Re: NULL terminated strings

["Joe Teff" <joeteff@xxxxxxxxxxxxx>]

Thanks for the responses. The light did come on. I'm thinking more and
more that I need to learn C. There are too many times now where at
least a working knowledge of C is neccessary even in RPG.

Joe Teff


I wrote:

>I'm not a C programmer and I'm a little confused with NULL terminated
>strings. If I see the following line of code, this is supposed to create a
>NULL terminated string, correct?
>
>  C       Eval     SomeFld = SomeText + x'00'
>
>I guess NULL doesn't mean the absence of a value, rather just all zeros.
>
>Where I get confused is the Options(*string) keyword on a procedure.
>Does that automatically append the x'00' onto the end of whatever
>non-blank text is in the field?


Hans wrote:
>Almost.  Yes, it automatically adds the X'00' onto the end of
>your string, but all characters, including blanks.  If you
>don't want blanks included in your string value, you need to
>use %trim or %trimr.
>
>A bit more precisely, options(*string) gives you a pointer to
>some temporary storage location that contains your string
>value with the x'00' character at the end.
>
>To illustrate, here's the example I posted last week:
>------------------------------------------------------------
>H dftactgrp(*no) bnddir('QC2LE')
>D atol            pr            10i 0 extproc('atol')
>D                                 *   value options(*string)
>D num             s             10i 0
>C                   eval      num = atol('1234')
>C     num           dsply
>C                   eval      *inlr = *on
>------------------------------------------------------------
>
>C function atol expects a null-terminated string as its only
>parameter.  Strings in C are often implemented by pointers to
>null-terminated strings, thus a pointer value needs to be
>passed.  The string passed as parameter in RPG is saved in a
>compiler-generated temporary variable, the null is appended to
>the end, and the pointer to that location is passed to atol,
>which doesn't even realize it's being called by a non-C program.
>
>You can also use null-terminated strings in other ways in RPG.
>Let's say you have a pointer to a null-terminated string.  You
>can use BIF %str to treat that pointer as an RPG string value.
>If pointer variable PTR contains the address of a null
>terminated string, %STR(PTR) gives you the value of the string
>in RPG character varying format.

Jon Paris wrote:

>Normally terminating with a null character (hex zeros) is done
>prior to passing a pointer to the resulting string to a C function.
>Those nice folks Hans and Barbara made this simpler by
>introducing the Option(*String) as a qualifier to a pointer in a
>prototype. When this is specified you have the option of doing
>the work yourself and passing the pointer, or if you specify not
>a pointer but a variable or expression, the compiler will do the
>null termination etc. for you.

>The complementary function is %Str which will convert a C type
>string into a "real" RPG one.

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