.Dd December 19, 2018
.Dt SQLITE3_GET_AUXDATA 3
.Os
.Sh NAME
.Nm sqlite3_get_auxdata ,
.Nm sqlite3_set_auxdata
.Nd Function Auxiliary Data
.Sh SYNOPSIS
.Ft void *
.Fo sqlite3_get_auxdata
.Fa "sqlite3_context*"
.Fa "int N"
.Fc
.Ft void 
.Fo sqlite3_set_auxdata
.Fa "sqlite3_context*"
.Fa "int N"
.Fa "void*"
.Fa "void (*)(void*)"
.Fc
.Sh DESCRIPTION
These functions may be used by (non-aggregate) SQL functions to associate
metadata with argument values.
If the same value is passed to multiple invocations of the same SQL
function during query execution, under some circumstances the associated
metadata may be preserved.
An example of where this might be useful is in a regular-expression
matching function.
The compiled version of the regular expression can be stored as metadata
associated with the pattern string.
Then as long as the pattern string remains the same, the compiled regular
expression can be reused on multiple invocations of the same function.
.Pp
The sqlite3_get_auxdata(C,N) interface returns a pointer to the metadata
associated by the sqlite3_set_auxdata(C,N,P,X) function with the Nth
argument value to the application-defined function.
N is zero for the left-most function argument.
If there is no metadata associated with the function argument, the
sqlite3_get_auxdata(C,N) interface returns a NULL pointer.
.Pp
The sqlite3_set_auxdata(C,N,P,X) interface saves P as metadata for
the N-th argument of the application-defined function.
Subsequent calls to sqlite3_get_auxdata(C,N) return P from the most
recent sqlite3_set_auxdata(C,N,P,X) call if the metadata is still valid
or NULL if the metadata has been discarded.
After each call to sqlite3_set_auxdata(C,N,P,X) where X is not NULL,
SQLite will invoke the destructor function X with parameter P exactly
once, when the metadata is discarded.
SQLite is free to discard the metadata at any time, including: 
.Bl -bullet
.It
when the corresponding function parameter changes  , or 
.It
when sqlite3_reset() or sqlite3_finalize()
is called for the SQL statement  , or 
.It
when sqlite3_set_auxdata() is invoked again on the same parameter 
, or 
.It
during the original sqlite3_set_auxdata() call when a memory allocation
error occurs.
.El
.Pp
Note the last bullet in particular.
The destructor X in sqlite3_set_auxdata(C,N,P,X) might be called immediately,
before the sqlite3_set_auxdata() interface even returns.
Hence sqlite3_set_auxdata() should be called near the end of the function
implementation and the function implementation should not make any
use of P after sqlite3_set_auxdata() has been called.
.Pp
In practice, metadata is preserved between function calls for function
parameters that are compile-time constants, including literal values
and parameters and expressions composed from the same.
.Pp
The value of the N parameter to these interfaces should be non-negative.
Future enhancements may make use of negative N values to define new
kinds of function caching behavior.
.Pp
These routines must be called from the same thread in which the SQL
function is running.
.Sh SEE ALSO
.Xr sqlite3_finalize 3 ,
.Xr sqlite3_reset 3