Sysprog 12

Sysprog 12

• 1.
  C/C++ Linux SystemProgramming Session 12 User-space System Programming – session 2
• 2.
  Outline Signals JobControl Scheduling IPC Intro
• 3.
  Sending Signals intkill(pid_t pid, int sig); int raise(int sig); // = kill(getpid(), sig); int sigqueue(pid_t pid, int sig, const union sigval value); // value is a payload (IPC w/data!!)
• 4.
  Handling Signals –old school Handler typedef void (*sighandler_t)(int); sighandler_t signal(int signum, sighandler_t handler); SIG_IGN / SIG_DFL Example from sshd.c: static void main_sigchld_handler(int sig) { int save_errno = errno; pid_t pid; int status; while ((pid = waitpid(-1, &status, WNOHANG)) > 0 || (pid < 0 && errno == EINTR)) ; signal(SIGCHLD, main_sigchld_handler); errno = save_errno; }
• 5.
  Signal sets intsigemptyset(sigset_t *set); // Init int sigfillset(sigset_t *set); // Init with all int sigaddset(sigset_t *set, int signum); int sigdelset(sigset_t *set, int signum); int sigismember(const sigset_t *set, int signum); Non-POSIX: int sigisemptyset (sigset_t *set); int sigorset (sigset_t *dest, sigset_t *left, sigset_t *right); int sigandset (sigset_t *dest, sigset_t *left, sigset_t *right);
• 6.
  Masking Mask/Unmask intsigprocmask(int how, const sigset_t *set, sigset_t *oldset); how: SIG_BLOCK/SIG_UNBLOCK/SIG_SETMASK Check pending (temporarily masked) int sigpending(sigset_t *set); Waiting for a signal int pause(void); int sigsuspend(const sigset_t *mask);
• 7.
  Masking Example -ssh/server_loop.c /* block SIGCHLD while we check for dead children */ sigemptyset(&nset); sigaddset(&nset, SIGCHLD); sigprocmask(SIG_BLOCK, &nset, &oset); if (child_terminated) { debug(&quot;Received SIGCHLD.&quot;); while ((pid = waitpid(-1, &status, WNOHANG)) > 0 || (pid < 0 && errno == EINTR)) if (pid > 0) session_close_by_pid(pid, status); child_terminated = 0; } sigprocmask(SIG_SETMASK, &oset, NULL); }
• 8.
  Versatile Signal HandlingInterface int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact); Important sigaction fields: void (*sa_handler)(int); void (*sa_sigaction)(int, siginfo_t *, void *); sigset_t sa_mask; Important siginfo_t: si_signo , si_uid, si_value, si_addr
• 9.
  Example - inetdmemset(&sa, 0, sizeof(sa)); sigaddset(&sa.sa_mask, SIGALRM); sigaddset(&sa.sa_mask, SIGCHLD); sigaddset(&sa.sa_mask, SIGHUP); sa.sa_handler = retry_network_setup; sigaction_set(SIGALRM, &sa); sa.sa_handler = reread_config_file; sigaction_set(SIGHUP, &sa); sa.sa_handler = reap_child; sigaction_set(SIGCHLD, &sa); sa.sa_handler = clean_up_and_exit; sigaction_set(SIGTERM, &sa); sa.sa_handler = clean_up_and_exit; sigaction_set(SIGINT, &sa); sa.sa_handler = SIG_IGN; sigaction(SIGPIPE, &sa, &saved_pipe_handler); static void clean_up_and_exit(int sig UNUSED_PARAM){ ,,,, remove_pidfile(_PATH_INETDPID); exit(EXIT_SUCCESS); } int FAST_FUNC sigaction_set(int signum, const struct sigaction *act){ return sigaction(signum, act, NULL); }
• 10.
  Some Signal NotesBehavior in fork Behavior in exec Process Group relevance System call interruptions Intro to race conditions Critical region Reentrancy Minimal work Sigatomic_t
• 11.
  system/popen fork/exec(/bin/sh) Security hole Signal blocking
• 12.
  Scheduling Time-sharing (timeslices)States Process table Context switch Priorities Preemptive vs Cooperative Multitasking Idle process
• 13.
  2.4 Scheduling Nicevalues [-20,19], -20 is not nice, i.e. high priority Time slice length & run queue order Superuser to decrement int nice(int inc); // nice(0) is current get/set priority int getpriority (int which, int who); int setpriority (int which, int who, int prio); Absolute PRIO_RPOCESS, PRIO_PGRP, PRIO_USER
• 14.
  Relinquishing CPU intsched_yield why? 2.4 vs 2.6 (RT vs non-RT) Sleep unsigned int sleep(unsigned int seconds); int nanosleep(const struct timespec *rqtp, struct timespec *rmtp); int usleep(useconds_t usec);
• 15.
  Real-time Scheduling Real-time- soft/hard determinism Priorities: 1-99, 0 non-RT Scheduling classes Round-robin FIFO Chrt Be “nice” !!! struct sched_param { /* ... */ int sched_priority; /* ... */ }; int sched_getparam (pid_t pid, struct sched_param *sp); int sched_setparam (pid_t pid, const struct sched_param *sp);
• 16.
  Processor Affinity SMPAffinity: Which processor? By default, try the same and hereditary Do I care? -- Cache coherency Do I really care? http://www.linuxjournal.com/article/6799 pid = atol(argv[1]); sscanf(argv[2], &quot;%08lx&quot;, &new_mask); if (sched_getaffinity(pid, len, &cur_mask) < 0) { perror(&quot;sched_getaffinity&quot;); return -1; } printf(&quot;pid %d's old affinity: %08lx\n&quot;, pid, cur_mask); if (sched_setaffinity(pid, len, &new_mask)) { perror(&quot;sched_setaffinity&quot;); return -1; } if (sched_getaffinity(pid, len, &cur_mask) < 0) { perror(&quot;sched_getaffinity&quot;); return -1; }
• 17.
  Time Monotonic time/ real time / process time POSIX int clock_getres(clockid_t clk_id, struct timespec *res); int clock_gettime(clockid_t clk_id, struct timespec *tp); int clock_settime(clockid_t clk_id, const struct timespec *tp);
• 18.
  System time get/setint gettimeofday(struct timeval *tv, struct timezone *tz); int settimeofday(const struct timeval *tv, const struct timezone *tz); time_t time(time_t *t); // seconds Printable char *ctime(const time_t *timep); //And family int adjtime(const struct timeval *delta, struct timeval *olddelta);
• 19.
  Examples from NTPgettimeofday(&tv, 0); epoch = tv.tv_sec; ... fprintf(stdout, &quot;# %s\n# %s&quot;, filename, ctime(&epoch)); curtime = time(0); printf(&quot;Starting: %s&quot;, ctime(&curtime));
• 20.
  Interval Timers Seta timer Signal when expired int getitimer(int which, struct itimerval *value); int setitimer(int which, const struct itimerval *value, struct itimerval *ovalue); struct itimerval { struct timeval it_interval; /* next value */ struct timeval it_value; /* current value */ }; struct timeval { long tv_sec; /* seconds */ long tv_usec; /* microseconds */ }; unsigned int alarm(unsigned int seconds);
• 21.
  Example – ping.cstatic void noresp(int ign UNUSED_PARAM) { printf(&quot;No response from %s\n&quot;, hostname); exit(EXIT_FAILURE); } main() { ... signal(SIGALRM, noresp); alarm(5); /* give the host 5000ms to respond */ ... }
• 22.
  IPC Intro Sharedata Synchronize Mutual Exclusion Critical region Semaphores