Re: constification for lang_specific_driver

["Joseph S. Myers" <jsm28 at cam dot ac dot uk>]

Zack Weinberg wrote:

> Does anyone know why the second argument of execvp() is declared as
> char * const *, instead of const char *const * ?

So that a char ** can be passed as an argument.

See B.3.1.2 in POSIX.1:1996; the following text is pasted from the latest
Austin Group draft and seems much the same.

	The statement about argv[] and envp[] being constants is included
	to make explicit to future writers of language bindings that these
	objects are completely constant.  Due to a limitation of the ISO C 
	standard, it is not possible to state that idea in standard C.  
	Specifying two levels of const-qualification for the argv[] and
	envp[] parameters for the exec functions may seem to be the
	natural choice, given that these functions do not modify either
	the array of pointers or the characters to which the function
	points, but this would disallow existing correct code.  Instead,
	only the array of pointers is noted as constant.  The table of
	assignment compatibility for dst=src, derived from the ISO C
	standard summarizes the compatibility:

     dst:           char *[] const char *[] char *const[] const char *const[]
src:
  char *[]            VALID    -               VALID        -
  const char *[]       -      VALID             -          VALID
  char * const []      -       -               VALID        -
  const char *const[]  -       -                -          VALID

	Since all existing code has a source type matching the first row,
	the column that gives the most valid combinations is the third
	column.  The only other possibility is the fourth column, but
	using it would require a cast on the argv or envp arguments.  It
	is unfortunate that the fourth column cannot be used, because the
	declaration a non-expert would naturally use would be that in the
	second row.

(The existing code for these purposes being pre-standard C knowing nothing
of const.)

-- 
Joseph S. Myers
jsm28@cam.ac.uk