NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/2216

Internet Message Format | 1996-08-05 | 3.0 KB

Path: eql.caltech.edu!rankin
From: [e-mail] (Pat Rankin)
Newsgroups: comp.lang.c
Subject: Re: Problem calling LIB$FIND_FILE on VAX
Date: 19 Jan 1996 14:02 PST
Organization: California Institute of Technology
Distribution: world
Message-ID: <[e-mail]>
References: <[e-mail]>
NNTP-Posting-Host: bear.eql.caltech.edu
News-Software: VAX/VMS VNEWS 1.41x8

In article <[e-mail]>, [e-mail] writes...
> I'm having difficulty trying to call the lib$find_file(,,,,,,) VAX
> Run-Time-Library (RTL) command from within a C program. I think my
> problem is not with the parameters of the actual lib$ call itself,
> but with the "$descriptor" part that actually holds the values, I've
> been through the VAX manuals and the "descriptor" part is still unclear
> to me. Am I accessing it correctly?...

     This topic would be more appropriate in comp.os.vms.

>%SYSTEM-F-ACCVIO, access violation, reason mask=01, virtual address=010E0102,
[...]

     That virtual "address" looks like the value of the first 32 bits of
one of your string descriptors.

[...]
>   struct dsc$descriptor_s FileFound;
[...]
>    /* Call library function  Lib$find_file(,,,,) */
>   status = lib$find_file( descriptor( FName ), FileFound,
>                                ContexVar, 0, 0, 0, 1 );

     You are passing `FileFound' by value but lib$find_file expects to
receive it by reference.  You need to use `&FileFound' to pass a pointer
to your variable.  `ContexVar' and those integer constants also have
the same problem.  (Actually worse; you're using unadorned 0 where you
ought to be using null pointers, and clearly there's no prototype in
scope or the `FileFound' mismatch would have been caught by the compiler.)
lib$XXX routines take arguments by reference so that they're relatively
straightforward to use with any programming language; C code needs to
construct pointers rather than pass immediate values.

>   if (status == SS$_NORMAL) {
>      FileFound.dsc$a_pointer[ FileFound.dsc$w_length ] = '\0';
>   }

     You should not be checking for an explicit value; check for an odd
number, which is the most generic way to test VMS condition codes for
success.  And since `FileFound' defines a static string, lib$find_file
will not touch its length field, so your '\0' would always end up being
stored at `instr[255]' and not actually trim the resulting file name.
All lib$XXX routines which write to static strings will pad their output
values with trailing spaces to fill out the full length defined for the
string.  Since VMS file names never have trailing spaces in them, you
can trim the string by starting at the end of the buffer and backing up
until you hit any non-space character or the beginning of the buffer.
Or you could switch to dynamic strings, but I'd advise against that.
With dynamic allocation you would have the exact length, but there'd be
no guarantee of room in the allocated buffer to append a trailing NUL
byte for use as a normal C-style string.

        Pat Rankin, [e-mail]