File Equations in MPE

File Equations in MPE

On MPE, :File equations are commands that redefine the attributes of a file, including perhaps the actual filename. When a program attempts to open a file named XXX, MPE looks for a file equation of the form :FILE XXX= which could redirect the file to another one or change the way it is accessed. If the program opens a file named *XXX, then there must be a :File equation for XXX or the FOPEN call fails.

UNIX does not have :File Equations, mostly because it does not have file attributes, but it does have symbolic links which allow more than one name for the same file.

When something mysterious happens with an MPE file, use the :Listeq command to check for :File equations and the :Listftemp command to check for temporary files. Although all :File parameters correspond to some FOPEN or FCLOSE parameters, not all FOPEN and FCLOSE parameters can be redefined with :File. For instance, you cannot redefine the number of user labels or the flag indicating whether to retain space between the end-of-file and the file-limit on close.