Why sbrk clear the increased memory region

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Recently, I'm using barebox in a deep embedded project, and need some funtctions
in libc/libm, So I linked barebox with newlib

But whenever newlib call sbrk with negtive `increment`, barebox crash.

The reason is sbrk clean memory region using `increment`.

Why sbrk must clear the increased memory region? 

--------------------------------8<----------------------------- 
>From dbaa658b60fe413e1d1818afad13a53e7ad000ce Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E5=BC=A0=E5=BF=A0=E5=B1=B1?= <zzs213@xxxxxxx>
Date: Mon, 22 Apr 2019 11:01:33 +0800
Subject: [PATCH] only clear memory when increasing head size
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Signed-off-by: 张忠山 <zzs213@xxxxxxx>
---
 common/memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/common/memory.c b/common/memory.c
index 8f0b59891..0a8700d28 100644
--- a/common/memory.c
+++ b/common/memory.c
@@ -122,7 +122,7 @@ void *sbrk(ptrdiff_t increment)
 	void *old = sbrk_no_zero(increment);
 
 	/* Only clear increment, if valid address was returned */
-	if (old != NULL)
+	if ((old != NULL) && (increment > 0))
 		memset(old, 0, increment);
 
 	return old;
-- 
2.21.0

-- 
Best Regards,
zzs


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