* [PATCH] uml: flush stdout before forking
@ 2015-12-18 20:28 Vegard Nossum
2016-01-10 15:49 ` [uml-devel] " Richard Weinberger
0 siblings, 1 reply; 2+ messages in thread
From: Vegard Nossum @ 2015-12-18 20:28 UTC (permalink / raw)
To: Richard Weinberger
Cc: Jeff Dike, user-mode-linux-devel, linux-kernel, Vegard Nossum
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 <vegard.nossum@oracle.com>
---
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
^ permalink raw reply related [flat|nested] 2+ messages in thread* Re: [uml-devel] [PATCH] uml: flush stdout before forking
2015-12-18 20:28 [PATCH] uml: flush stdout before forking Vegard Nossum
@ 2016-01-10 15:49 ` Richard Weinberger
0 siblings, 0 replies; 2+ messages in thread
From: Richard Weinberger @ 2016-01-10 15:49 UTC (permalink / raw)
To: Vegard Nossum
Cc: Richard Weinberger, Jeff Dike, LKML,
user-mode-linux-devel@lists.sourceforge.net
On Fri, Dec 18, 2015 at 9:28 PM, Vegard Nossum <vegard.nossum@oracle.com> wrote:
> 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 <vegard.nossum@oracle.com>
Applied and queued for stable!
--
Thanks,
//richard
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