msg555 · September 10, 2024 12:44 · Apr 19, 2013 · Apr 19, 2013 · Apr 19, 2013
diff --git a/gistfile1.c b/gistfile1.c
@@ -5,20 +5,23 @@
 #define STACK_PAD 128
 
 int rec(int n) {
-  if(n == 0) return 0;
-  int r = rec(n - 1);
-  ++r;
-  return r;
-}
-
-void nmain() {
-  printf("%d\n", rec(1000000));
-  exit(0);
+  return n == 0 ? 0 : rec(n - 1) + 1;
 }
 
 int main() {
+  /* Allocate a new stack to use first.  Because stacks grow to lower addresses
+   * we want to use just a little before the end of the allocation. */
   char* nstack = (char*)malloc(STACK_SIZE) + STACK_SIZE - STACK_PAD;
-  char lols[(char*)&nstack - nstack];
-  nmain();
+
+  /* Adjust the stack pointer to fall right in our newly allocated stack.
+   * "char stack_hack[sz];" has the effect of subtracting sz from the stack
+   * pointer. */
+  char stack_hack[(char*)&nstack - nstack];
+
+  /* Now we can recurse deeply.  If you comment out the definition of stack_hack
+   * then (on my system's limits, anyway) the second one will segfault. */
+  printf("%d\n", rec(100000));
+  printf("%d\n", rec(1000000));
+
   return 1;
 }
diff --git a/gistfile1.c b/gistfile1.c
@@ -1,6 +1,5 @@
 #include <stdio.h>
 #include <stdlib.h>
-#include <alloca.h>
 
 #define STACK_SIZE (1 << 26)
 #define STACK_PAD 128
@@ -19,7 +18,7 @@ void nmain() {
 
 int main() {
   char* nstack = (char*)malloc(STACK_SIZE) + STACK_SIZE - STACK_PAD;
-  alloca((char*)&nstack - nstack);
+  char lols[(char*)&nstack - nstack];
   nmain();
   return 1;
 }
diff --git a/gistfile1.c b/gistfile1.c
@@ -0,0 +1,25 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <alloca.h>
+
+#define STACK_SIZE (1 << 26)
+#define STACK_PAD 128
+
+int rec(int n) {
+  if(n == 0) return 0;
+  int r = rec(n - 1);
+  ++r;
+  return r;
+}
+
+void nmain() {
+  printf("%d\n", rec(1000000));
+  exit(0);
+}
+
+int main() {
+  char* nstack = (char*)malloc(STACK_SIZE) + STACK_SIZE - STACK_PAD;
+  alloca((char*)&nstack - nstack);
+  nmain();
+  return 1;
+}