Hello!
On Fri, Dec 30, 2016 at 02:36:00PM +0100, Witold Filipczyk wrote:
> Hi,
>
> there is map:
> map $http_host $hosts {
> hostnames;
> default 0;
> .example.com 1;
> .example.net 1;
> ...
> .example.org 1;
> }
>
> How to read the value of $hosts in C module code?
If you only need a single variable, most effective approach is to
use indexed variables. The ngx_http_get_variable_index() function
is used to obtain an index during configuration parsing / module
initialization, and ngx_http_get_indexed_variable() to obtain a
value at runtime. E.g., memcached module uses this to access
$memcached_key variable, see here:
http://hg.nginx.org/nginx/file/tip/src/http/modules/ngx_http_memcached_module.c#l716
http://hg.nginx.org/nginx/file/tip/src/http/modules/ngx_http_memcached_module.c#l243
More universal approach is to use complex values. Complex values
can contain arbitrary combinations of variables and static
strings. During configuration parsing complex values are compiled
using ngx_http_compile_complex_value()
(ngx_http_set_complex_value_slot can do it automatically if there
is a dedicated configuration directive), and then
ngx_http_complex_value() can be used to obtain a value. E.g., map
itself uses complex value to handle the first parameter,
"$http_host" in your case, see here:
http://hg.nginx.org/nginx/file/tip/src/http/modules/ngx_http_map_module.c#l209
http://hg.nginx.org/nginx/file/tip/src/http/modules/ngx_http_map_module.c#l120
> How to make it right, that's mean do not evaluate it more than once per
> request?
Map results are automatically cached by nginx, much like all
variables not explicitly declared as non-cacheable, so you don't
really need to worry. On the other hand, usually it's a good idea
to remember the value obtained in your module data if you are
going to use it again.
--
Maxim Dounin
http://nginx.org/
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