threads / patch / 12510

patchrun-command: Redirect stderr to a pipe before redirecting stdout to stderr

Subject: [PATCH] run-command: Redirect stderr to a pipe before redirecting stdout to stderr

## tl;dr

5 messages between Mar 5, 2008 and Mar 6, 2008. Diffs are folded; open one to read it.

replies: 4people: 4as markdown or json

Christian Couder· Mar 5, 2008, 07:35 UTC · lore

With this patch, in the 'start_command' function after forking we now take care of stderr in the child process before stdout.

This way if 'start_command' is called with a 'child_process' argument like this:

	.err = -1;
	.stdout_to_stderr = 1;

then stderr will be redirected to a pipe before stdout is redirected to stderr. So we can now get the process' stdout from the pipe (as well as its stderr).

Earlier such a call would have redirected stdout to stderr before stderr was itself redirected, and therefore stdout would not have followed stderr, which would not have been very useful anyway.

Update documentation in 'api-run-command.txt' accordingly.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
---
 Documentation/technical/api-run-command.txt |    7 ++++---
 run-command.c                               |   14 +++++++-------
 2 files changed, 11 insertions(+), 10 deletions(-)

	Changes since previous version:

	- improved patch justification as requested by Junio,
	- s/before/after/ in 'api-run-command.txt' (ooops).

Show changes to 2 files +11 −9

Documentation/technical/api-run-command.txt, run-command.c

diff --git a/Documentation/technical/api-run-command.txt b/Documentation/technical/api-run-command.txt
index fde3b45..c364a22 100644
--- a/Documentation/technical/api-run-command.txt
+++ b/Documentation/technical/api-run-command.txt
@@ -111,9 +111,10 @@ stderr as follows:
 	.no_stdin, .no_stdout, .no_stderr: The respective channel is
 		redirected to /dev/null.
 
-	.stdout_to_stderr: stdout of the child is redirected to the
-		parent's stderr (i.e. *not* to what .err or
-		.no_stderr specify).
+	.stdout_to_stderr: stdout of the child is redirected to its
+		stderr. This happens after stderr is itself redirected.
+		So stdout will follow stderr to wherever it is
+		redirected.
 
 To modify the environment of the sub-process, specify an array of
 string pointers (NULL terminated) in .env:
diff --git a/run-command.c b/run-command.c
index 743757c..44100a7 100644
--- a/run-command.c
+++ b/run-command.c
@@ -91,6 +91,13 @@ int start_command(struct child_process *cmd)
 			close(cmd->in);
 		}
 
+		if (cmd->no_stderr)
+			dup_devnull(2);
+		else if (need_err) {
+			dup2(fderr[1], 2);
+			close_pair(fderr);
+		}
+
 		if (cmd->no_stdout)
 			dup_devnull(1);
 		else if (cmd->stdout_to_stderr)
@@ -103,13 +110,6 @@ int start_command(struct child_process *cmd)
 			close(cmd->out);
 		}
 
-		if (cmd->no_stderr)
-			dup_devnull(2);
-		else if (need_err) {
-			dup2(fderr[1], 2);
-			close_pair(fderr);
-		}

Johannes Sixt· Mar 5, 2008, 07:36 UTC · re: Christian Couder · lore

Re: [PATCH] run-command: Redirect stderr to a pipe before redirecting stdout to stderr

Christian Couder schrieb:

Show 21 quoted lines

> With this patch, in the 'start_command' function after forking
> we now take care of stderr in the child process before stdout.
> 
> This way if 'start_command' is called with a 'child_process'
> argument like this:
> 
> 	.err = -1;
> 	.stdout_to_stderr = 1;
> 
> then stderr will be redirected to a pipe before stdout is
> redirected to stderr. So we can now get the process' stdout
> from the pipe (as well as its stderr).
> 
> Earlier such a call would have redirected stdout to stderr
> before stderr was itself redirected, and therefore stdout
> would not have followed stderr, which would not have been
> very useful anyway.
> 
> Update documentation in 'api-run-command.txt' accordingly.
> 
> Signed-off-by: Christian Couder <chriscool@tuxfamily.org>

FWIW:

Acked-by: Johannes Sixt <johannes.sixt@telecom.at>

-- Hannes

Xavier Maillard· Mar 6, 2008, 01:00 UTC · re: Christian Couder · lore

Re: [PATCH] run-command: Redirect stderr to a pipe before redirecting stdout to stderr

   Signed-off-by: Christian Couder <chriscool@tuxfamily.org>

Tested successfully.

Acked-by: Xavier Maillard <xma@gnu.org>

	Xavier

-- 
http://www.gnu.org
http://www.april.org
http://www.lolica.org

Christian Couder· Mar 6, 2008, 05:44 UTC · re: Christian Couder · lore

Re: [PATCH] run-command: Redirect stderr to a pipe before redirecting stdout to stderr

Hi Junio and Shawn,

On Wed, 5 Mar 2008, I wrote:

> With this patch, in the 'start_command' function after forking
> we now take care of stderr in the child process before stdout.
> ...

I am afraid I forgot the

From: "Shawn O. Pearce" <spearce@spearce.org>

at the top of the patch in my last resend, so the patch was attributed to me instead of Shawn.

Is it possible to do something about it ? (I fear not since it is now in 'next' but maybe I am wrong.)

Thanks in advance, Christian.

Shawn O. Pearce· Mar 6, 2008, 06:42 UTC · re: Christian Couder · lore

Re: [PATCH] run-command: Redirect stderr to a pipe before redirecting stdout to stderr

Christian Couder <chriscool@tuxfamily.org> wrote:

Show 12 quoted lines

> On Wed, 5 Mar 2008, I wrote:
> > With this patch, in the 'start_command' function after forking
> > we now take care of stderr in the child process before stdout.
> > ...
> 
> I am afraid I forgot the 
> 
> From: "Shawn O. Pearce" <spearce@spearce.org>
> 
> at the top of the patch in my last resend, so the patch was attributed to me instead of Shawn.
> 
> Is it possible to do something about it ? (I fear not since it is now in 'next' but maybe I am wrong.)

Nope, its done and over with.

Don't sweat it. It was a very short patch, you wrote it apparently independently from me, so the author line from you is just as valid.

I say its fine as is. Maybe better. Your version had an API doc update. My version is what triggered the "maybe we should document the API before we modify it further" discussion. ;-)

-- 
Shawn.