GlobalAlloc

GlobalAlloc is a Windows API function that allocates memory from the process’s global heap. It returns a handle to the allocated block (an HGLOBAL) or NULL on failure. The function is part of the legacy Global/Local memory management API that originated in 16-bit Windows and is retained mainly for backward compatibility and for use with certain APIs such as the clipboard.

The function has the signature HGLOBAL GlobalAlloc(UINT uFlags, SIZE_T dwBytes). The uFlags parameter specifies the allocation type and attributes. Common flags include GMEM_FIXED (a fixed block), GMEM_MOVEABLE (a movable block), and GMEM_ZEROINIT (initialize the block to zero). Other flags historically influenced how the system managed the memory, such as discardable behavior. The exact flags and their effects are largely a product of the original 16-bit API but are still recognized today for compatibility.

Accessing the memory depends on the flags. If GMEM_MOVEABLE is specified, GlobalAlloc returns a movable handle; you must call GlobalLock to obtain a pointer to the memory and GlobalUnlock when finished. If GMEM_FIXED is specified, the return value can be used directly as a pointer without locking. When the memory is no longer required, GlobalFree releases it. GlobalSize can be used to query the size of an allocated block.

Typical usage pattern: h = GlobalAlloc(GMEM_MOVEABLE | GMEM_ZEROINIT, 1024); if (h) { p = GlobalLock(h); / use p / GlobalUnlock(h); GlobalFree(h); }

Notes: GlobalAlloc is largely obsolete for new code; modern alternatives such as HeapAlloc or VirtualAlloc are preferred. Some APIs still require HGLOBAL handles, so the function remains for compatibility.