annosbr.c: free(3) takes void pointer; don't cast non-const pointer.

[nmh] / sbr / done.c

diff --git a/sbr/done.c b/sbr/done.c
index c703aba5b654e42983cf987b1c310b544bc935a6..cd76fac5b849dbf04cdd3149be7eef2ca39d4274 100644 (file)
--- a/sbr/done.c
+++ b/sbr/done.c
@@ -7,4 +7,39 @@
 
 #include <h/mh.h>
 
-void (*done) (int) NORETURN = exit;
+/* It's undefined behaviour in C99 to convert from a function pointer to
+ * a data-object pointer, e.g. void pointer.  gcc's -pedantic warns of
+ * this and can stop compilation.  POSIX requires the operation however,
+ * e.g. for dlsym(3), and so we know it's safe on POSIX platforms, e.g.
+ * the pointers are of the same size.  Thus use a union to subvert gcc's
+ * check.  The function-pointer equivalent of a void pointer is any
+ * function-pointer type as all function pointers are defined to be
+ * convertible from one another;  use the simplest available. */
+typedef union {
+    void *v;
+    void (*f)(void);
+} generic_pointer;
+
+static void (*altexit)(int) NORETURN = exit;
+
+/* set_done changes the path of done() from exit(3), or back to exit(3).
+ * Anything else will work, but a standard-error warning will report the
+ * old non-exit() value has been trampled. */
+void
+set_done(void (*new)(int) NORETURN)
+{
+    generic_pointer gpo, gpn;
+
+    if (altexit != exit && new != exit) {
+        gpo.f = (void (*)(void))altexit;
+        gpn.f = (void (*)(void))new;
+        inform("altexit trampled: %p %p", gpo.v, gpn.v);
+    }
+    altexit = new;
+}
+
+void NORETURN
+done(int status)
+{
+    (*altexit)(status);
+}