path: root/src/lib/bsdsignal.c.pamphlet

\documentclass{article}
\usepackage{axiom}

\newcommand{\var}[1]{\textsl{#1}}

\title{\File{src/lib/bsdsignal.c} Pamphlet}
\author{The Axiom Team}

\begin{document}
\maketitle

\begin{abstract}
\end{abstract}
\eject

\tableofcontents
\newpage

\section{Executive Overview}

\section{Signals}

The system defines a set of signals that may be delivered to a process. 
Signal 
delivery resembles the occurrence of a hardware interrupt: the signal is 
normally blocked from further occurrence, the current process context is 
saved, 
and a new one is built. A process may specify a \emph{handler} to
which a signal 
is delivered, or specify that a signal is to be \emph{ignored}. A process may 
also specify that a default action is to be taken by the system when a signal 
occurs. A signal may also be \emph{blocked}, in which case its delivery is 
postponed until it is \emph{unblocked}. The action to be taken on delivery is 
determined at the time of delivery. Normally, signal handlers execute on the 
current stack of the process. This may be changed, on a per-handler basis, so 
that signals are taken on a special \emph{signal stack}.

Signal routines normally execute with the signal that caused their invocation 
\emph{blocked}, but other signals may yet occur. A global \emph{signal mask} 
defines the set of signals currently blocked from delivery to a process. 
The signal mask for a process is initialized from that of its parent 
(normally empty). It may be changed with a \emph{[[sigprocmask]](2)} call, or 
when a signal is delivered to the process.

When a signal condition arises for a process, the signal is added to a set of 
signals pending for the process. If the signal is not currently \emph{blocked} 
by the process then it is delivered to the process. Signals may be delivered 
any time a process enters the operating system (e.g., during a system call, 
page fault or trap, or clock interrupt). If muliple signals are ready to be 
delivered at the same time, any signals that could be caused by traps are 
delivered first. Additional signals may be processed at the same time, with 
each appearing to interrupt the handlers for the previous signals before 
their first instructions. The set of pending signals is retuned by the 
\emph{[[sigpending]](2)} system call. When a caught signal is delivered, 
the current 
state of the process is saved, a new signal mask is calculated (as described 
below), and the signal handler is invoked. The call to the handler is arranged 
so that if the signal handling routine returns normally the process will resume 
execution in the context from before the signal's delivery. If the process 
wishes to resume in a different context, then it must arrange to restore 
the previous context itself.

When a signal is delivered to a proces a new signal mask is installed for the 
duration of the process's signal handler (or until a \emph{[[sigprocmask]](2)} 
system call is made). This mask is formed by taking the union of the current 
signal mask set, the signal to be delivered, and the signal mask associated 
with the handler to be invoked.

The \emph{[[sigaction]]()} system call assigns an action for a signal 
specified by \var{sig}. If \var{act} is non-zero, it specifies an action 
([[SIG_DFL]], [[SIG_IGN]],  or a handler routine) and mask to be used when 
delivering the specified signal. 
If \var{oact} is non-zero, the previous handling information for the signal is 
returned to the user.

Once a signal handler is installed, it normally  remains installed until 
another 
[[sigaction()]] system call is made, or an \emph{[[execve]](2)} is performed. 
A 
signal-specific default action may be reset by setting [[sa_handler]] to 
[[SIG_DFL]]. The defaults are process termination, possibly with core dump; 
no action; stopping the process; or continuing the process. See the signal 
list below for each signal's default action. If [[sa_handler]] is [[SIG_DFL]], 
the default action for the signal is to discard the signal, and if a signal 
is pending, the pending signal is discarded even if the signal is masked. If 
[[sa_handler]] is set to [[SIG_IGN]] current and pending instances of the 
signal 
are ignored and discarded.

Options may be specified by setting [[sa_flags]]. The meaning of the various 
bits is as follows:
\begin{tabular}{ll}
SA\_NOCLDSTOP & If this bit is set when installing a catching function for\\
             & the SIGCHLD signal, the SIGCHLD signal will be generated only\\
             & when a child process exits, not when a child process stops.\\
SA\_NOCLDWAIT & If this bit is set when calling {\sl sigaction()} for the\\
             & SIGCHLD signal, the system will not create zombie processes\\
             & when children of the calling process exit. If the calling\\
             & process subsequently issues a {\sf wait()} (or equivalent),\\
             & it blocks until all of the calling process's child processes\\
             & terminate, and then returns a value of -1 with errno set to\\
             & ECHILD.\\
SA\_ONSTACK   & If this bit is set, the system will deliver the signal to\\
             & the process on a {\sl signal stack}, specified with\\
             & {\bf sigaltstack(2)}.\\
SA\_NODEFER   & If this bit is set, further occurrences of the delivered\\
             & signal are not masked during the execution of the handler.\\
SA\_RESETHAND & If this bit is set, the handler is reset back to SIG\_DFL\\
             & at the moment the signal is delivered.\\
SA\_RESTART   & See the paragraph below\\
SA\_SIGINFO   & If this bit is set, the handler function is assumed to be\\
             & pointed to by the sa\_sigaction member of struct sigaction\\
             & and should match the prototype shown above or as below in\\
             & EXAMPLES. This bit should not be set when assigning SIG\_DFL\\
             & or SIG\_IGN
\end{tabular}

If a signal is caught during the system calls listed below, the call may be 
forced to terminate with the error [[EINTR]], the call may return with a data 
transfer shorter than requested, or the call may be restarted. Restart of 
pending calls is requested by setting the SA\_RESTART bit in {\sl sa\_flags}. 
The affected system calls include {\bf open(2)}, {\bf read(2)}, {\bf write(2)}, 
{\bf sendto(2)}, {\bf recvfrom(2)}, {\bf sendmsg(2)} and {\bf recvmsg(2)} 
on a communications channel or a slow device (such as a terminal, but not a 
regular file) and during a {\bf wait(2)} or {\bf ioctl(2)}. However, calls 
that have already committed are not restarted, but instead return a partial 
success (for example, a short read count).

After a {\bf fork(2)} or {\bf vfork(2)} all signals, the signal mask, the 
signal stack, and the restart/interrupt flags are inherited by the child.

The {\bf execve(2)} system call reinstates the default action for all signals 
which were caught and resets all signals to be caught on the user stack. 
Ignored signals remain ignored; the signal mask remains the same; signals 
that restart pending system calls continue to do so.

The following is a list of all signals with names as in the include file
{\sl <signal.h>}:

\begin{tabular}{lll}
{\bf NAME} & {\bf Default Action} & Description\\
SIGHUP     & terminate process    & terminal line hangup\\
SIGINT     & terminate process    & interrupt program\\
SIGQUIT    & create core image    & quit program\\
SIGILL     & create core image    & illegal instruction\\
SIGTRAP    & create core image    & trace trap\\
SIGABRT    & create core image    & {\bf abort(3)} call (formerly SIGIOT)\\
SIGEMT     & create core image    & emulate instruction executed\\
SIGFPE     & create core image    & floating-point exception\\
SIGKILL    & terminate process    & kill program\\
SIGBUS     & create core image    & bus error\\
SIGSEGV    & create core image    & segmentation violation\\
SIGSYS     & create core image    & non-existent system call invoked\\
SIGPIPE    & terminate process    & write on a pipe with no reader\\
SIGALRM    & terminate process    & real-time timer expired\\
SIGTERM    & terminate process    & software termination signal\\
SIGURG     & discard signal       & urgent condition present on socket\\
SIGSTOP    & stop process         & stop (cannot be caught or ignored)\\
SIGSTP     & stop process         & stop signal generated from keyboard\\
SIGCONT    & discard signal       & continue after stop\\
SIGCHLD    & discard signal       & child status has changed\\
SIGTTIN    & stop process         & background read attempted from \\
           &                      & control terminal\\
SIGTTOU    & stop process         & background write attempted from\\
           &                      & control terminal\\
SIGIO      & discard signal       & I/O is possible on a descriptor {\bf fcntl(2)}\\
SIGXCPU    & terminate process    & cpu time limit exceeded {\bf setrlimit(2)}\\
SIGXFSZ    & terminate process    & file size limit exceeded {\bf setrlimit(2)}\\
SIGVTALRM  & terminate process    & virtual time alarm {\bf setitimer(2)}\\
SIGPROF    & terminate process    & profiling timer alarm {\bf setitimer(2)}\\
SIGWINCH   & discard signal       & Window size change\\
SIGINFO    & discard signal       & status request from keyboard\\
SIGUSR1    & terminate process    & User defined signal 1\\
SIGUSR2    & terminate process    & User defined signal 2
\end{tabular}

The {\sl sigaction()} function returns the value 0 if successful; otherwise 
the value -1 is returned and the global variable {\sl errno} is set to indicate 
the error.

Signal handlers should have either the ANSI C prototype:
\begin{verbatim}
  void handler(int);
\end{verbatim}
or the POSIX SA\_SIGINFO prototype:
\begin{verbatim}
  void handler(int, siginfo\_t *info, ucontext\_t *uap);
\end{verbatim}

The handler function should match the SA\_SIGINFO prototype if the SA\_SIGINFO 
bit is set in flags. It then should be pointed to by the sa\_sigaction member 
of struct sigaction. Note that you should not assign SIG\_DFL or SIG\_IGN this way.

If the SA\_SIGINFO flag is not set, the handler function should match either 
the ANSI C or traditional BSD prototype and be pointed to by the sa\_handler 
member of struct sigaction. In practice, FreeBSD always sends the three 
arguments of the latter and since the ANSI C prototype is a subset, both 
will work. The sa\_handler member declaration in FreeBSD include files is 
that of ANSI C (as required by POSIX), so a function pointer of a BSD-style 
function needs to be casted to compile without warning. The traditional BSD 
style is not portable and since its capabilities are a full subset of a 
SA\_SIGNFO handler its use is deprecated.

The {\sl sig} argument is the signal number, one of the SIG\ldots values from 
{\sl <signal.h>}.

The {\sl code} argument of the BSD-style handler and the si\_code member of the
info argument to a SA\_SIGINFO handler contain a numeric code explaining the
cause of the signal, usually on of the SI\_\ldots values from {\sl <sys/signal.h>}
or codes specific to a signal, i.e. one of the FPE\_\ldots values for SIGFPE.

The {\sl uap} argument to a POSIX SA\_SIGINFO handler points to an instance of 
ucontext\_t.

The {\bf sigaction()} system call will fail and no new signal handler will be
installed if one of the following occurs:
\begin{tabular}{ll}
EFAULT & Either {\sl act} or {\sl oact} points to memory that is not a\\
         & valid part of the process address space\\
EINVAL & The {\sl sig} argument is not a valid signal number\\
EINVAL & An attempt is made to ignore or supply a handler for SIGKILL\\
         & or SIGSTOP
\end{tabular}
\section{MAC OSX and BSD platform change}
<<*>>=
<<license>>

#include "axiom-c-macros.h"
#include "bsdsignal.h"

@
The MACOSX platform is broken because no matter what you do it seems to
include files from [[/usr/include/sys]] ahead of [[/usr/include]]. On linux
systems these files include themselves which causes an infinite regression
of includes that fails. GCC gracefully steps over that problem but the
build fails anyway. On MACOSX the [[/usr/include/sys]] versions 
of files are badly broken with respect to the [[/usr/include]] versions.
<<*>>=
#if defined(MACOSXplatform)
#include "/usr/include/signal.h"
#else
#include <signal.h> 
#endif

#include "bsdsignal.H1"


SignalHandlerFunc
bsdSignal(int sig, SignalHandlerFunc action, int restartSystemCall)
{
#if HAVE_DECL_SIGACTION

  struct sigaction in,out;
  in.sa_handler = action;
  /* handler is reinstalled - calls are restarted if restartSystemCall */
@

We needed to change [[SIGCLD]] to [[SIGCHLD]] for the [[MAC OSX]] platform
and we need to create a new platform variable. This change is made to 
propogate that platform variable.
<<*>>=
#ifdef SA_RESTART
  if(restartSystemCall) in.sa_flags = SA_RESTART;
  else in.sa_flags = 0;
#elif defined(SA_INTERRUPT)
  if (restartSystemCall) in.sa_flags = 0;
  else in.sa_flags = SA_INTERRUPT;
#else
  in.sa_flags = 0; 
#endif
  
  return (sigaction(sig, &in, &out) ? (SignalHandlerFunc) -1 : 
	  (SignalHandlerFunc) out.sa_handler);
#else /* !HAVE_DECL_SIGACTION */
  return (SignalHandlerFunc) -1;
#endif /* HAVE_DECL_SIGACTION */
}


@
\section{License}
<<license>>=
/*
Copyright (c) 1991-2002, The Numerical ALgorithms Group Ltd.
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

    - Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.

    - Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in
      the documentation and/or other materials provided with the
      distribution.

    - Neither the name of The Numerical ALgorithms Group Ltd. nor the
      names of its contributors may be used to endorse or promote products
      derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
@
\newpage
\begin{thebibliography}{99}
\bibitem{1} nothing
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\end{document}