Error Handler

OS SWI Calls

» OS_ChangeEnvironment

» Error Handler

Error Handler

The error buffer (R3 from OS_ChangeEnvironment) will contain the following:

Use

This handler is called by the default claimant of ErrorV? when an error occurs. It should restore all handlers to their original values and then call the non-X form of OS_GenerateError to pass the error to the previous handler.

Notes

This handler is entered in USER mode with the workspace pointer in R0, as opposed to the conventional R12.

The block should be at least 260 bytes in size: 4 bytes for the erroring program counter plus 256 for a standard RISC OS error block. Note that the printed PRM is vague in specifying a “256 byte error buffer”, currently the kernel truncates any error message text to 252 bytes, totalling 4 + 4 + 252 = 260 in the worst case.

There is no guarantee that R13 will contain a valid stack pointer. Even if the foreground task was using R13 as a stack in the usual manner, there is no guarantee that the pointer is still valid – e.g. an RTSupport routine running in SYS mode, or a non-transient callback, may have been using R13 for some other purpose at the time the error was generated. Therefore if a stack is required, it’s recommended that you validate R13, or use a different stack private to the error handler.

It’s also possible that R1-R12 and R14 do not correspond to the values they had when the error was generated.

See also

• OS_ChangeEnvironment
• OS_GenerateError
• ErrorV?