Hi, When -DSQLITE_THREADSAFE=0 is used then unixShmBarrier becomes a noop because unixEnterMutex/unixLeaveMutex are empty functions. Digging through the SQLite source code I found this comment:
/* Read the header. This might happen concurrently with a write to the ** same area of shared memory on a different CPU in a SMP, ** meaning it is possible that an inconsistent snapshot is read ** from the file. If this happens, return non-zero. ** There are two copies of the header at the beginning of the wal-index. ** When reading, read [0] first then [1]. Writes are in the reverse order. ** Memory barriers are used to prevent the compiler or the hardware from ** reordering the reads and writes. */ IIUC then with SQLITE_THREADSAFE=0 on an SMP system with a database in WAL mode SQLite wouldn't be able to detect that an inconsistent snapshot is read. Should this be documented, and how serious would be the consequences of reading an inconsistent snapshot there? Would it be recommended to always build applications that use WAL with SQLITE_THREADSAFE!=0? Best regards, --Edwin
