Limitations of Mapped Files
The "Mapped Files" studio outlines the basic use of mapped files but fails to explicitly mention a number of limitations.
Size Should Not Change
The examples in the studio show how the file can be changed by manipulating the mapped variable but uses an example that does not change the size of the variable or the file. So, this works as advertised:
'1234567' fappend flnm=. 'NewFile.txt' require 'jmf' JCHAR map_jmf_ 'var';flnm var=: 3|.var fread flnm 4567123
But this fails:
var=: var,'8910' |allocation error | var =:var,'8910'
because it attempts to increase the size of the variable. Attempting to decrease the size does not give an error but the result may be unexpected:
var=: 2}.var var 67123 fread flnm 6712323
It looks like the new, shorter value of the variable simply overwrites the initial part of the file.
Similarly, changing the size of the file is not reflected in the mapped variable:
'XXXX' fappend flnm 4 var 23 fread flnm 2312323XXXX
Once the variable is re-mapped to the file, it will reflect the changes:
unmap_jmf_ 'var' 0 JCHAR map_jmf_ 'var';flnm var 2312323XXXX
Temporarily Decreasing Size
A further complication is that the variable may be shortened, then lengthened back to the original size:
var=: '12345',~5}.var var 23XXXX12345 fread flnm 23XXXX12345
This does not have to be done all at once:
var=: 3}.var var=: var,'foo' fread flnm XXX12345foo