Error List

In a DBL program, an I/O error list is associated with a file I/O statement. It details how the program should handle specific I/O errors by redirecting program control to error-handling labels. Here’s the general syntax structure:

IO_statement(arguments)[error_list]

Where:

• IO_statement: This is the file I/O statement that the error list applies to, such as READ, WRITE, OPEN, FIND, etc.
• error_list: This is a comma-separated list of error handlers in the form error_code=label. You can also specify a catch-all error handler if you just put a label at the end of the list.

Components of the error list

• error_code: This is a specific error condition you want to handle, such as $ERR_EOF, $ERR_LOCKED, $ERR_NODUPS, etc. It represents a class of error that the I/O operation might encounter.
• label: This is a user-defined label in the program that execution will jump to if the corresponding error occurs.

Example of an I/O error list:

READ(channel, recordArea, key) [$ERR_EOF=notfound_label, $ERR_LOCKED=retry_label, catch_all_label]

In the above example,

• READ: DBL I/O statement that attempts to read a record.
• recordArea: The variable into which the read data will be placed.
• key: The key value to use for the read operation.
• channel: The channel from which the data is being read.
• $ERR_EOF=notfound_label: If an end-of-file condition is encountered, control transfers to notfound_label.
• $ERR_LOCKED=retry_label: If a record lock error occurs, control transfers to retry_label.
• catch_all_label: If any other error occurs, control transfers to catch_all_label.

Extended example with multiple I/O statements:

main
    record myData
        fld1, d10
        fld2, a100
    endrecord
    record
        chn, int
proc
    open(chn = 0, I:I, "myfile")[$ERR_FNF=file_not_found]
    repeat
    begin
retry_label,
        READS(chn, myData)[$ERR_EOF=end_label, $ERR_LOCKED=retry_label, catch_all]
        nextloop
end_label,
        Console.WriteLine("Reached the end of the file")
        exitloop
catch_all,
        Console.WriteLine("something went wrong the error code was: " + %string(syserr))
        nextloop
    end

    exite
file_not_found,
    Console.WriteLine("The file wasnt found")
endmain