LocalAlloc function (winbase.h)

Článek
07/27/2022

Allocates the specified number of bytes from the heap.

Note The local functions have greater overhead and provide fewer features than other memory management functions. New applications should use the heap functions unless documentation states that a local function should be used. For more information, see Global and Local Functions.

Syntax

DECLSPEC_ALLOCATOR HLOCAL LocalAlloc(
  [in] UINT   uFlags,
  [in] SIZE_T uBytes
);

Parameters

[in] uFlags

The memory allocation attributes. The default is the LMEM_FIXED value. This parameter can be one or more of the following values, except for the incompatible combinations that are specifically noted.

Value	Meaning
LHND 0x0042	Combines LMEM_MOVEABLE and LMEM_ZEROINIT.
LMEM_FIXED 0x0000	Allocates fixed memory. The return value is a pointer to the memory object.
LMEM_MOVEABLE 0x0002	Allocates movable memory. Memory blocks are never moved in physical memory, but they can be moved within the default heap. The return value is a handle to the memory object. To translate the handle to a pointer, use the LocalLock function. This value cannot be combined with LMEM_FIXED.
LMEM_ZEROINIT 0x0040	Initializes memory contents to zero.
LPTR 0x0040	Combines LMEM_FIXED and LMEM_ZEROINIT.
NONZEROLHND	Same as LMEM_MOVEABLE.
NONZEROLPTR	Same as LMEM_FIXED.

The following values are obsolete, but are provided for compatibility with 16-bit Windows. They are ignored.

LMEM_DISCARDABLE

LMEM_NOCOMPACT

LMEM_NODISCARD

[in] uBytes

The number of bytes to allocate. If this parameter is zero and the uFlags parameter specifies LMEM_MOVEABLE, the function returns a handle to a memory object that is marked as discarded.

Return value

If the function succeeds, the return value is a handle to the newly allocated memory object.

If the function fails, the return value is NULL. To get extended error information, call GetLastError.

Remarks

Windows memory management does not provide a separate local heap and global heap. Therefore, the LocalAlloc and GlobalAlloc functions are essentially the same.

The movable-memory flags LHND, LMEM_MOVABLE, and NONZEROLHND add unnecessary overhead and require locking to be used safely. They should be avoided unless documentation specifically states that they should be used.

New applications should use the heap functions unless the documentation specifically states that a local function should be used. For example, some Windows functions allocate memory that must be freed with LocalFree.

If the heap does not contain sufficient free space to satisfy the request, LocalAlloc returns NULL. Because NULL is used to indicate an error, virtual address zero is never allocated. It is, therefore, easy to detect the use of a NULL pointer.

If the LocalAlloc function succeeds, it allocates at least the amount requested. If the amount allocated is greater than the amount requested, the process can use the entire amount. To determine the actual number of bytes allocated, use the LocalSize function.

To free the memory, use the LocalFree function. It is not safe to free memory allocated with LocalAlloc using GlobalFree.

Examples

The following code shows a simple use of LocalAlloc and LocalFree.

#include <windows.h>
#include <stdio.h>
#include <tchar.h>

void _cdecl _tmain()
{
    LPTSTR pszBuf=NULL;

    pszBuf = (LPTSTR)LocalAlloc(
              LPTR,
              MAX_PATH*sizeof(TCHAR));

    // Handle error condition
    if( pszBuf == NULL )
    {
       _tprintf(TEXT("LocalAlloc failed (%d)\n"), GetLastError());
       return;
    }

    //see how much memory was allocated
    _tprintf(TEXT("LocalAlloc allocated %d bytes\n"), LocalSize(pszBuf));

    // Use the memory allocated

    // Free the memory when finished with it
    LocalFree(pszBuf);
}

Requirements

Requirement	Value
Minimum supported client	Windows XP [desktop apps \| UWP apps]
Minimum supported server	Windows Server 2003 [desktop apps \| UWP apps]
Target Platform	Windows
Header	winbase.h (include Windows.h)
Library	Kernel32.lib
DLL	Kernel32.dll

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