Memory: Copy On Write

CS 321 Lecture, Dr. Lawlor, 2006/02/22

See Silberschatz Chapter 9.3 for Copy-On-Write info.

Bad Memory Handlers

When a program tries to access bad memory (memory whose pagetable entry is marked "bad"), the CPU executes the segfault interrupt handler. This OS code gets the following information:

What process was running (the OS can keep track of this itself)
What address the program was trying to access (the CPU passes this)
What the program was trying to do at that address--read, write, etc.

See the Microsoft Windows SetUnhandledExceptionFilter and ExceptionInformation for details on how this information can be delivered to a normal program. It boils down to this code:

/**
 Signal handler, called by the OS when any exception occurs.
*/
LONG WINAPI
badMemSignal(
    struct _EXCEPTION_POINTERS *ExceptionInfo
    )
{
    EXCEPTION_RECORD *er=ExceptionInfo->ExceptionRecord;
    if (er->ExceptionCode==EXCEPTION_ACCESS_VIOLATION) {
       bool isWrite=er->ExceptionInformation[0];
       void *badAddress=(void *)er->ExceptionInformation[1];
       /* ... check address, fix memory, etc. ... */
       return EXCEPTION_CONTINUE_EXECUTION;
    }
    return EXCEPTION_CONTINUE_SEARCH;
}

/* Call at the beginning of the program */
void badMemSetup {
    SetUnhandledExceptionFilter(badMemSignal);
}

See GNU libsigsegv for a decent interface for UNIX systems.