src/port/pqsignal.c

   1 /*-------------------------------------------------------------------------
   2  *
   3  * pqsignal.c
   4  *        reliable BSD-style signal(2) routine stolen from RWW who stole it
   5  *        from Stevens...
   6  *
   7  * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
   8  * Portions Copyright (c) 1994, Regents of the University of California
   9  *
  10  *
  11  * IDENTIFICATION
  12  *        src/port/pqsignal.c
  13  *
  14  *      This is the signal() implementation from "Advanced Programming in the UNIX
  15  *      Environment", with minor changes.  It was originally a replacement needed
  16  *      for old SVR4 systems whose signal() behaved as if sa_flags = SA_RESETHAND |
  17  *      SA_NODEFER, also known as "unreliable" signals due to races when the
  18  *      handler was reset.
  19  *
  20  *      By now, all known modern Unix systems have a "reliable" signal() call.
  21  *      We still don't want to use it though, because it remains
  22  *      implementation-defined by both C99 and POSIX whether the handler is reset
  23  *      or signals are blocked when the handler runs, and default restart behavior
  24  *      is also unspecified.  Therefore we take POSIX's advice and call sigaction()
  25  *      so we can provide explicit sa_flags, but wrap it in this more convenient
  26  *      traditional interface style.  It also provides a place to set any extra
  27  *      flags we want everywhere, such as SA_NOCLDSTOP.
  28  *
  29  *      Windows, of course, is resolutely in a class by itself.  In the backend,
  30  *      this relies on pqsigaction() in src/backend/port/win32/signal.c, which
  31  *      provides limited emulation of reliable signals.
  32  *
  33  *      Frontend programs can use this version of pqsignal() to forward to the
  34  *      native Windows signal() call if they wish, but beware that Windows signals
  35  *      behave quite differently.  Only the 6 signals required by C are supported.
  36  *      SIGINT handlers run in another thread instead of interrupting an existing
  37  *      thread, and the others don't interrupt system calls either, so SA_RESTART
  38  *      is moot.  All except SIGFPE have SA_RESETHAND semantics, meaning the
  39  *      handler is reset to SIG_DFL each time it runs.  The set of things you are
  40  *      allowed to do in a handler is also much more restricted than on Unix,
  41  *      according to the documentation.
  42  *
  43  * ------------------------------------------------------------------------
  44  */
  45
  46 #include "c.h"
  47
  48 #include <signal.h>
  49
  50 #ifndef FRONTEND
  51 #include "libpq/pqsignal.h"
  52 #endif
  53
  54 /*
  55  * Set up a signal handler, with SA_RESTART, for signal "signo"
  56  *
  57  * Returns the previous handler.
  58  */
  59 pqsigfunc
  60 pqsignal(int signo, pqsigfunc func)
  61 {
  62 #if !(defined(WIN32) && defined(FRONTEND))
  63         struct sigaction act,
  64                                 oact;
  65
  66         act.sa_handler = func;
  67         sigemptyset(&act.sa_mask);
  68         act.sa_flags = SA_RESTART;
  69 #ifdef SA_NOCLDSTOP
  70         if (signo == SIGCHLD)
  71                 act.sa_flags |= SA_NOCLDSTOP;
  72 #endif
  73         if (sigaction(signo, &act, &oact) < 0)
  74                 return SIG_ERR;
  75         return oact.sa_handler;
  76 #else
  77         /* Forward to Windows native signal system. */
  78         return signal(signo, func);
  79 #endif
  80 }