Abnormal process termination

A process can be terminated abnormally for a number of reasons. Ultimately, all of these reasons will result in a signal's being set on the process. A signal is something that can interrupt the flow of your threads at any time. The default action for most signals is to terminate the process.

Note: Note that what causes a particular signal to be generated is sometimes processor-dependent.

Here are some of the reasons that a process might be terminated abnormally:

If any thread in the process tries to use a pointer that doesn't contain a valid virtual address for the process, then the hardware will generate a fault and the kernel will handle the fault by setting the SIGSEGV signal on the process. By default, this will terminate the process.
A floating-point exception will cause the kernel to set the SIGFPE signal on the process. The default is to terminate the process.
If you create a shared memory object and then map in more than the size of the object, when you try to write past the size of the object you'll be hit with SIGBUS. In this case, the virtual address used is valid (since the mapping succeeded), but the memory cannot be accessed.

To get the kernel to display some diagnostics whenever a process terminates abnormally, configure procnto with multiple -v options. If the process has fd 2 open, then the diagnostics are displayed using (stderr); otherwise; you can specify where the diagnostics get displayed by using the -D option to your startup. For example, the -D as used in this buildfile excerpt will cause the output to go to a serial port:

[virtual=x86,bios +compress] .bootstrap = {
    startup-bios -D 8250..115200
    procnto -vvvv
}

You can also have the current state of a terminated process written to a file so that you can later bring up the debugger and examine just what happened. This type of examination is called postmortem debugging. This happens only if the process is terminated due to one of these signals:

Signal	Description
SIGABRT	Program-called abort function
SIGBUS	Parity error
SIGEMT	EMT instruction (emulation trap) Note that SIGEMT and SIGDEADLK (mutex deadlock; see SyncMutexEvent() ) use the same signal number.
SIGFPE	Floating-point error or division by zero
SIGILL	Illegal instruction executed One possible cause for this signal is trying to perform an operation that requires I/O privileges. To request these privileges: Enable the PROCMGR_AID_IO ability. For more information, see procmgr_ability() . Call ThreadCtl() , specifying the _NTO_TCTL_IO flag: ThreadCtl( _NTO_TCTL_IO, 0 );
SIGQUIT	Quit
SIGSEGV	Segmentation violation
SIGSYS	Bad argument to a system call
SIGTRAP	Trace trap (not reset when caught)
SIGXCPU	Exceeded the CPU limit
SIGXFSZ	Exceeded the file size limit

The process that dumps the state to a file when the process terminates is called dumper , which must be running when the abnormal termination occurs. This is extremely useful, because embedded systems may run unassisted for days or even years before a crash occurs, making it impossible to reproduce the actual circumstances leading up to the crash.