3 files changed, 19 insertions, 9 deletions

diff --git a/bin/sh/histedit.c b/bin/sh/histedit.c
index 5698a8f9c5db..f91d98a91f7e 100644
--- a/bin/sh/histedit.c
+++ b/bin/sh/histedit.c
@@ -411,7 +411,7 @@ fc_replace(const char *s, char *p, char *r)
 		} else
 			STPUTC(*s++, dest);
 	}
-	STACKSTRNUL(dest);
+	STPUTC('\0', dest);
 	dest = grabstackstr(dest);
 
 	return (dest);
diff --git a/bin/sh/memalloc.c b/bin/sh/memalloc.c
index 115eea03df7b..d1903e585191 100644
--- a/bin/sh/memalloc.c
+++ b/bin/sh/memalloc.c
@@ -295,6 +295,13 @@ grabstackblock(int len)
  * is space for at least one character.
  */
 
+static char *
+growstrstackblock(int n)
+{
+	growstackblock();
+	sstrnleft = stackblocksize() - n;
+	return stackblock() + n;
+}
 
 char *
 growstackstr(void)
@@ -304,12 +311,10 @@ growstackstr(void)
 	len = stackblocksize();
 	if (herefd >= 0 && len >= 1024) {
 		xwrite(herefd, stackblock(), len);
-		sstrnleft = len - 1;
+		sstrnleft = len;
 		return stackblock();
 	}
-	growstackblock();
-	sstrnleft = stackblocksize() - len - 1;
-	return stackblock() + len;
+	return growstrstackblock(len);
 }
 
 
@@ -323,9 +328,7 @@ makestrspace(void)
 	int len;
 
 	len = stackblocksize() - sstrnleft;
-	growstackblock();
-	sstrnleft = stackblocksize() - len;
-	return stackblock() + len;
+	return growstrstackblock(len);
 }
 
 
diff --git a/bin/sh/memalloc.h b/bin/sh/memalloc.h
index 347544211f51..571c42952cd0 100644
--- a/bin/sh/memalloc.h
+++ b/bin/sh/memalloc.h
@@ -65,9 +65,16 @@ void ungrabstackstr(char *, char *);
 #define stackblock() stacknxt
 #define stackblocksize() stacknleft
 #define STARTSTACKSTR(p)	p = stackblock(), sstrnleft = stackblocksize()
-#define STPUTC(c, p)	(--sstrnleft >= 0? (*p++ = (c)) : (p = growstackstr(), *p++ = (c)))
+#define STPUTC(c, p)	(--sstrnleft >= 0? (*p++ = (c)) : (p = growstackstr(), --sstrnleft, *p++ = (c)))
 #define CHECKSTRSPACE(n, p)	{ if (sstrnleft < n) p = makestrspace(); }
 #define USTPUTC(c, p)	(--sstrnleft, *p++ = (c))
+/*
+ * STACKSTRNUL's use is where we want to be able to turn a stack
+ * (non-sentinel, character counting string) into a C string,
+ * and later pretend the NUL is not there.
+ * Note: Because of STACKSTRNUL's semantics, STACKSTRNUL cannot be used
+ * on a stack that will grabstackstr()ed.
+ */
 #define STACKSTRNUL(p)	(sstrnleft == 0? (p = growstackstr(), *p = '\0') : (*p = '\0'))
 #define STUNPUTC(p)	(++sstrnleft, --p)
 #define STTOPC(p)	p[-1]