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5.7 Controlling return place after handler
Motivation
* Synchronous events, e.g. divide by zero, null pointer * Returning to the instruction causing error - implies infinite loop * Default: exit() or abort() in handler * Q? What if we don't want to terminate process/thread? Change place of return after handler * Increment program counter (not portable) * Goto - missing stack management * siglongjmp => proper stack management System calls: Siglongjmp(), sigsetjmp() (pp. 192) * Like "goto" and "set label" but * Unravel function call stack properly! * Portable across POSIX.1 operating systems
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5.7 Controlling return place after handler
Synopsis (pp. 192) #include <setjmp.h> int sigsetjmp(sigjmp_buf env, int savemask); void siglongjmp(sigjmp_buf env, int val);
Details of sigsetjmp() * sigsetjmp() - set the "label" in "env" * argument 1 = stores <program counter, state of stack> * nonzero argument 2 => save signal_mask as well! * returns 0 if called directly * returns "val" when returning from siglongjmp() - like exit(v) -- wait(&status) * Call sigsetjmp() at the return point after handler Details of siglongjmp() * siglongjmp(env, val) - similar to "goto" label * argument 1 to restores <program counter, stack, mask> * argument 2 used to set return value for sigsetjmp() * Call inside the handler as the last step * Must have called sigsetjmp() before calling siglongjmp()
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5.7 Controlling return place after handler
Example: Handling SIGSEGV, SIGFPE void handler( sig int ) { /* code fragment */ siglongjmp(jmpbuf, 2); } void test() { int a = 0; a = a/ a ; /* SIGFPE */ } int main() { /* install handler for SIGFPE, SIGSEGV */ /* unmask SIGFPE, SIGSEGV */ switch (status = sigsetjmp(jmpbuf, 1) ) { case -1: exit(0); case 0 : test() ; case 2 : /* code after return from handler */ } }
Ex. Program 5.2, pp. 192-3 * Q? Where do we return after int_handler()? * Q? What will the program print? * Q? Why use "jumpok" ?
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5.7 Unwinding the Stack
objects on stack are destroyed
* local variables, * local class objects destructors are called Program state (stack, mask) goes back to a previous state * global data may have changed * existing objects may have changed Issues * What recovery is possible? Can data be saved? * What dynamic memory items need to be released? * Can intermediate changes to global data be undone?
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5.7 Controlling return place after handler
Alternatives ways for unwindi...