The OS Interface

   

  The Ultrix implementation of the type OS.Dir is a text string giving a path name (without symbolic links) to the directory. The Ultrix and Taos implementations behave differently when an open directory is renamed. Compared to Taos, the Ultrix implementation also causes OpenDir to be slower and GetPath to be faster. The Ultrix implementation of OpenDir is faster than usual if the path does not contain any / characters.

The Ultrix implementation of OS does not support remote file access via path names beginning with the # character. Thus such literal path names should not be coded within Topaz applications. Programs should either reference files in GF (the Topaz global file name space), or should use environment variables or other parameterization.

There are many obstacles to sharing open files between an Ultrix Topaz process and another process. Sharing a disk file that isn't in append mode will work correctly if the Topaz process uses OS.Read and OS.Write rather than OS.FRead and OS.FWrite. But there are other problems due to the use of several Ultrix features in the Ultrix implementation of the OS file operations:

• All devices other than terminals and sockets are placed into non-blocking mode, in order to avoid having an operation performed by one thread block within the Ultrix kernel and therefore prevent other threads within the same process from executing. Placing a terminal into non-blocking mode would cause unacceptable interference with the parent process, which is sharing the same open-file object, so it is not done.
• All files are placed in asynchronous mode, so that SigIO signals can be used to schedule I/O even in the presence of lower-priority compute-bound threads. Note that not all device types recognize asynchronous mode.
• All files are placed in non-append mode. This is required to implement OS.FWrite.

When OS.StartProcess is called, each file in the process template (set by OS.SetDescriptor) is unconditionally placed in blocking mode and in synchronous mode. Additionally, if the file had originally come from OS.GetDescriptor and had been in append mode, it is returned to append mode.

For documentation of the Ultrix file modes, see fcntl(2).