man acquire_bitmap () - Locks the bitmap before drawing onto it. Allegro game programming library.
NAME
acquire_bitmap - Locks the bitmap before drawing onto it. Allegro game programming library.
SYNOPSIS
#include <allegro.h>
void acquire_bitmap(BITMAP *bmp);
DESCRIPTION
Locks the specified video memory bitmap prior to drawing onto it. This does not apply to memory bitmaps, and only affects some platforms (Windows needs it, DOS does not). These calls are not strictly required, because the drawing routines will automatically acquire the bitmap before accessing it, but locking a DirectDraw surface is very slow, so you will get much better performance if you acquire the screen just once at the start of your main redraw function, and only release it when the drawing is completely finished. Multiple acquire calls may be nested, and the bitmap will only be truly released when the lock count returns to zero. Be warned that DirectX programs activate a mutex lock whenever a surface is locked, which prevents them from getting any input messages, so you must be sure to release all your bitmaps before using any timer, keyboard, or other non-graphics routines!
Note that if you are using hardware accelerated VRAM->VRAM blits, you should not call acquire_bitmap().