Add p-allocexit program.

GNUmakefile

@@ -49,9 +49,11 @@ KERNEL_OBJS = $(OBJDIR)/k-exception.ko \
 	$(OBJDIR)/k-memviewer.ko $(OBJDIR)/lib.ko
 
 PROCESS_LIB_OBJS = $(OBJDIR)/lib.o $(OBJDIR)/p-lib.o
-PROCESS_OBJS = $(OBJDIR)/p-allocator.o $(PROCESS_LIB_OBJS)
+PROCESS_OBJS = $(PROCESS_LIB_OBJS) \
+	$(OBJDIR)/p-allocator.o \
+	$(OBJDIR)/p-allocexit.o
 
-FLATFS_CONTENTS = obj/p-allocator
+FLATFS_CONTENTS = obj/p-allocator obj/p-allocexit
 
 
 # How to make object files

p-allocexit.cc

+#include "p-lib.hh"
+#define ALLOC_SLOWDOWN 100
+
+extern uint8_t end[];
+
+uint8_t* heap_top;
+uint8_t* stack_bottom;
+
+void process_main(void) {
+    // Process 1 never allocates; it alternates between forking children
+    // and yielding. Each forked child allocates.
+    while (1) {
+        if (rand(0, ALLOC_SLOWDOWN - 1) == 0) {
+            if (sys_fork() == 0) {
+                break;
+            }
+        } else {
+            sys_yield();
+        }
+    }
+
+    pid_t p = sys_getpid();
+    srand(p);
+
+    // The heap starts on the page right after the 'end' symbol,
+    // whose address is the first address not allocated to process code
+    // or data.
+    heap_top = ROUNDUP((uint8_t*) end, PAGESIZE);
+
+    // The bottom of the stack is the first address on the current
+    // stack page (this process never needs more than one stack page).
+    stack_bottom = ROUNDDOWN((uint8_t*) read_rsp() - 1, PAGESIZE);
+
+    // Allocate heap pages until (1) hit the stack (out of address space)
+    // or (2) allocation fails (out of physical memory).
+    while (1) {
+        int x = rand(0, 8 * ALLOC_SLOWDOWN - 1);
+        if (x < 8 * p && heap_top != stack_bottom) {
+            if (sys_page_alloc(heap_top) >= 0) {
+                *heap_top = p;      // check we have write access to new page
+                heap_top += PAGESIZE;
+            }
+        } else if (x == 8 * p) {
+            if (sys_fork() == 0) {
+                p = sys_getpid();
+            }
+        } else if (x == 8 * p + 1
+                   || (x < p && heap_top == stack_bottom)) {
+            sys_exit(0);
+        } else {
+            sys_yield();
+        }
+    }
+}