Hello,

you have to put this from the perspective of: changes to the SIP message (headers and body) are not immediately reflected. So even if you do a replace or subst operation, changes are not visible. If you do remove_hf() or append_hf(), it happens the same.

The FAQ has an entry for it:

  - https://www.kamailio.org/wiki/tutorials/faq/main#why_changes_made_to_headers_or

$fu/U/d used to be read only, we made them r/w for convenience, as a variable based operation instead of uac_replace_from().

The r-uri variables ($ru, $rU, $rd, ...) are not pointing to the SIP message buffer, there is a special field (and buffer) inside the internal structure of Kamailio, that's why changes to it are visible. The corresponding variables pointing to the SIP message buffer are $ou, $oU, ...

Henning,

Thanks for the explanation.  This does clear it up for me.

Do you happen to know if there is a list of pseudo vars that fall under the non special case?   (a list for those psedo vars where msg_apply_changes needs to be called for the update to be reflected while in routing file processing that is.)

Thanks,

Karthik

On Wed, Jun 13, 2018 at 1:39 PM, Henning Westerholt <hw@kamailio.org> wrote:

Am Mittwoch, 13. Juni 2018, 20:28:13 CEST schrieb Alex Balashov:
> On Wed, Jun 13, 2018 at 01:26:07PM -0500, Karthik Srinivasan wrote:
> > Could you explain why we need to call this function when manipulating $fU
> > ?
>
> Some PV manipulations work that way, others don't. :-) "Because
> Kamailio".

Don't want to dig into to much technical details here..

But to give a bit more context, the Kamailio architecture related to SIP
message processing is optimized to avoid re-parsing of the message during
configuration processing. This works with so called "lumps" which are more or
less like a programming patch file (e.g. change, delete parts). This lumps are
applied shortly before sending the message out or if you call
msg_apply_changes().

Some parts of the SIP message are accessed directly, because they are "more
important" (like the request URI) are handled specially, some like the From
user are done like a normal SIP header part as described above.

For a bit more details and to look into the details, have a look to the
dbg_sip_msg([log_level], [facility]) function in the debugger module.

Best regards,

Henning

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