If you are opening an existing file, all file systems linked into the runtime are searched for the named file. If, however, you are creating a new file, you must tell the runtime which file system to use. You accomplish this with one of two configuration file variables; the first is:
DEFAULT_HOST filesystem
This designates the file system to be used for newly created files which are not individually assigned. For example,
DEFAULT_HOST ORACLE
means that all new files will be Oracle files unless otherwise specified by the second configuration variable, which is:
filename_HOST filesystem
where filename is the file name, without any extension, named in the ASSIGN TO clause of your SELECT statement. This configuration variable is used to assign an individual data file to a file system. Any file so assigned will use the designated file system, and not the one specified by DEFAULT_HOST. For example,
myfile_HOST VISION
means that myfile will be under the Vision file system.
You can use these configuration file entries in combination to assign your new files in a default with exceptions manner; for example, this set of entries:
DEFAULT_HOST ORACLE
afile_HOST VISION
bfile_HOST VISION
means that all new files except afile and bfile will be assigned to Oracle, and those two files will be assigned to Vision.
You can change the values of these variables during program execution by including in your code:
SET ENVIRONMENT "filename_HOST" TO filesystem
or
SET ENVIRONMENT "DEFAULT_HOST" TO filesystem
This enables you to change file systems during the execution of your program. (This is not the typical way to specify a file system; normally it is designated in the runtime configuration file and is not changed in the COBOL program.)