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SQLITE3_BUSY_HANDLER(3)    Library Functions Manual    SQLITE3_BUSY_HANDLER(3)

NAME
     sqlite3_busy_handler - Register A Callback To Handle SQLITE_BUSY Errors

SYNOPSIS
     int
     sqlite3_busy_handler(sqlite3*, int(*)(void*,int), void*);

DESCRIPTION
     The sqlite3_busy_handler(D,X,P) routine sets a callback function X that
     might be invoked with argument P whenever an attempt is made to access a
     database table associated with database connection D when another thread
     or process has the table locked.  The sqlite3_busy_handler() interface is
     used to implement sqlite3_busy_timeout() and PRAGMA busy_timeout.

     If the busy callback is NULL, then SQLITE_BUSY is returned immediately
     upon encountering the lock.  If the busy callback is not NULL, then the
     callback might be invoked with two arguments.

     The first argument to the busy handler is a copy of the void* pointer
     which is the third argument to sqlite3_busy_handler().  The second
     argument to the busy handler callback is the number of times that the
     busy handler has been invoked previously for the same locking event.  If
     the busy callback returns 0, then no additional attempts are made to
     access the database and SQLITE_BUSY is returned to the application.  If
     the callback returns non-zero, then another attempt is made to access the
     database and the cycle repeats.

     The presence of a busy handler does not guarantee that it will be invoked
     when there is lock contention.  If SQLite determines that invoking the
     busy handler could result in a deadlock, it will go ahead and return
     SQLITE_BUSY to the application instead of invoking the busy handler.
     Consider a scenario where one process is holding a read lock that it is
     trying to promote to a reserved lock and a second process is holding a
     reserved lock that it is trying to promote to an exclusive lock.  The
     first process cannot proceed because it is blocked by the second and the
     second process cannot proceed because it is blocked by the first.  If
     both processes invoke the busy handlers, neither will make any progress.
     Therefore, SQLite returns SQLITE_BUSY for the first process, hoping that
     this will induce the first process to release its read lock and allow the
     second process to proceed.

     The default busy callback is NULL.

     There can only be a single busy handler defined for each database
     connection.  Setting a new busy handler clears any previously set
     handler.  Note that calling sqlite3_busy_timeout() or evaluating PRAGMA
     busy_timeout=N will change the busy handler and thus clear any previously
     set busy handler.

     The busy callback should not take any actions which modify the database
     connection that invoked the busy handler.  In other words, the busy
     handler is not reentrant.  Any such actions result in undefined behavior.

     A busy handler must not close the database connection or prepared
     statement that invoked the busy handler.

SEE ALSO
     sqlite3(3), sqlite3_stmt(3), sqlite3_busy_timeout(3), SQLITE_OK(3)

NetBSD 8.0                      March 11, 2017                      NetBSD 8.0