sel/sig.c: Use `DISCARD' to ignore errors from the signal-pipe `write'.

[mLib] / sel / sig.c

/* -*-c-*-
 *
 * Signal handling
 *
 * (c) 1999 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------*
 *
 * This file is part of the mLib utilities library.
 *
 * mLib is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Library General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or (at your option) any later version.
 *
 * mLib is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with mLib; if not, write to the Free
 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
 * MA 02111-1307, USA.
 */

/*----- Header files ------------------------------------------------------*/

#include <errno.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <fcntl.h>

#include "alloc.h"
#include "fdflags.h"
#include "macros.h"
#include "report.h"
#include "sel.h"
#include "sig.h"

/*----- Data structures ---------------------------------------------------*/

typedef struct sigstate {
  struct sigaction sa;
  sig *list;
} sigstate;

/*----- Static variables --------------------------------------------------*/

static sel_file sigsel;
static int sigfd;
static sigstate *sigs;
static sigset_t ss_all, ss_caught;
static unsigned nsig;

/*----- Main code ---------------------------------------------------------*/

/* --- @sig_handler@ --- *
 *
 * Arguments:	@int n@ = signal number
 *
 * Returns:	---
 *
 * Use:		Generic signal handler.  Writes a single byte to the
 *		signal pipe.  The byte contains the signal number.  Also
 *		sets the appropriate bit in @ss_caught@ to indicate which
 *		signals are pending.
 */

static void sig_handler(int n)
{
  int e = errno;
  unsigned char sch = (unsigned char)n;
  sigprocmask(SIG_BLOCK, &ss_all, 0);
  sigaddset(&ss_caught, n);
  DISCARD(write(sigfd, &sch, 1));
  /* The system should reset the signal mask here. */
  errno = e;
}

/* --- @sig_read@ --- *
 *
 * Arguments:	@int fd@ = file descriptor to read
 *		@unsigned mode@ = thing to do with descriptor
 *		@void *p@ = uninteresting argument
 *
 * Returns:	---
 *
 * Use:		Dispatches signals to their handlers safely.
 */

static void sig_read(int fd, unsigned mode, void *p)
{
  sigset_t ss;

  /* --- Read the currently caught signals --- *
   *
   * Block signals while the mask is being copied.
   */

  {
    sigset_t oss;
    unsigned char buf[256];

    sigprocmask(SIG_BLOCK, &ss_all, &oss);
    ss = ss_caught;
    sigemptyset(&ss_caught);
    while (read(fd, buf, sizeof(buf)) > 0)
      /* Do nothing */;
    sigprocmask(SIG_SETMASK, &oss, 0);
  }

  /* --- Process the caught signals --- */

  {
    int i;
    for (i = 0; i < nsig; i++) {
      sig *s;
      if (!sigismember(&ss, i))
	continue;
      s = sigs[i].list;
      while (s) {
	sig *ss = s;
	s = s->next;
	ss->proc(i, ss->p);
      }
    }
  }
}

/* --- @sig_add@ --- *
 *
 * Arguments:	@sig *s@ = pointer to signal handler block
 *		@int n@ = number of the signal
 *		@void (*proc)(int n, void *p)@ = signal handler function
 *		@void *p@ = argument to pass to handler
 *
 * Returns:	---
 *
 * Use:		Adds a signal handler.
 */

void sig_add(sig *s, int n, void (*proc)(int /*n*/, void */*p*/), void *p)
{
  sigstate *ss = &sigs[n];

  /* --- Initialize the block --- */

  s->proc = proc;
  s->p = p;
  s->sig = n;
  s->next = ss->list;
  s->prev = 0;

  /* --- Engage a signal handler, maybe --- */

  if (!ss->list) {
    struct sigaction sa;
    sa.sa_handler = sig_handler;
    sa.sa_flags = SA_NOCLDSTOP;
#ifdef SA_RESTART
    sa.sa_flags |= SA_RESTART;
#endif
    sigemptyset(&sa.sa_mask);
    sigaddset(&ss_all, n);
    sigaction(n, &sa, &ss->sa);
  }

  /* --- Link the block into the list --- */

  if (ss->list)
    ss->list->prev = s;
  ss->list = s;
}

/* --- @sig_remove@ --- *
 *
 * Arguments:	@sig *s@ = pointer to signal handler block
 *
 * Returns:	---
 *
 * Use:		Removes the signal handler from the list.
 */

void sig_remove(sig *s)
{
  sigstate *ss = &sigs[s->sig];

  /* --- Unlink the handler block --- */

  if (s->next)
    s->next->prev = s->prev;
  if (s->prev)
    s->prev->next = s->next;
  else
    ss->list = s->next;

  /* --- Maybe remove the handler --- */

  if (!ss->list) {
    sigaction(s->sig, &ss->sa, 0);
    sigdelset(&ss_all, s->sig);
  }
}

/* --- @sig_init@ --- *
 *
 * Arguments:	@sel_state *s@ = pointer to select state
 *
 * Returns:	---
 *
 * Use:		Initializes the signal handling system ready for use.
 */

void sig_init(sel_state *s)
{
  int fd[2];

  /* --- Work out how many signals there are --- */

  {
    sigset_t ss;
    unsigned min = 0, max = 0;

    sigemptyset(&ss);

    /* --- Get a good upper bound --- *
     *
     * Cap the search at 256.  I'll be sending signal numbers as bytes.
     */

    nsig = 1;
    while (sigaddset(&ss, nsig) == 0) {
      if (nsig == 256)
	goto counted;
      nsig <<= 1;
    }

    /* --- Now binary search until I find the actual limit --- */

    min = nsig >> 1;
    max = nsig;
    for (;;) {
      nsig = (min + max) >> 1;
      if (nsig == min)
	break;
      if (sigaddset(&ss, nsig))
	max = nsig;
      else
	min = nsig;
    }
  counted:;
  }

  /* --- Initialize the signal state table --- */

  {
    unsigned i;

    sigs = xmalloc(nsig * sizeof(*sigs));
    for (i = 0; i < nsig; i++)
      sigs[i].list = 0;
  }

  /* --- Create the signal pipe --- */

  if (pipe(fd))
    die(1, "couldn't create pipe for signal handling");

  /* --- Set both ends to nonblocking and close-on-exec --- */

  fdflags(fd[0], O_NONBLOCK, O_NONBLOCK, FD_CLOEXEC, FD_CLOEXEC);
  fdflags(fd[1], O_NONBLOCK, O_NONBLOCK, FD_CLOEXEC, FD_CLOEXEC);

  /* --- Set everything up for the future --- */

  sigfd = fd[1];
  sel_initfile(s, &sigsel, fd[0], SEL_READ, sig_read, 0);
  sel_addfile(&sigsel);
  sigemptyset(&ss_all);
  sigemptyset(&ss_caught);
}

/*----- That's all, folks -------------------------------------------------*/