Exception handling

Description

Sometimes code attempts operations that are not legal on the current hardware, for example dividing by zero, or accessing data through a pointer that is not properly aligned. When this happens the hardware will raise an exception. This is very similar to an interrupt, but happens synchronously with code execution rather than asynchronously and hence can be tied to the thread that is currently running.

The exceptions that can be raised depend very much on the hardware, especially the processor. The corresponding documentation should be consulted for more details. Alternatively the architectural HAL header file hal_intr.h, or one of the variant or platform header files it includes, will contain appropriate definitions. The details of how to handle exceptions, including whether or not it is possible to recover from them, also depend on the hardware.

Exception handling is optional, and can be disabled through the configuration option CYGPKG_KERNEL_EXCEPTIONS. If an application has been exhaustively tested and is trusted never to raise a hardware exception then this option can be disabled and code and data sizes will be reduced somewhat. If exceptions are left enabled then the system will provide default handlers for the various exceptions, but these do nothing. Even the specific type of exception is ignored, so there is no point in attempting to decode this and distinguish between say a divide-by-zero and an unaligned access. If the application installs its own handlers and wants details of the specific exception being raised then the configuration option CYGSEM_KERNEL_EXCEPTIONS_DECODE has to be enabled.

An alternative handler can be installed using cyg_exception_set_handler. This requires a code for the exception, a function pointer for the new exception handler, and a parameter to be passed to this handler. Details of the previously installed exception handler will be returned via the remaining two arguments, allowing that handler to be reinstated, or null pointers can be used if this information is of no interest. An exception handling function should take the following form:

void
my_exception_handler(cyg_addrword_t data, cyg_code_t exception, cyg_addrword_t info)
{
    …
}
      

The data argument corresponds to the new_data parameter supplied to cyg_exception_set_handler. The exception code is provided as well, in case a single handler is expected to support multiple exceptions. The info argument will depend on the hardware and on the specific exception.

cyg_exception_clear_handler can be used to restore the default handler, if desired. It is also possible for software to raise an exception and cause the current handler to be invoked, but generally this is useful only for testing.

By default the system maintains a single set of global exception handlers. However, since exceptions occur synchronously it is sometimes useful to handle them on a per-thread basis, and have a different set of handlers for each thread. This behaviour can be obtained by disabling the configuration option CYGSEM_KERNEL_EXCEPTIONS_GLOBAL. If per-thread exception handlers are being used then cyg_exception_set_handler and cyg_exception_clear_handler apply to the current thread. Otherwise they apply to the global set of handlers.

Valid contexts

If the system is configured with a single set of global exception handlers then cyg_exception_set_handler and cyg_exception_clear_handler may be called during initialization or from thread context. If instead per-thread exception handlers are being used then it is not possible to install new handlers during initialization because the functions operate implicitly on the current thread, so they can only be called from thread context. cyg_exception_call_handler should only be called from thread context.