avoid user-level segfault if a process doesn't call exit()

Makefile

@@ -90,7 +90,7 @@ tags: $(OBJS) _init
 ULIB = $U/ulib.o $U/usys.o $U/printf.o $U/umalloc.o
 
 _%: %.o $(ULIB)
-	$(LD) $(LDFLAGS) -N -e main -Ttext 0 -o $@ $^
+	$(LD) $(LDFLAGS) -N -e _main -Ttext 0 -o $@ $^
 	$(OBJDUMP) -S $@ > $*.asm
 	$(OBJDUMP) -t $@ | sed '1,/SYMBOL TABLE/d; s/ .* / /; /^$$/d' > $*.sym
 

user/ulib.c

@@ -3,6 +3,17 @@
 #include "kernel/fcntl.h"
 #include "user/user.h"
 
+//
+// wrapper so that it's OK if main() does not call exit().
+//
+void
+_main()
+{
+  extern int main();
+  main();
+  exit(0);
+}
+
 char*
 strcpy(char *s, const char *t)
 {