From 0a1cfca0f41ca4ee5d981253b8f151c67aacf4f6 Mon Sep 17 00:00:00 2001
From: Michael Forney <mforney@mforney.org>
Date: Wed, 9 Jun 2021 14:34:57 -0700
Subject: [PATCH] Avoid creating out-of-bounds pointer when checking for
 seprintf truncation

It is technically undefined behavior (see C11 6.5.6p8) to construct
a pointer more than one past the end of an array. To prevent this,
compare n with the remaining space in the array before adding to
ptr.
---
 chat.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/chat.h b/chat.h
index e75112a..327262b 100644
--- a/chat.h
+++ b/chat.h
@@ -52,8 +52,8 @@ static inline char *seprintf(char *ptr, char *end, const char *fmt, ...) {
 	int n = vsnprintf(ptr, end - ptr, fmt, ap);
 	va_end(ap);
 	if (n < 0) return NULL;
-	ptr += n;
-	return (ptr > end ? end : ptr);
+	if (n > end - ptr) return end;
+	return ptr + n;
 }
 
 enum Attr {