Exception Control

Event I/O can generate two types of exceptions: I/O errors and EOFs. How these are handled is specified in the TEIO bank and can be controlled by the following symbolic commands:-

$end_on_eof

This can be set to $ENABLED or $DISABLED. If enabled then, when an EOF is generated (which may not be simply when an EOF is reached - see section 3.9.3), the program switches to the termination phase. If disabled, the INP processor returns signalling EOF and allows the user to handle it. The default is enabled.

$return_eof

This can be set to $ENABLED or $DISABLED. If enabled then, each time a file reaches it EOF or its event limit (as specified by the MAX option) EIO returns an ``internal EOF'' that is to say it returns QUIT with LMAST set to zero. This feature is used in conjunction with the Random Access Analyser (see section 8.1. The default is disabled.

$skip_on_error

This can be set to $ENABLED or $DISABLED. If enabled then, when an error is encountered, the program aborts the current event and proceeds to the next. If disabled, the INP or OUT processors return signalling an error and allows the user to handle it. If the error occurs while the program is obeying a SKIP option then it is ignored, although it is reported and is included in the I/O error count. The default is enabled.

$io_err_lim

This defines the maximum value of the I/O error count per file to be accepted before aborting the run. The default is 0.

The user may wish to consider overriding the defaults if:-

• Trying to read a damaged file.

• Calling the input and output processors from user code and wanting to retain control when exceptions occur.