111CMSC 212 – S07 (lect 17)AnnouncementsProgram #4 is on the web– Change is submit setup• 2 tokens/24 hours• Release tests will show outputMidterm #2– Tuesday, April 10 (6:00-7:30pm in ARM 0126)– No class on April 10 Reading– Bryant & O’Hallaron 8.5, 11.{6,7,8} (Today)– Bryant & O’Hallaron Chapter 12 (Thursday)2CMSC 212 – S07 (lect 17)File Descriptor Vs. FILE*File Descriptors– Permit unbuffered I/O– Are integers– Used or returned by - open, read, writeFile Pointers– FILE*– Are used for buffered I/O– Are pointers to a struct• Struct contains a file descriptor and buffers– Are used or returned by fopen, fwrite, fprintf, ….223CMSC 212 – S07 (lect 17)File DescriptorsAll UNIX I/O is done via file descriptors– They are integer numbers– Represent an abstract data type used by the operating system– open system call returns a new file descriptor– Read and write system call use file descriptors0123File posRef CntFile posRef CntDescriptor Table(per process)File SizeFile AccessV-node table(per file)File SizeFile Access4CMSC 212 – S07 (lect 17)File Table After Forking a Process0123File posRef CntFile posRef CntDescriptor Table(parent)File SizeFile AccessV-node table(per file)File SizeFile Access0123Descriptor Table(child)335CMSC 212 – S07 (lect 17)Pipe System Callint pipe(int fd[2]);– Creates a communication channel between two file descriptors– Data written to fd[1] can be read from fd[0]– Data written to fd[0] can be read from fd[1]0123FIFORef CntDescriptor Table(per process)46CMSC 212 – S07 (lect 17)Changing Standard Input/OutputUseful to combine pipe, dup2, and exec….pipe(pipefds);pid = fork();if (pid == 0) {dup2(0, pipefd[0]);close(pipefd[1]);ret = execvp("ls", args);….else if (pid > 0) {dup2(1, pipefd[1]);close(pipefd[0]);} else {….}447CMSC 212 – S07 (lect 17)A Simple Shell Programwhile (1) {ret = fgets(line, sizeof(line), stdin);…/* convert line into array of arguments */pid = fork();if (pid == 0) {ret = execvp(args[0], args);if (ret) {printf("Command not found\n");exit(-1);} } else if (pid > 0) {ret = waitpid(pid, &status, NULL);} else { … }}8CMSC 212 – S07 (lect 17)Adding a Pipe Command to the Shell/* skipped: split line into an array of commands to run */for (i=0; i < num; i++) {if (i < num-1) pipe(outfds);cmd[i].pid = fork();if (pid == 0) {if (i > 0) dup2(0, infds[0]);if (i < num-1) dup2(1, outfds[1]);close(infds[1]);close(outfds[0]);ret = execvp(cmd[i].args[0], cmd[i].args);if (ret) {printf("Command not found\n");exit(-1);}} else if (pid > 0) {memcpy(infds, outfds, sizeof(int)*2));}}/* wait for all forked commands to exit */559CMSC 212 – S07 (lect 17)SignalsA way for processes to communicate exceptional eventsCan orignate from:– Hardware events: • SIGFPE – floating point exception• SIGILL – illegal instruction• SIGSEGV – segmentation violation– User interaction: • SIGKILL – kill the program• SIGINT – suspend process (control-z)• SIGCHLD – child process stopped or terminated10CMSC 212 – S07 (lect 17)Process GroupsEvery process is the member of one group– A small integer of type pid_tA child process inherits its parent’s groupA process can change its group– Setpgid(pid_t pid, pid_t pgid)• setpgid(0,0) – create a new group for the current proccess6611CMSC 212 – S07 (lect 17)Sending SignalsKill command sends a signal– From C: int kill(pid_t pid, int sig);– From command line: kill –signal <pid>A negative pid– Sends a signal to all processes in group abs(pid)When a process receives a signal– Might terminate– Might ignore signal– Might run a function12CMSC 212 – S07 (lect 17)Action Taken on a signalHandler_t *signal(int signum, handler_t *handler)– Defines that on signal signum, handler function will be called– Returns • previous handler on success,• SIG_ERR on failure– Special handlers• SIG_IGN – ignore the signal• SIG_DFL – restore default action for this signal number7713CMSC 212 – S07 (lect 17)Timer Driven SignalsCan schedule a signal to be delivered– Unsigned int alarm(unsigned int secs);• In secs seconds a SIGALARM will be send to this process• Return value is the time remaining on a previous alarmExample:void alarmHandler(int sig) {static int nag = 0;Printf(“NAG!\n”);If (++nag < 5) {alarm(1);} else {exit(0);}}int main(int argc, char *argv[]) {signal(SIGALARM, alarmHandler);while (1) {…. Do some other
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