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From: Dale Stanbrough <dale@goanna.cs.rmit.edu.au>
Newsgroups: comp.lang.ada
Subject: Re: Interfacing Ada to Unix/execl var. arg. list function?
Date: 19 Oct 1994 03:30:11 GMT
Organization: RMIT, Melbourne, Australia
Distribution: world
Message-ID: <3823s3$ioq@goanna.cs.rmit.oz.au>
References: <37ioh5$h9v@goanna.cs.rmit.oz.au> <37j8pd$ovu@gnat.cs.nyu.edu>
 <37m2kd$cp3@watnews1.watson.ibm.c> <37uf6e$7ik@theopolis.orl.mmc.com>
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Date: 1994-10-19T03:30:11+00:00
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In article <37uf6e$7ik@theopolis.orl.mmc.com> Bob Gilbert,
rgilbert@orl.mmc.com writes:
>  function C_Excel(Path     : System.ADDRESS;
>                   Arg_List : ARG_LIST := Null_Arg_List) return
Interfaces.c.int is
>
>    type ADDRESS_ARRAY is array (NATURAL range Arg_List'range) of
System.ADDRESS;
>    Arg_Address_List : ADDRESS_ARRAY;
>
>  begin
>    for i in Arg_List'range loop          -- Build Arg_Address_List for C call
>      case Arg_List is
>          Arg_List(i) := Arg_List(i).Arg_1'address;
>        when Arg_2 =>
>          Arg_List(i) := Arg_List(i).Arg_2'address;
>         .
>         .
>        when Arg_n =>
>          Arg_List(i) := Arg_List(i).Arg_n'address;
>        when Null_Arg => null;
>      end case;
>    end loop;
>      .
>      .

The main problem lies in here - how do you assemble the various
arguments into a call to a pragma import'ed C routine?

The only solution appears to be to direct pass parameters via 
aggregates (as your solution suggests) and then call an equivalent
non-variable argument list function (e.g. execl - execv).

The code below works, but I am unsure of whether it is memory safe,
and whether all memory gets reclaimed at the end of the function.


function execv( path    :string;
                argv    :string_array := null_arg_list) return integer is

        C_path  :constant interfaces.c.char_array(1..path'length+1)
                                := interfaces.c.to_C(path);
        type address_array is array(1..argv'length + 1) of system.address;

        C_argv  :address_array;
        index   :integer;

        ------------------------------------------------------------
        function C_execv(       path    :system.address;
                                C_arg_list:system.address)
                                        return interfaces.c.int;
        pragma import(C, C_execv, "execv");
        ------------------------------------------------------------
begin
        -- set up the array of pointers to the strings
        index := 0;
        for i in argv'range loop
                index := index + 1;
                -- can Ada release the memory pointed returned by 
                -- interfaces.c.to_c after the loop (leaving the pointer
                -- pointing at nothing), or will it wait until the end of
                -- the function, or will it never free up the memory?

                C_argv(index) := interfaces.c.to_c(argv(i).all)(0)'address;
        end loop;

        -- append C style null to the end of the array of addresses
        -- (presumes that system.null_address = C style null).

        C_argv(C_argv'last) := system.null_address;

        -- pass the address of the first element of each
        -- parameter, as C expects.

        return integer(C_execv( C_path(1)'address,
                                C_argv(1)'address));
end execv;


Thanks,

Dale

-------------------------------------------------------------
Dale Stanbrough, RMIT, Melbourne, Australia, dale@rmit.edu.au
GNU Ada 94 (GNAT) => the best $0 you'll ever spend.
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