HP OpenVMS Systems

Ada and String Descriptors?

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The Question is:

 
When using Lib.Find_File in VAX Ada, the Resultant_Filespec data is returned as
 a 'string' but with no information about the number of valid characters put
 into the string by Find_File.
 
Languages such as VAX Pascal allow us to play around with the descriptor used
 to pass information to LIB$Find_File (so we can use a CLASS_VS descriptor
 instead of CLASS_S).
 
What is the recommended (VAX Ada) way of determining the number of valid
 characters in the Resultant_Filespec?  Search for trailing whitespace
 characters?
 
TIA

The Answer is :

 
  Lib.Find_File is merely a entry in the Ada Predefined Library for the
  system service routine.
 
  You could write your own wrapper based on the LIB package's declaration,
  if you want to use class VS descriptors, since Ada's import pragmas
  do not appear to support that particular class (LRM 13.9a.1.1).
 
  You could also write a jacket routine for LIB.FIND_FILE in a language
  that supports dynamic descriptors with an interface that adds a
  "returned-length" argument.
 
  Or you could use RMS services $PARSE and $SEARCH directly, instead of
  FIND_FILE
 
  The simplest is to trim trailing whitespace from your resultant
  filename - LIB$FIND_FILE will always fill the remainder of a fixed
  length string with spaces. You can also be sure that even under ODS-5,
  no valid OpenVMS file spec contains trailing spaces.
 
 
 
    procedure print_directory is
       FILENAME   : STRING (1..255);
       RESNAME    : STRING (1..255);
       STAT       : CONDITION_HANDLING.COND_VALUE_TYPE;
       CTX        : SYSTEM.UNSIGNED_LONGWORD;
       LENGTH     : INTEGER;
       RESLEN     : SYSTEM.UNSIGNED_WORD;
    begin
       TEXT_IO.PUT_LINE ("Enter a filename: ");
       TEXT_IO.GET_LINE (FILENAME, LENGTH);
       CTX := 0;
       STAT := 1;
       while (STAT /= STARLET.RMS_NMF) loop
          LIB.FIND_FILE(STAT,FILENAME(1..LENGTH),RESNAME,CTX);
          if ((not CONDITION_HANDLING.SUCCESS(STAT))
               and (STAT /= STARLET.RMS_NMF)) then
             CONDITION_HANDLING.SIGNAL(STAT);
          end if;
          if (STAT /= STARLET.RMS_NMF) then
             STR.TRIM (STAT, RESNAME,RESNAME,RESLEN);
             TEXT_IO.PUT_LINE (RESNAME(1..INTEGER(RESLEN)));
             TEXT_IO.NEW_LINE;
          end if;
       end loop;
       LIB.FIND_FILE_END (STAT,CT);
       if (not CONDITION_HANDLING.SUCCESS(ISTAT)) then
          CONDITION_HANDLING.SIGNAL(ISTAT);
       end if;
    end print_directory;
 
 

  

     

     
answer written or last revised on ( 9-FEB-2004 )
     
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