COBOL and C linkages with FFI -- help needed..

[Anonymous]

Hi again,
Hoping you can help me get C to COBOL to C linkages working. I am having some challenges, have tried lots of things and still have no idea where the issue is.

https://github.com/adelosa/gnucob-ffi

I have simplified and recreated the issue in the above GitHub repo.
It also has some explanations on what I am trying to do.

I can't work out why the parameter parsing works on the first C->COBOL call but on the subsequent call (C->COBOL-C->COBOL) it says that the second linkage area has not been passed.. but I have.. And I can't work it out because the first linkage area is fine and I use the same process for both.

I suspect its just my bad programming.. but I just can't see it.
Any pointers would be appreciated. Feel free to ask any questions..

Anthony

[Simon Sobisch]

Found what should be in the Manual included in the testsuite. Using your original program and GnuCOBOL 3 your "C2" program must explicit tell the COBOL runtime that it did not get the same number of parameters that were used in the last COBOL CALL.

To do so:

cob_get_global_ptr()->cob_call_params = 2;

We still should have a look at your second example.

[A Delosa]

Sorry.. forgot to log in before positing.. It's me!

[Simon Sobisch]

Please have a look at "C interface" in the GnuCOBOL manual, source and generated PDF.

[Simon Sobisch]

Hm, that was less helpful than I've thought.

For this scenario the easiest setup would be to use cob_call, but this does not seem to be possible directly as you want to use it with a function pointer.

In this case - using GnuCOBOL 3 - the easiest option is to use a C wrapper passing that's function pointer - and from there use cob_call.

Background: first COBOL module "sees" that there is no COBOL in stack and internally sets the number of linkage arguments depending on "not null", then it does a CALL and in there sets "I pass only 1 argument", then C comes, calls the second COBOL program which sees COBOL in stack and then takes "there was one item used for CALL".

[A Delosa]

Hi Simon -- thanks for the response. From what I can tell the cob_call function has the same signature as my call_cobmodule function.

I replaced the call but I get the same outcome.. Have I understood what you are saying correctly or am I not using cob_call correctly..

The following is my changes to ccall.c

#include <ffi.h>
#include <libcob.h>
#include <stdio.h>

static void (*cobmodule)(void);
int call_cobmodule(char * module, int num_linkage_areas, void ** linkage_areas);

struct linkage1 {
    void * linkage_area;         // pointer linkage_area
    char data[20];              // tran number (FW)
};

void ccall(struct linkage1 * linkage1) {

    printf("%s\n", linkage1->data);
    void ** linkage_area = linkage1->linkage_area;

    // Setup call to ffi
    void * prog_linkage_ptr[2];           // Alloc list of pointers for prog linkage
    prog_linkage_ptr[0] = &linkage_area[0];
    prog_linkage_ptr[1] = &linkage_area[1];

    call_cobmodule("cob2", 2, prog_linkage_ptr);
}

int call_cobmodule(char * module, int num_linkage_areas, void ** linkage_areas) {

    int rc = cob_call(module, num_linkage_areas, linkage_areas);  
    //rc now holds the result of the call
    return rc;
}

Output is the same:

❯ ./build.sh && LD_LIBRARY_PATH=. COB_LIBRARY_PATH=. ./main
IN COB1
COB1-LINKAGE1       
COB1-LINKAGE2       
COB1-LINKAGE1       
IN COB2
COB1-LINKAGE1       
libcob: ./cob2.cob:21: error: LINKAGE item 'LINKAGE2' (accessed by 'LINKAGE2-DATA') not passed by caller

 Last statement of cob2 was at line 21 of ./cob2.cob
 Last statement of cob1 was at line 24 of ./cob1.cob

[A Delosa]

Hi Simon -- Thanks for the pull request. That fixed the issue perfectly..
Thanks for your time and assistance - it is really appreciated!