf("OK - end of heap\n");
        break;
    case _HEAPBADPTR:
        printf("ERROR - bad pointer to heap\n");
        break;
    case _HEAPBADBEGIN:
        printf("ERROR - bad start of heap\n");
        break;
    case _HEAPBADNODE:
        printf("ERROR - bad node in heap\n");
        break;
    }
}

    USED block at 00310650 of size 0100
    USED block at 00310758 of size 0800
    USED block at 00310F60 of size 0080
    FREE block at 00310FF0 of size 0398
    USED block at 00311390 of size 000D
    USED block at 003113A8 of size 00B4
    USED block at 00311468 of size 0034
    USED block at 003114A8 of size 0039
...
    USED block at 00312228 of size 0010
    USED block at 00312240 of size 1000
    FREE block at 00313250 of size 1DB0
OK - end of heap