Initial version. Production quality, fully complete source but contains known bugs (lots!)

mcron.c.template

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+/*
+ *   Copyright (C) 2003 Dale Mellor
+ * 
+ *   This program is free software; you can redistribute it and/or modify
+ *   it under the terms of the GNU General Public License as published by
+ *   the Free Software Foundation; either version 2, or (at your option)
+ *   any later version.
+ * 
+ *   This program 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 General Public License for more details.
+ * 
+ *   You should have received a copy of the GNU General Public License
+ *   along with this program; if not, write to the Free Software
+ *   Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
+ *   USA.
+ */
+
+
+/*
+    This C code represents the thinnest possible wrapper around the Guile code
+    which constitutes all the functionality of the mcron program. There are two
+    plus one reasons why we need to do this, and one very unfortunate
+    consequence.
+
+        Firstly, SUID does not work on an executable script. In the end, it is
+        the execution of the translator, in our case guile, which determines the
+        effective user, and it is not wise to make the system guile installation
+        SUID root!
+
+        Secondly, executable scripts show up in ugly ways in listings of the
+        system process table. Guile in particular, with its multi-line
+        #! ...\ \n -s ...!#
+        idiosyncracies shows up in process listings in a way that is difficult
+        to determine what program is actually running.
+
+        A third reason for the C wrapper which might be mentioned is that a
+        security-conscious system administrator can choose to only install a
+        binary, thus removing the possibility of a user studying a guile script
+        and working out ways of hacking it to his own ends, or worse still
+        finding a way to modify it to his own ends.
+
+        Unfortunately, running the guile script from inside a C program means
+        that the sigaction function does not work. Instead, it is necessary to
+        perform the signal processing in C.
+
+    The guile code itself is substituted for the GU1LE_PROGRAM_GOES_HERE (sic)
+    token by the makefile, which processes the scheme to make it look like one
+    big string.
+*/
+
+
+
+#include <signal.h>
+#include <libguile.h>
+
+
+
+/* This is a function designed to be installed as a signal handler, for signals
+   which are supposed to initiate shutdown of this program. It calls the scheme
+   procedure (see mcron.scm for details) to do all the work, and then exits. */
+
+void react_to_terminal_signal (int sig)
+{
+    scm_eval_string (scm_take0str ("(delete-run-file)") );
+    exit (1);
+}
+
+
+
+/* This is a function designed to be installed as a signal handler. It calls the
+   scheme procedure to do all the work (see mcron.scm for details). */
+
+void react_to_hup_signal (int sig)
+{
+    scm_eval_string (scm_take0str ("(process-hup)") );
+}
+
+
+
+/* This is a function designed to be callable from scheme, and sets up all the
+   signal handlers required by the cron personality.  */
+
+SCM set_cron_signals ()
+{
+    static struct sigaction sa;
+    memset (&sa, 0, sizeof (sa));
+    sa.sa_handler = react_to_terminal_signal;
+    sigaction (SIGTERM, &sa, 0);
+    sigaction (SIGINT,  &sa, 0);
+    sigaction (SIGQUIT, &sa, 0);
+
+    static struct sigaction hup; hup = sa;
+    hup.sa_handler = react_to_hup_signal;
+    sigaction (SIGHUP, &hup, 0);
+    
+    return SCM_BOOL_T;
+}
+
+
+
+/* The effective main function (i.e. the one that actually does some work). We
+   register the function above with the guile system, and then execute the mcron
+   guile program. */
+
+void inner_main ()
+{
+    scm_c_define_gsubr ("c-set-cron-signals", 0, 0, 0, set_cron_signals);
+    
+    scm_eval_string (scm_take0str (
+                                    GUILE_PROGRAM_GOES_HERE
+                                                            ) );
+}
+
+
+
+/* The real main function. Does nothing but start up the guile subsystem. */
+
+int main (int argc, char **argv)
+{
+    scm_boot_guile (argc, argv, inner_main, 0);
+    return 0;
+}