Add [caml_fatal_error_hook].

When not NULL, this hook is called when a fatal error is encountered
instead of printing an error message on stderr.

Changes

@@ -36,7 +36,8 @@ Working version
    OCaml heap.
    (Jacques-Henri Jourdan, review by Damien Doligez)
 
-- #8630: Use abort() instead of exit(2) in caml_fatal_error.
+- #8630: Use abort() instead of exit(2) in caml_fatal_error, and add
+  the new hook caml_fatal_error_hook.
   (Jacques-Henri Jourdan, review by Xavier Leroy)
 
 ### Standard library:

runtime/caml/misc.h

@@ -27,6 +27,7 @@
 
 #include <stddef.h>
 #include <stdlib.h>
+#include <stdio.h>
 
 /* Basic types and constants */
 
@@ -125,6 +126,15 @@ CAMLnoreturn_end;
 #define CAMLassert(x) ((void) 0)
 #endif
 
+/* This hook is called when a fatal error occurs in the OCaml
+   runtime. It is given arguments to be passed to the [vprintf]-like
+   functions in order to synthetize the error message.
+   If it returns, the runtime calls [abort()].
+
+   If it is [NULL], the error message is printed on stderr and then
+   [abort()] is called. */
+extern void (*caml_fatal_error_hook) (char *msg, va_list args);
+
 CAMLnoreturn_start
 CAMLextern void caml_fatal_error (char *, ...)
 #ifdef __GNUC__

runtime/misc.c

@@ -76,14 +76,20 @@ void caml_gc_message (int level, char *msg, ...)
   }
 }
 
+void (*caml_fatal_error_hook) (char *msg, va_list args) = NULL;
+
 CAMLexport void caml_fatal_error (char *msg, ...)
 {
   va_list ap;
   va_start(ap, msg);
-  fprintf (stderr, "Fatal error: ");
-  vfprintf (stderr, msg, ap);
+  if(caml_fatal_error_hook != NULL) {
+    caml_fatal_error_hook(msg, ap);
+  } else {
+    fprintf (stderr, "Fatal error: ");
+    vfprintf (stderr, msg, ap);
+    fprintf (stderr, "\n");
+  }
   va_end(ap);
-  fprintf (stderr, "\n");
   abort();
 }