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int sqlite3_set_authorizer( sqlite3*, int (*xAuth)(void*,int,const char*,const char*,const char*,const char*), void *pUserData );
This routine registers an authorizer callback with a particular database connection, supplied in the first argument. The authorizer callback is invoked as SQL statements are being compiled by sqlite3_prepare() or its variants sqlite3_prepare_v2(), sqlite3_prepare16() and sqlite3_prepare16_v2(). At various points during the compilation process, as logic is being created to perform various actions, the authorizer callback is invoked to see if those actions are allowed. The authorizer callback should return SQLITE_OK to allow the action, SQLITE_IGNORE to disallow the specific action but allow the SQL statement to continue to be compiled, or SQLITE_DENY to cause the entire SQL statement to be rejected with an error. If the authorizer callback returns any value other than SQLITE_IGNORE, SQLITE_OK, or SQLITE_DENY then the sqlite3_prepare_v2() or equivalent call that triggered the authorizer will fail with an error message.
When the callback returns SQLITE_OK, that means the operation requested is ok. When the callback returns SQLITE_DENY, the sqlite3_prepare_v2() or equivalent call that triggered the authorizer will fail with an error message explaining that access is denied.
The first parameter to the authorizer callback is a copy of the third parameter to the sqlite3_set_authorizer() interface. The second parameter to the callback is an integer action code that specifies the particular action to be authorized. The third through sixth parameters to the callback are either NULL pointers or zero-terminated strings that contain additional details about the action to be authorized. Applications must always be prepared to encounter a NULL pointer in any of the third through the sixth parameters of the authorization callback.
If the action code is SQLITE_READ and the callback returns SQLITE_IGNORE then the prepared statement statement is constructed to substitute a NULL value in place of the table column that would have been read if SQLITE_OK had been returned. The SQLITE_IGNORE return can be used to deny an untrusted user access to individual columns of a table. When a table is referenced by a SELECT but no column values are extracted from that table (for example in a query like "SELECT count(*) FROM tab") then the SQLITE_READ authorizer callback is invoked once for that table with a column name that is an empty string. If the action code is SQLITE_DELETE and the callback returns SQLITE_IGNORE then the DELETE operation proceeds but the truncate optimization is disabled and all rows are deleted individually.
An authorizer is used when preparing SQL statements from an untrusted source, to ensure that the SQL statements do not try to access data they are not allowed to see, or that they do not try to execute malicious statements that damage the database. For example, an application may allow a user to enter arbitrary SQL queries for evaluation by a database. But the application does not want the user to be able to make arbitrary changes to the database. An authorizer could then be put in place while the user-entered SQL is being prepared that disallows everything except SELECT statements.
Applications that need to process SQL from untrusted sources might also consider lowering resource limits using sqlite3_limit() and limiting database size using the max_page_count PRAGMA in addition to using an authorizer.
Only a single authorizer can be in place on a database connection at a time. Each call to sqlite3_set_authorizer overrides the previous call. Disable the authorizer by installing a NULL callback. The authorizer is disabled by default.
The authorizer callback must not do anything that will modify the database connection that invoked the authorizer callback. Note that sqlite3_prepare_v2() and sqlite3_step() both modify their database connections for the meaning of "modify" in this paragraph.
When sqlite3_prepare_v2() is used to prepare a statement, the statement might be re-prepared during sqlite3_step() due to a schema change. Hence, the application should ensure that the correct authorizer callback remains in place during the sqlite3_step().
Note that the authorizer callback is invoked only during sqlite3_prepare() or its variants. Authorization is not performed during statement evaluation in sqlite3_step(), unless as stated in the previous paragraph, sqlite3_step() invokes sqlite3_prepare_v2() to reprepare a statement after a schema change.