From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Vegard Nossum Date: Fri, 18 Dec 2015 21:28:53 +0100 Message-Id: <1450470533-13115-1-git-send-email-vegard.nossum@oracle.com> Sender: linux-kernel-owner@vger.kernel.org Subject: [PATCH] uml: flush stdout before forking To: Richard Weinberger Cc: Jeff Dike , user-mode-linux-devel@lists.sourceforge.net, linux-kernel@vger.kernel.org, Vegard Nossum List-ID: I was seeing some really weird behaviour where piping UML's output somewhere would cause output to get duplicated: $ ./vmlinux | head -n 40 Checking that ptrace can change system call numbers...Core dump limits : soft - 0 hard - NONE OK Checking syscall emulation patch for ptrace...Core dump limits : soft - 0 hard - NONE OK Checking advanced syscall emulation patch for ptrace...Core dump limits : soft - 0 hard - NONE OK Core dump limits : soft - 0 hard - NONE This is because these tests do a fork() which duplicates the non-empty stdout buffer, then glibc flushes the duplicated buffer as each child exits. A simple workaround is to flush before forking. Signed-off-by: Vegard Nossum --- arch/um/os-Linux/start_up.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c index 47f1ff0..22a358e 100644 --- a/arch/um/os-Linux/start_up.c +++ b/arch/um/os-Linux/start_up.c @@ -94,6 +94,8 @@ static int start_ptraced_child(void) { int pid, n, status; + fflush(stdout); + pid = fork(); if (pid == 0) ptrace_child(); -- 1.9.1