Class Notes (838,386)
CSC209H1 (41)
Karen Reid (20)
Lecture

# mar13c.docx

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Department
Computer Science
Course
CSC209H1
Professor
Karen Reid
Semester
Winter

Description
Function Pointers and Signals Signals  Pointers to Functions  Unexpected/unpredictable asynchronous events o Since a pointer is just an address we can have pointers to functionso Floating point error, death of a child o Ex.1 o Interval timer expired (alarm clock) int cube(int x) { o Terminate, Suspend, Quit and Kill a process return x*x*x; }  Events are called interrupts int (*f)(int); // Declare a function pointer o When the kernel recognizes an event, it sends a signal to the process f = cube; // assign it to cube o Normal processes may send signals  What are signals for? For of communications btwn processes // Call the function that f points to o When a program forks into 2 or more processes, rarely do they printf ("%d\n", (*f)(5)); execute independently  The processes usually requires some form of synchronization, o Ex.2 develop a map function that takes an array of integers and a function to apply to each element often handled by signals 1. #include  To transfer data between processes, we will use pipes and 2. sockets 3. void map(int *nums, int size, int (*f) (int)) { 4. int i; o Signals are generated by 5. for (i = 0; i < size; i++ ) {  Machine interrupts 6. nums[i] = (*f)(nums[i]);  The program itself, other programs or the user 7. } 8. }  Sofware interrupts 9. o lists the signal types on CDF o "man 7 signal" gives some description of various signals 10. int square(int x) { 11. return x * x;  SIGTERM, SIGABRT, SIGKILL, SIGSEGV, SIGBUS, SIGSTOP, 12. } SIGCONT, SIGCHLD, SIGPIPE, SIGUSR1, SIGUSR2 13.  Signal handlers 14. int decrement(int x) { 15. return x-1; o When a C program receives a signal, control is immediately passed to 16. } a function called a signal handler 17. o The signal handler function can execute some C statements and exit 18. int main() { in 3 different ways: 19. int a[3] = {2, 4, 6}; 20. int i;  Return control to the place in the program which was executing 21. printf("original:\n") when the signal occurred 22. for (i = 0; i < 3; i++) {  Return control to some other point in the program 23. printf("a[%d] = %d\n", i, a[i]); 24. }  Terminate the program by calling exit o Default actions 25. map(a, 3, square);  Each signal has a default action 26. printf("\nsquared:\n")  Terminate – kill process w/ an optional core dump 27. for (i = 0; i < 3; i++) { 28. printf("a[%d] = %d\n", i, a[i]);  Stop – supended, process to be resumed later 29. }  Ignore – thrown away the signal 30. map(a, 3, decrement);  The default action can be changed for most signal types using 31. printf("\ndecremented:\n") 32. for (i = 0; i < 3; i++) { the sigaction() function 33. printf("a[%d] = %d\n", i, a[i]);  The exceptions are SIGKILL and SIGSTOP 34. } 35. return 0; int sigaction(int sig, const struct sigaction *act, 36. } struct sigaction *oldact);  Install a signal handler act, for the signal sig  Output \$ gcc -Wall -g -o fpointer fpointer.c  Struct defined in to fill in to pass in for act \$ ./fpointer struct sigaction { original: // SIG_DFL, SIG_IGN, or pointer to function a[0] = 2 void (*sa_handler)(int); a[1] = 4 sigset_t sa_mask; // sigs to block during handler a[2] = 6 int sa_flags; // flags and options }; squared:  You may come across various extensions, including another filed a[0] = 4 in the signaction struct for a function to catch signals a[1] = 16 a[2] = 36 o For each process, UNIX maintains a table of actions that should be performed for each kind of signal decremented: SIGNALS Default Shortcut Comment a[0] = 3 Action a[1] = 15 a[2] = 35 SIGKILL Terminate Terminate a process SIGINT Terminate Ctrl-C Terminate a process by user o Ex.3 qsort() used for generic quick sort routine input SIGQUIT Terminate, Ctrl-\ Terminate a process by user void qsort(void *base, size_t nmeb, size_t size, Dump Core input, then core dump int (* compar)(const void *, const void *));  compar is a pointer to a comparison function for the SIGSTOP Stop Ctrl-Z Suspend a process to be resumed later elements to be sorted SIGSEGV Terminate, Segmentation fault,  Just give a pointer to a function like strcmp Dump Core Invalid memory ref  Similar to bserach() for binary search
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