signal.c

   1 /*
   2  * Copyright (C) 2004-2013 Andre Noll <maan@systemlinux.org>
   3  *
   4  * Licensed under the GPL v2. For licencing details see COPYING.
   5  */
   6 /** \file signal.c Signal handling functions. */
   7
   8 #include <signal.h>
   9 #include <sys/types.h>
  10
  11 #include "para.h"
  12 #include "error.h"
  13 #include "fd.h"
  14
  15 static int signal_pipe[2];
  16
  17 /**
  18  * Initialize the paraslash signal subsystem.
  19  *
  20  * This function creates a pipe, the signal pipe, to deliver pending
  21  * signals to the application (Bernstein's trick). It should be called
  22  * during the application's startup part, followed by subsequent calls
  23  * to para_install_sighandler() for each signal that should be caught.
  24  *
  25  * A generic signal handler is used for all signals simultaneously. When a
  26  * signal arrives, the signal handler writes the number of the signal received
  27  * to one end of the signal pipe. The application can test for pending signals
  28  * by checking if the file descriptor of the other end of the signal pipe is
  29  * ready for reading, see select(2).
  30  *
  31  * \return This function either succeeds or calls exit(2) to terminate the
  32  * current process. On success, the file descriptor of the read end of the
  33  * signal pipe is returned.
  34  */
  35 int para_signal_init(void)
  36 {
  37         int ret;
  38         if (pipe(signal_pipe) < 0) {
  39                 ret = -ERRNO_TO_PARA_ERROR(errno);
  40                 goto err_out;
  41         }
  42         ret = mark_fd_nonblocking(signal_pipe[0]);
  43         if (ret < 0)
  44                 goto err_out;
  45         ret = mark_fd_nonblocking(signal_pipe[1]);
  46         if (ret < 0)
  47                 goto err_out;
  48         return signal_pipe[0];
  49 err_out:
  50         PARA_EMERG_LOG("%s\n", para_strerror(-ret));
  51         exit(EXIT_FAILURE);
  52 }
  53
  54 /* Write the signal number to signal pipe. */
  55 static void generic_signal_handler(int s)
  56 {
  57         /*
  58          * Signal handlers that make system calls must save a copy of errno on
  59          * entry to the handler and restore it on exit, to prevent the
  60          * possibility of overwriting a errno value that had previously been
  61          * set in the main program.
  62          */
  63         int save_errno = errno;
  64         ssize_t ret = write(signal_pipe[1], &s, sizeof(int));
  65
  66         if (ret == sizeof(int)) {
  67                 errno = save_errno;
  68                 return;
  69         }
  70         if (ret < 0)
  71                 PARA_EMERG_LOG("%s\n", strerror(errno));
  72         else
  73                 PARA_EMERG_LOG("short write to signal pipe\n");
  74         exit(EXIT_FAILURE);
  75 }
  76
  77 /**
  78  * Reap one child.
  79  *
  80  * \param pid In case a child died, its pid is returned here.
  81  *
  82  * Call waitpid() and print a log message containing the pid and the cause of
  83  * the child's death.
  84  *
  85  * \return A (negative) paraslash error code on errors, zero, if no child died,
  86  * one otherwise. If and only if the function returns one, the content of \a
  87  * pid is meaningful.
  88  *
  89  * \sa waitpid(2).
  90  */
  91 int para_reap_child(pid_t *pid)
  92 {
  93         int status;
  94         *pid = waitpid(-1, &status, WNOHANG);
  95
  96         if (!*pid)
  97                 return 0;
  98         if (*pid < 0)
  99                 return -ERRNO_TO_PARA_ERROR(errno);
 100         if (WIFEXITED(status))
 101                 PARA_DEBUG_LOG("child %i exited. Exit status: %i\n", (int)*pid,
 102                         WEXITSTATUS(status));
 103         else if (WIFSIGNALED(status))
 104                 PARA_DEBUG_LOG("child %i was killed by signal %i\n", (int)*pid,
 105                         WTERMSIG(status));
 106         else
 107                 PARA_WARNING_LOG("child %i terminated abormally\n", (int)*pid);
 108         return 1;
 109 }
 110
 111 /**
 112  * Install the given handler for the given signal.
 113  *
 114  * \param sig The number of the signal to catch.
 115  * \param handler to be installed, \p SIG_IGN, or \p SIG_DFL.
 116  *
 117  * This either succeeds or calls exit(EXIT_FAILURE).
 118  *
 119  * \sa sigaction(2).
 120  */
 121 void para_sigaction(int sig, void (*handler)(int))
 122 {
 123         struct sigaction act;
 124
 125         PARA_DEBUG_LOG("catching signal %d\n", sig);
 126         act.sa_handler = handler;
 127         sigemptyset(&act.sa_mask);
 128         act.sa_flags = 0;
 129         if (sig == SIGALRM) {
 130                 #ifdef SA_INTERRUPT /* SunOS */
 131                         act.sa_flags |= SA_INTERRUPT;
 132                 #endif
 133         } else {
 134                 #ifdef SA_RESTART /* BSD */
 135                         act.sa_flags |= SA_RESTART;
 136                 #endif
 137         }
 138         if (sigaction(sig, &act, NULL) >= 0)
 139                 return;
 140         PARA_EMERG_LOG("failed to install signal handler for signal %d\n",
 141                 sig);
 142         exit(EXIT_FAILURE);
 143 }
 144
 145 /**
 146  * Install the generic signal handler for the given signal number.
 147  *
 148  * \param sig The number of the signal to catch.
 149  *
 150  * \sa signal(2), sigaction(2).
 151  */
 152 void para_install_sighandler(int sig)
 153 {
 154         para_sigaction(sig, &generic_signal_handler);
 155 }
 156
 157 /**
 158  * Block a signal for the caller.
 159  *
 160  * \param sig The signal to block.
 161  *
 162  * This sets the given signal in the current signal mask of the calling process
 163  * to prevent this signal from delivery.
 164  *
 165  * \sa \ref para_unblock_signal(), sigprocmask(2), sigaddset(3).
 166  */
 167 void para_block_signal(int sig)
 168 {
 169         sigset_t set;
 170
 171         PARA_DEBUG_LOG("blocking signal %d\n", sig);
 172         sigemptyset(&set);
 173         sigaddset(&set, sig);
 174         sigprocmask(SIG_BLOCK, &set, NULL);
 175 }
 176
 177 /**
 178  * Unblock a signal.
 179  *
 180  * \param sig The signal to unblock.
 181  *
 182  * This function removes the given signal from the current set of blocked
 183  * signals.
 184  *
 185  * \sa \ref para_block_signal(), sigprocmask(2), sigaddset(3).
 186  */
 187 void para_unblock_signal(int sig)
 188 {
 189         sigset_t set;
 190
 191         PARA_DEBUG_LOG("unblocking signal %d\n", sig);
 192         sigemptyset(&set);
 193         sigaddset(&set, sig);
 194         sigprocmask(SIG_UNBLOCK, &set, NULL);
 195 }
 196
 197 /**
 198  * Return the number of the next pending signal.
 199  *
 200  * \param rfds The fd_set containing the signal pipe.
 201  *
 202  * \return On success, the number of the received signal is returned. If there
 203  * is no signal currently pending, the function returns zero. On read errors
 204  * from the signal pipe, the process is terminated.
 205  */
 206 int para_next_signal(fd_set *rfds)
 207 {
 208         size_t n;
 209         int s, ret = read_nonblock(signal_pipe[0], &s, sizeof(s), rfds, &n);
 210
 211         if (ret < 0) {
 212                 PARA_EMERG_LOG("%s\n", para_strerror(-ret));
 213                 exit(EXIT_FAILURE);
 214         }
 215         if (n == 0)
 216                 return 0;
 217         assert(n == sizeof(s));
 218         PARA_DEBUG_LOG("next signal: %d\n", s);
 219         return s;
 220 }
 221
 222 /**
 223  * Close the write end of the signal pipe.
 224  */
 225 void para_signal_shutdown(void)
 226 {
 227         close(signal_pipe[1]);
 228 }