String Allocation when Interfacing C with Ada

String Allocation when Interfacing C with Ada

[Paul Chardon]

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

Re: String Allocation when Interfacing C with Ada

[Stephen Leake]

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