GlobalAlloc function (winbase.h)

Article
07/27/2022

Allocates the specified number of bytes from the heap.

Note The global 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 global function should be used. For more information, see Global and Local Functions.

Syntax

DECLSPEC_ALLOCATOR HGLOBAL GlobalAlloc(
  [in] UINT   uFlags,
  [in] SIZE_T dwBytes
);

Parameters

[in] uFlags

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

Value	Meaning
GHND 0x0042	Combines GMEM_MOVEABLE and GMEM_ZEROINIT.
GMEM_FIXED 0x0000	Allocates fixed memory. The return value is a pointer.
GMEM_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 into a pointer, use the GlobalLock function. This value cannot be combined with GMEM_FIXED.
GMEM_ZEROINIT 0x0040	Initializes memory contents to zero.
GPTR 0x0040	Combines GMEM_FIXED and GMEM_ZEROINIT.

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

GMEM_DDESHARE

GMEM_DISCARDABLE

GMEM_LOWER

GMEM_NOCOMPACT

GMEM_NODISCARD

GMEM_NOT_BANKED

GMEM_NOTIFY

GMEM_SHARE

[in] dwBytes

The number of bytes to allocate. If this parameter is zero and the uFlags parameter specifies GMEM_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 GlobalAlloc and LocalAlloc functions are essentially the same.

The movable-memory flags GHND and GMEM_MOVABLE 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 to allocate and manage memory unless the documentation specifically states that a global function should be used. For example, the global functions are still used with Dynamic Data Exchange (DDE), the clipboard functions, and OLE data objects.

If the GlobalAlloc function succeeds, it allocates at least the amount of memory requested. If the actual 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 GlobalSize function.

If the heap does not contain sufficient free space to satisfy the request, GlobalAlloc 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.

Memory allocated with this function is guaranteed to be aligned on an 8-byte boundary. To execute dynamically generated code, use the VirtualAlloc function to allocate memory and the VirtualProtect function to grant PAGE_EXECUTE access.

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

Examples

The following code shows a simple use of GlobalAlloc and GlobalFree.

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

void _cdecl main()
{
    PSECURITY_DESCRIPTOR pSD;

    pSD = (PSECURITY_DESCRIPTOR) GlobalAlloc(
           GMEM_FIXED,
           sizeof(PSECURITY_DESCRIPTOR));

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

    //see how much memory was allocated
    _tprintf(TEXT("GlobalAlloc allocated %d bytes\n"), GlobalSize(pSD));

    // Use the memory allocated

    // Free the memory when finished with it
    GlobalFree(pSD);
}

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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