HP OpenVMS Systems

C RTL I/O stream and record context?

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The Question is:

 
The C RTL fopen() and creat() calls support optional RMS options specified as
 additional parameters to the call. What, preciesly, do the CTX=STM and CTX=REC
 options do? The CRTL Ref Manual (Table REF-3) simply states that they cause
 they force stream or r
ecord mode access, and refers the reader to the RMS Reference Manual for
 further information. The RMS manual has nothing about stream or record access
 mode that I could find. Of course, the CTX field in RMS is used for user- or
 application-specific data,
so presumably the C RTL routines are using CTX as a placeholder to indicate the
 access mode here.
 
Context: I'm porting an O-O language processor and development system from UNIX
 to VMS, and I'm having some problems with illegal file specs that might be
 resolved by using the VMS Porting library ("THE JACKETS"). However, when I use
 the fopen jacket, whi
ch specifies CTX=STM, the system does not read the same stream of characters
 that it gets when using an unmodified fopen() call. I'm trying to understand
 precisely how the CTX=STM and CTX=REC options affect the data returned when
 reading RMS files.
 

The Answer is :

 
  Current versions of the C RTL when combined with the current compiler
  and ODS-5 can translate many native UNIX file specifications -- though
  not all -- into the specification format native to OpenVMS.
 
  As you are undoubtedly aware, illegal file specifications are unlikely
  related to the file access mode.  As no details of the illegal file
  specifications were included, no specific options can be provided.
 
  Chapter one of the C RTL Reference Manual contains a detailed discussion
  of the access context options.  The core text:
 
    "RMS sequential files can be opened in record mode or stream mode. By
    default, STREAM_LF files are opened in stream mode; all other file
    types are opened in record mode. When opening a file, you can override
    these defaults by specifying the optional argument "ctx=rec" to force
    record mode, or "ctx=stm" to force stream mode. RMS relative and
    indexed files are always opened in record mode.  The access mode
    determines the behavior of various I/O functions in the Compaq C
    RTL."
 
  Stream files are the closest organization to the native UNIX file system.
  Files with record structures are native to OpenVMS, and provide for RMS
  caching and various file organizations not necessarily found integrated
  directly in other operating system platforms.
 
  The OpenVMS Wizard often prefers direct RMS access when specific control
  over RMS activities is required, as the C RTL provides an emulation of
  UNIX I/O on C and mechanisms to control this emulation.  While the C
  RTL approach is quite functional, direct RMS access can be easier to
  understand as there is no need to explicitly disable the RTL capabilities
  and assumptions provided to support C norms and C applications written
  for other platforms.
 

  

     

     
answer written or last revised on ( 17-SEP-2002 )
     
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