On Wednesday, 3 July, 2019 11:59, Peng Yu <pengyu...@gmail.com> wrote:
>As I said in my original message "I am trying to understand how lock >is implemented in sqlite3". from os_unix.c: /* The following describes the implementation of the various locks and ** lock transitions in terms of the POSIX advisory shared and exclusive ** lock primitives (called read-locks and write-locks below, to avoid ** confusion with SQLite lock names). The algorithms are complicated ** slightly in order to be compatible with Windows95 systems simultaneously ** accessing the same database file, in case that is ever required. ** ** Symbols defined in os.h indentify the 'pending byte' and the 'reserved ** byte', each single bytes at well known offsets, and the 'shared byte ** range', a range of 510 bytes at a well known offset. ** ** To obtain a SHARED lock, a read-lock is obtained on the 'pending ** byte'. If this is successful, 'shared byte range' is read-locked ** and the lock on the 'pending byte' released. (Legacy note: When ** SQLite was first developed, Windows95 systems were still very common, ** and Widnows95 lacks a shared-lock capability. So on Windows95, a ** single randomly selected by from the 'shared byte range' is locked. ** Windows95 is now pretty much extinct, but this work-around for the ** lack of shared-locks on Windows95 lives on, for backwards ** compatibility.) ** ** A process may only obtain a RESERVED lock after it has a SHARED lock. ** A RESERVED lock is implemented by grabbing a write-lock on the ** 'reserved byte'. ** ** A process may only obtain a PENDING lock after it has obtained a ** SHARED lock. A PENDING lock is implemented by obtaining a write-lock ** on the 'pending byte'. This ensures that no new SHARED locks can be ** obtained, but existing SHARED locks are allowed to persist. A process ** does not have to obtain a RESERVED lock on the way to a PENDING lock. ** This property is used by the algorithm for rolling back a journal file ** after a crash. ** ** An EXCLUSIVE lock, obtained after a PENDING lock is held, is ** implemented by obtaining a write-lock on the entire 'shared byte ** range'. Since all other locks require a read-lock on one of the bytes ** within this range, this ensures that no other locks are held on the ** database. */ So the original advise to read the os.c/os.h, os_common.h, os_unix.c and os_win.c/os_win.h was valid. Only knowing how to read English prose is required. note: /os.h indentify/os.h identify/ /Widnows95/Windows95/ /selected by from/selected byte from/ -- The fact that there's a Highway to Hell but only a Stairway to Heaven says a lot about anticipated traffic volume. _______________________________________________ sqlite-users mailing list sqlite-users@mailinglists.sqlite.org http://mailinglists.sqlite.org/cgi-bin/mailman/listinfo/sqlite-users
