bmp_unwrite_line man page
bmp_unwrite_line — Direct access bank switching line release. Allegro game programming library.
void bmp_unwrite_line(BITMAP *bmp);
Releases the bitmap memory after you are finished with it. You only need to call this once at the end of a drawing operation, even if you have called bmp_write_line() or bmp_read_line() several times before it.