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