The meaning of ngx_http_request_t.out ?

邓尧 torshie at
Wed Aug 28 09:54:39 UTC 2013

On Tue, Aug 27, 2013 at 10:21 PM, Maxim Dounin <mdounin at> wrote:

> Hello!
> On Tue, Aug 27, 2013 at 11:21:38AM +0800, 邓尧 wrote:
> > Hi,
> > I'm writing an nginx module, it does something similar to the sub module.
> > After some research I succeeded in handling the ngx_chain_t pointer
> passed
> > to my body filter. My module seems to work well for static files.
> > When my module is handling PHP responses (fastcgi), sometimes the
> > ngx_chain_t pointer is NULL. I simply call the next body filter in such
> > situation like the sub module body filter. However function
> > ngx_http_write_filter() will fail, because r->out isn't NULL
> > and the buf's in r->out are of zero size.
> > My questions are: what's the meaning of r->out? how should I modify it in
> > my body filter ? Could I simply return NGX_OK in my body filter without
> > calling the next body filter if the ngx_chain_t pointer is NULL ?
> The r->out is write filter's private data, you shouldn't touch it
> in your module.  And it shouldn't contain zero size non-special
> buffers, if it does - there is a bug somewhere (most likely in
> your filter).
> --
> Maxim Dounin
> _______________________________________________
> nginx-devel mailing list
> nginx-devel at

What are the most likely reasons for a buffer in r->out to be zero sized
and non-special ? I do believe the bug is somewhere in my body filter, but
I checked many times, cannot find it. My body filter's work flow is like
the following:
1. create a private empty chain, ctx->out.
2. search the input chain for patterns
3. if a matched pattern is found, append three ngx_buf_t structures (one
for the substitution string, two for the data surrounding the matched
pattern) to ctx->out, the one used for the substitution string is zerorized
then only three members are set: pos, last, memory. The other two are
memcpy()ed from the original buffer then:
    * pos, last, file_pos, file_last are modified accordingly
    * last_buf, last_in_chain are cleared (last_buf & last_in_chain of the
final chain are still set)
    * shadow is set to NULL
4. if no pattern is found, append the ngx_buf_t structure to ctx->out, then
clear last_buf & last_in_chain.
5. when a pattern is across two buffers, well, it's ignored, should be no
6. feed ctx->out to the next body filter
7. return the code from the above function call after some cleaning up.
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