ON UNDEFINEDFILE PL/1 怎么使用

UNDEFINEDFILE is a PL/1 compiler option that specifies the action to be taken when a program attempts to read from a file that has not been opened or has been closed unexpectedly.

To use UNDEFINEDFILE in PL/1, you need to specify it as a compiler option when compiling your program. Here's an example:

/* PL/1 program to demonstrate UNDEFINEDFILE */

OPTIONS UNDEFINEDFILE(ERROR);

DCL MYFILE FILE RECORD INPUT;
DCL MYVAR CHAR(10);

OPEN MYFILE(FILENAME('myfile.txt'));

READ MYFILE INTO(MYVAR);

CLOSE MYFILE;

END;

In this example, we have specified the UNDEFINEDFILE option as ERROR, which means that the program will terminate with an error message if it attempts to read from a file that has not been opened or has been closed unexpectedly.

You can also specify UNDEFINEDFILE as WARNING or IGNORE. If you specify it as WARNING, the program will issue a warning message but will continue executing. If you specify it as IGNORE, the program will ignore the error and continue executing as if nothing happened.

Note that UNDEFINEDFILE is a compiler option and not a PL/1 statement. Therefore, you need to specify it separately from your program code.