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From: Stephen Leake <Stephen.Leake@gsfc.nasa.gov>
Subject: Re: String Allocation when Interfacing C with Ada
Date: 1996/12/06
Message-ID: <32A82389.35AC@gsfc.nasa.gov>#1/1
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references: <32A6D464.41C67EA6@avions.aerospatiale.fr>
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organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA
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Date: 1996-12-06T00:00:00+00:00
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Paul Chardon wrote:
> 
> Hello,
>         I'm writing a C program using a string, this string is passed as
> parameter to an Ada subprogram in order to modify its value. Is there
> any problem of allocation or deallocation in this case.
> 
> For instance:
> 
> -- The main C program
> void Handle_String(char * s);
> main ()
> {
>    char *a_string
> 
>    strcpy(a_string,"high");

Since a_string is not initialized, this writes "high" to an arbitrary
location in ram, possibly zero.

>    printf("String Value : %s \n", A_String);
>    adainit();

This runs Ada elaboration, which could easily overwrite the location
pointed to by the un-initialized a_string.

>    Handle_String(A_String);
>    printf("String Value : %s \n",A_String);
>    /* Print only value token in account when using strcpy in the Ada
>       subprogram, not when using "Ada allocation" */
>    adafinal();
> }
> -- The imported Ada subprogram
> with Interfaces.C.Strings;
> with Text_Io;
> procedure Handle_String(S : in out Interfaces.C.Strings.Chars_ptr) is
> 
>         procedure C_Strcpy
>         (Target :    out Interfaces.C.Strings.Chars_Ptr;
>          Source : in     Interfaces.C.Strings.Chars_Ptr);
>         pragma import(C, C_Strcpy,"strcpy");
> 
> begin
>         -- THAT CALL
>         S := Interfaces.C.Strings.New_String
>              ("Value not token in account");
>         -- when I put only this call, no value is printed in the C        >         -- program

This should place a valid address in a_string, so I don't see why it
wouldn't work.

>         -- OR THIS OTHER CALL
>         C_Strcpy(S,
>                  Interfaces.C.Strings.New_String
>                  ("Value token in account"));
>         -- When I do that copy instead of the first allocation call, the  >         -- good value is printed in C main function

This should write "Value ..." at the original uninitialized location
pointed to by a_string, which might get clobbered.

> end Handle_String;
> pragma Export(C, Handle_String, "Handle_String");
> 
>         If anyone can explain why it doesnt works in the first call case.
> Thanks in advance, Paul.

I seem to conclude that what doesn't work should, and what should
doesn't. Hope you can sort it out!

-- 
- Stephe