zero and null in asciz pointers

[Chris Holbrook]

This comes under the headings of "things I should have done years ago" and "pitfalls for the unwary".

Some C compilers treat a pointer value of zero as a pointer to a null string, and I have got into the lazy habit of letting the compiler sort out a null pointer, and allowing a zero pointer to be equivalent to a null string. This results in an ERROR 9 in PowerBasic, as #DEBUG DISPLAY will show at run time - though without #DEBUG DISPLAY or some other form of error trapping you might not notice it!

So I'm using this macro instead of testing the pointer every time:

#compile exe
#dim all
#debug error on
#debug display on  '<===================== see what happens if you comment it out?
'--------------------------------------
macro mZP2S(p,sz)
    macrotemp stemp
    dim stemp as asciz * 1
    stemp = ""
    if p = 0 then
        sz = stemp
    else
        sz = @p
    end if
end macro
'--------------------------------------
function pbmain () as long
    local i, n as long
    local p as asciz ptr
    local sz as asciz * 64
    local s as string

    s = "hello, hello"
    ? @p              '<============= gives an error due to a null pointer
    mZP2S(p, sz)
    ? sz              '<============= no error - string derived by macro from null ptr is null
    p = strptr(s)
    MZP2S(p, sz)
    ? sz
end function

Is there a better way?

[José Roca]

MACRO mZP2S(p) = IIF$(p, @p, "")

Usage:

sz = mZP2S(p)

[Chris Holbrook]

Thanks, I'll take it!

[Michael Mattias]

Related: Passing BYVAL %NULL when an asciiz string is empty, or the address when it's not:

MACRO bvaz(asciizString)          = BYVAL IIF&(lstrlen(asciizString), VARPTR(AsCIIZString), %NULL)

  LOCAL sz  AS ASCIIZ * something 
  CALL FunctionWhichNeedsNullWhenStringIsEmptry   (bvaz (sz))

Functions which REQUIRE 'BYVAL %NULL' and will not work with the address of a null string include most of the ODBC API functions.

MCM