what does the "main" pointor in request struct mean?

4everpan nginx-forum at nginx.us
Thu May 27 10:55:57 MSD 2010

I found in Nginx that:
struct ngx_http_request_s {
    ngx_http_request_t               *main;
what does this argument mean?

I also found a similar argument in Apache:
request_rec* request_rec::main  
//Pointer to the main request if this is a sub-request (see http_request.h) 

Do they have the same meaning?
and if yes, what dose a main request and a sub-request mean?  What's the difference between the 2 http requests?

hope anybody's help. Thx~~

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,90805,90805#msg-90805

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