Re: CStringGetTextDatum and other conversions in server-side code

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On 03/02/2016 03:06 PM, david(at)andl(dot)org wrote:
> Writing a language handler: pl_language_handler. Need to do a variety of
> data conversions. One of them is char* C-string to and from Text/Varchar.
>
> The include file postgres.h has the macro CStringGetDatum but this is of
> no use: it’s just a cast.
>
> There is a builtin macro CStringGetTextDatum which calls a function and
> looks like it might work. Questions:
>
> 1. Is this the right function?
> 2. Is it OK to use, or are there restrictions?
> 3. Does it have friends: are there other conversion functions like
> this for other data types (decimal, time in particular)?
> 4. Is there any particular documentation I can read?

Your best bet is to look at examples. The code in the source tree under
contrib has many examples, and more specifically src/pl/* should have
even more relevant examples. Remember, grep is your friend -- get a copy
of the source tree and try:

cd <location-of-postgres-source-code>
grep -rni CStringGetTextDatum src/pl/* --include=*.c
grep -rni CStringGetTextDatum contrib/* --include=*.c
grep -rni TextDatumGetCstring contrib/* --include=*.c

Additionally there are external projects that implement postgres
procedural language handlers, e.g.:
https://github.com/jconway/plr
Lots of good examples there too. Google for others.

Basically any symbol exported from the postgres backend can be used by
your PL. There is not a lot of documentation other than in the source
code itself. Look at examples, see what they do, emulate it.

HTH,

Joe

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