diff --git a/src/makeint.h b/src/makeint.h
index f9de63d0..e345011b 100644
--- a/src/makeint.h
+++ b/src/makeint.h
@@ -658,8 +658,12 @@ int unload_file (const char *name);
 #ifdef MAKE_MAINTAINER_MODE
 # define SPIN(_s) spin (_s)
 void spin (const char* suffix);
+# define DBG(_f) dbg _f
+void dbg (const char *fmt, ...);
 #else
 # define SPIN(_s)
+/* Never put this code into Git or a release.  */
+# define DBG(_f) compile-error
 #endif
 
 /* We omit these declarations on non-POSIX systems which define _POSIX_VERSION,
diff --git a/src/misc.c b/src/misc.c
index 18728f35..4f52e78c 100644
--- a/src/misc.c
+++ b/src/misc.c
@@ -530,6 +530,22 @@ spin (const char* type)
     }
 }
 
+void
+dbg (const char *fmt, ...)
+{
+  FILE *fp = fopen ("/tmp/gmkdebug.log", "a+");
+  va_list args;
+  char buf[4096];
+
+  va_start (args, fmt);
+  vsprintf (buf, fmt, args);
+  va_end (args);
+
+  fprintf(fp, "%u: %s\n", (unsigned) make_pid (), buf);
+  fflush (fp);
+  fclose (fp);
+}
+
 #endif