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SQLite C Interface
Obtain Aggregate Function Context
void *sqlite3_aggregate_context(sqlite3_context*, int nBytes);
Implementations of aggregate SQL functions use this routine to allocate memory for storing their state.
The first time the sqlite3_aggregate_context(C,N) routine is called for a particular aggregate function, SQLite allocates N of memory, zeroes out that memory, and returns a pointer to the new memory. On second and subsequent calls to sqlite3_aggregate_context() for the same aggregate function instance, the same buffer is returned. Sqlite3_aggregate_context() is normally called once for each invocation of the xStep callback and then one last time when the xFinal callback is invoked. When no rows match an aggregate query, the xStep() callback of the aggregate function implementation is never called and xFinal() is called exactly once. In those cases, sqlite3_aggregate_context() might be called for the first time from within xFinal().
The sqlite3_aggregate_context(C,N) routine returns a NULL pointer when first called if N is less than or equal to zero or if a memory allocate error occurs.
The amount of space allocated by sqlite3_aggregate_context(C,N) is determined by the N parameter on first successful call. Changing the value of N in subsequent call to sqlite3_aggregate_context() within the same aggregate function instance will not resize the memory allocation. Within the xFinal callback, it is customary to set N=0 in calls to sqlite3_aggregate_context(C,N) so that no pointless memory allocations occur.
SQLite automatically frees the memory allocated by sqlite3_aggregate_context() when the aggregate query concludes.
The first parameter must be a copy of the SQL function context that is the first parameter to the xStep or xFinal callback routine that implements the aggregate function.
This routine must be called from the same thread in which the aggregate SQL function is running.