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Overview
Group
Quick Info

Windows NT
Yes
Win95
Yes
Win32s
Yes
Import Library
kernel32.lib
Header File
winbase.h
Unicode
No
Platform Notes
GMEM_FIXED is expensive (locks pages) on Win32s - use GMEM_MOVEABLE

GlobalAlloc

The GlobalAlloc function allocates the specified number of bytes from the heap. In the linear Win32 API environment, there is no difference between the local heap and the global heap.

HGLOBAL GlobalAlloc(

UINT uFlags,
// object allocation attributes
DWORD dwBytes
// number of bytes to allocate
);

Parameters

uFlags

Specifies how to allocate memory. If zero is specified, the default is GMEM_FIXED. Except for the incompatible combinations that are specifically noted, any combination of the following flags can be used. To indicate whether the function allocates fixed or movable memory, specify one of the first four flags:

Flag
Meaning
GMEM_FIXED
Allocates fixed memory. This flag cannot be combined with the GMEM_MOVEABLE or GMEM_DISCARDABLE flag. The return value is a pointer to the memory block. To access the memory, the calling process simply casts the return value to a pointer.
GMEM_MOVEABLE
Allocates movable memory. This flag cannot be combined with the GMEM_FIXED flag. The return value is the handle of the memory object. The handle is a 32-bit quantity that is private to the calling process. To translate the handle into a pointer, use the GlobalLock function.
GPTR
Combines the GMEM_FIXED and GMEM_ZEROINIT flags.
GHND
Combines the GMEM_MOVEABLE and GMEM_ZEROINIT flags.
GMEM_DDESHARE
Allocates memory to be used by the dynamic data exchange (DDE) functions for a DDE conversation. Unlike Windows version 3. x, this memory is not shared globally. However, this flag is available for compatibility purposes. It may be used by some applications to enhance the performance of DDE operations and should, therefore, be specified if the memory is to be used for DDE.
Only processes that use DDE or the clipboard for interprocess communications should specify this flag.
GMEM_DISCARDABLE
Allocates discardable memory. This flag cannot be combined with the GMEM_FIXED flag. Some Win32-based applications may ignore this flag.
GMEM_LOWER
Ignored. This flag is provided only for compatibility with Windows version 3. x.
GMEM_NOCOMPACT
Does not compact or discard memory to satisfy the allocation request.
GMEM_NODISCARD
Does not discard memory to satisfy the allocation request.
GMEM_NOT_BANKED
Ignored. This flag is provided only for compatibility with Windows version 3. x.
GMEM_NOTIFY
Ignored. This flag is provided only for compatibility with Windows version 3. x.
GMEM_SHARE
Same as the GMEM_DDESHARE flag.
GMEM_ZEROINIT
Initializes memory contents to zero.

dwBytes

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

Return Values

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

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

Remarks

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.

All memory is created with execute access; no special function is required to execute dynamically generated code.

Memory allocated with this function is guaranteed to be aligned on an 8-byte boundary.

The GlobalAlloc and LocalAlloc functions are limited to a combined total of 65,536 handles for GMEM_MOVEABLE and LMEM_MOVEABLE memory per process. This limitation does not apply to GMEM_FIXED or LMEM_FIXED memory.

If this 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.

See Also

GlobalFree
, GlobalLock, GlobalReAlloc, GlobalSize, LocalAlloc


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