Transmit Parameters to C-Program

[Jens]

Hi,
I'm experimenting with calls from M to C but my C-experience is very small.

I've done this:
S VAR1="BCB",VAR2="DEF"
D &test.hello(VAR1,VAR2)

test.xc:
/home/aut/test.so
hello: ydb_status_t hello(I:ydb_string_t* ,I:ydb_string_t* )

test.c:
#include <stdio.h>
#include "libyottadb.h"

int hello(int argc, ydb_string_t stringA, ydb_string_t stringB)
{
printf("\nHello\n");
printf("\n");
printf("%s%d", "\nLength 1 ", stringA.length);
printf("%s%d", "\nLength 2 ", stringB.length);
printf("%s%d", "\nArguments ", argc);
printf("%s%s", "\nText 1 ", stringA.address);
printf("%s%s", "\nText 2 ", stringB.address);
return 0;
}

Result:

Hello


Length 1 2073116
Length 2 3
Arguments 2
Text 1
Text 2 BCBDEF

Why is the first length incorrect and the first text not shown?
Can anyone help?
Does anyone have an example with parameter transfer from an to M?
Jens

[Sam Habiel]

Let's start with that your C parameters need to be pointers, but are not. You don't have * in the type name.

Some examples of call-outs:
https://gitlab.com/YottaDB/Util/YDBPosix
https://gitlab.com/YottaDB/Util/YDBZlib

Documentation here: https://docs.yottadb.com/ProgrammersGuide/extrout.html

The Call-out documentation I would admit is not satisfactory. Let me know how it can be improved.

--Sam

[Jens]

Sam, Thank you for your hints. The zlib-example helped me much. If there was a link to this C-code in the documentation it would be a help for others too I guess.

Jens

[K.S. Bhaskar]

On Tuesday, June 15, 2021 at 3:24:11 AM UTC-4, Jens wrote:
> Sam, Thank you for your hints. The zlib-example helped me much. If there was a link to this C-code in the documentation it would be a help for others too I guess.
>
> Jens

Thank you for the suggestion to improve the documentation, Jens. https://gitlab.com/YottaDB/DB/YDBDoc/-/issues/243 has been created to track the suggestion.

Regards
– Bhaskar

[Jens]

One extended question to this topic:
If I call the C-subroutine like this:

D &test.hello(VAR1,.VAR2)

int hello(int argc, ydb_string_t stringA, ydb_string_t stringB)

Is it possible to send back an array to VAR2?

I know about ydb_set_s(), but the C-program doesn't know the name "VAR2" to add nodes to it.

Jens

[K.S. Bhaskar]

Jens, can you pass the name of the variable in VAR2? By receiving the variable name in stringB, the C code can then populate the variable.

Regards
– Bhaskar

[Sam Habiel]

Jens,

I don't think that's possible. Back when I wrote this: https://github.com/shabiel/fis-gtm-plugins/blob/master/libcurl/libcurl_ydb_wrapper.c, I wish I could do that for GT.M code, but I couldn't; and I ran up against the limit for strings.

If you can't provide a delimited output that can be parsed back by M into an array, you will have to use ydb_set_s. It is somewhat difficult to use. You can send out the variable you would like to set in stringB, and use it.

I wrote this recently for my job: https://gitlab.com/shabiel/YDBAIM/-/blob/ydbaim-c-api/ydbaim.c#L109

You can read there exactly how to do what you want.

--Sam

[Jens]

Hi Bhaskar, hi Sam,

thank you for your quick replies. I was thinking about transferring the variable-name as string too, but it 'felt wrong'.
I'll try it this way now.

Have a nice weekend!

Jens