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[Qemu-trivial] [PATCH] Keep pty slave file descriptor open until the mas

From: Ashley Jonathan
Subject: [Qemu-trivial] [PATCH] Keep pty slave file descriptor open until the master is closed
Date: Fri, 11 Dec 2015 11:29:16 +0000

I have experienced a minor difficulty using QEMU with the "-serial pty" option:

If a process opens the slave pts device, writes data to it, then immediately 
closes it, the data doesn't reliably get delivered to the emulated serial port. 
This seems to be because a read of the master pty device returns EIO on Linux 
if no process has the pts device open, even when data is waiting "in the pipe".

A fix seems to be for QEMU to keep the pts file descriptor open until the pty 
is closed, as per the below patch.

 qemu-char.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/qemu-char.c b/qemu-char.c
index 2969c44..ed03ba0 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -1198,6 +1198,7 @@ typedef struct {
     int connected;
     guint timer_tag;
     guint open_tag;
+    int slave_fd;
 } PtyCharDriver;
 static void pty_chr_update_read_handler_locked(CharDriverState *chr);
@@ -1373,6 +1374,7 @@ static void pty_chr_close(struct CharDriverState *chr)
     pty_chr_state(chr, 0);
+    close(s->slave_fd);
     fd = g_io_channel_unix_get_fd(s->fd);
@@ -1401,7 +1403,6 @@ static CharDriverState *qemu_chr_open_pty(const char *id,
         return NULL;
-    close(slave_fd);
     chr = qemu_chr_alloc();
@@ -1422,6 +1423,7 @@ static CharDriverState *qemu_chr_open_pty(const char *id,
     chr->explicit_be_open = true;
     s->fd = io_channel_from_fd(master_fd);
+    s->slave_fd = slave_fd;
     s->timer_tag = 0;
     return chr;

Jonathan Ashley 
jonathan.ashley AT altran.com

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