213 RecitationJiri [email protected] shell hints• Executing programs• Understanding process tree• Reap all child processes• Avoid race hazard• I/O redirectionExecute programint execve(const char *fname, char *const argv[], char *const envp[]);Examples:execve(“/bin/ls”, NULL, NULL);execve(“./mytest”, argv, envp);Process tree for a shellFore-groundjobBack-groundjob #1Back-groundjob #2ShellChild Childpid=10pgid=10Foregroundprocess group 20Backgroundprocess group 32Backgroudprocess group 40pid=20pgid=20pid=32pgid=32pid=40pgid=40pid=21pgid=20pid=22pgid=20Each job has a unique process group idint setpgid(pid_t pid, pid_t pgid);setpgid(0, 0);Process tree for a shellFore-groundjobBack-groundjob #1Back-groundjob #2tshChild Childpid=10pgid=10Foregroundprocess group 20Backgroundprocess group 32Backgroudprocess group 40pid=20pgid=20pid=32pgid=32pid=40pgid=40pid=21pgid=20pid=22pgid=20Foreground jobreceives SIGINT, SIGTSTP,when you type ctrl-c, ctrl-zForward signalsint kill(pid_t pid, int sig)pid > 0: send sig to specified processpid = 0: send sig to all processes in same grouppid < -1: send sig to group -pidReaping child processwaitpid(pid_t pid, int *status, int options)pid: wait for child process with pid (process id or group id)-1: wait for any child processstatus: tell why child terminatedoptions:WNOHANG: return immediately if no children zombiedWUNTRACED: report status of terminated or stopped childrenIn Lab 4, use:waitpid(-1, &status, WNOHANG|WUNTRACED)In sigchld_handler(): while ((c_pid = waitpid(…)) > 0)Reaping child processint status;waitpid(pid, &status, WNOHANG|WUNTRACED)What to check in sigchld_handler: WIFEXITED(status): child exited normally (the child finished, and quit normally on its own) WEXITSTATUS(status): returns code when child exits WIFSIGNALED(status): child exited because a signal is not caught (SIGINT, or typing CTRL-C) WTERMSIG(status): gives the terminating signal number WIFSTOPPED(status): child is stopped by the receipt of a signal (SIGSTOP, or typing CTRL-Z) WSTOPSIG(status): gives the stop signal numberReaping child processWhere to put waitpid(…)? eval() vs. sigchld_handler()In sigchld_handler(): for botheval() should wait for a fg jobBusy wait for a fg jobIn eval():if(fork() != 0) { /* parent */ addjob(…); while(fg process still alive){ /* do nothing */ }}SleepIn eval():if(fork() != 0) { /* parent */ addjob(…); while(fg process still alive){ sleep(1); }}Race hazardA data structure is shared by twopieces of code that can runconcurrentlyDifferent behaviors of programdepending upon how the scheduleinterleaves the execution of code.An example of race hazardsigchld_handler() { while ((pid = waitpid(…)) > 0){ deletejob(pid); }}eval() { pid = fork(); if(pid == 0) { /* child */ execve(…); } /* parent */ /* signal handler may run BEFORE addjob()*/ addjob(…);}ShellSignal Handler Childfork()addjob()execve()exit()sigchld_handler()deletejob()timeShellSignal Handler Childfork()execve()exit()sigchld_handler()deletejob()timeaddjob()Job added to job list after the signal handler tried to delete it!Solution: blocking signalssigchld_handler() { pid = waitpid(…); deletejob(pid);}eval() { sigprocmask(SIG_BLOCK, …) pid = fork(); if(pid == 0) { /* child */ sigprocmask(SIG_UNBLOCK, …) execve(…); } /* parent */ /* signal handler might run BEFORE addjob() */ addjob(…); sigprocmask(SIG_UNBLOCK, …)}I/O redirection Do it before call execve() in child processeval() { pid = fork(); if(pid == 0) { /* child */ /* Redirect I/O */ if (redirect_input) dup2(…);/* redirect STDIN */ if (redirect_output) dup2(…);/* redirect STDOUT */ execve(…); } addjob(…);}dup2(int oldfd, int newfd) Covered in Chapter 11oldfd: old file descriptornewfd: new file descriptor Dup2 makes newfd be a copy of the oldfdSome examples:Get input from in_fd instead of standard inputdup2(in_fd, STDIN_FILENO);Print output to out_fd instead of standard outputdup2(out_fd, STDOUT_FILENO);Reminders Some important system calls: fork, execve, waitpid, sigprocmask, setpgid, kill Check man pages for details about system calls man 2 kill Check return values of all system calls STEP by STEP Test your shell by typing commands first Each trace worth the same (work on easy ones first!) Start
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