Databases Reference
In-Depth Information
Type
C type
SHOW_ARRAY
st_mysql_show_var *
SHOW_FUNC
int (*)(MYSQL_THD,
struct st_mysql_show_var*, char *)
The last two types are special. Elements of these types do not define rows in
SHOW STATUS
, they define other elements of the
st_mysql_show_var
structure.
SHOW_ARRAY
specifies that
value
is not really a value, but a pointer to another
zero-terminated array of status variables. The corresponding
name
of the
SHOW_ARRAY
element will be used as a
prefix
added to all the names of status variables in the
referenced array. The variables beginning with
Com_
in the normal
SHOW STATUS
output is an example of this in action.
The
SHOW_FUNC
function is more interesting. The value is interpreted as a pointer to a
function that generates the
st_mysql_show_var
structure. This function should have
the following prototype:
int my_status_var(MYSQL_THD thd,
struct st_mysql_show_var *var, char *buff)
The first argument is a pointer to the thread object of the current connection,
var
points to a
st_mysql_show_var
structure that we need to fill in, and
buff
is a
preallocated 1024-byte buffer. The function needs to set
var->type
and
var->value
(the
var->name
member will be ignored by MySQL) to form a valid status variable
structure. And if needed,
var->type
can be set to
SHOW_ARRAY
or even to
SHOW_FUNC
again, and MySQL will handle this situation correctly. A buffer
buff
is provided as
convenience storage; the function may store the value there and point
var->value
to
it. The return value of this function is ignored.
So, putting this together we can have something similar to the following to make our
plugin add new status variables to a server:
struct st_mysql_show_var my_status_vars[]=
{
{"data_size", (char *)&data_size, SHOW_LONG},
{"avg_text_size", (char *)&avg_text_size, SHOW_LONGLONG},
{0,0,0}
};
We would then use
my_status_vars
in the plugin declaration.
