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Subject:	EBADF from write() if foreign pty is opened via /proc/PID/fd/N symlink
To:	cygwin AT cygwin DOT com
Message-ID:	<82b490da-145a-1428-e253-6f47983794db@t-online.de>
Date:	Wed, 21 May 2025 12:14:31 +0200
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From:	Christian Franke via Cygwin <cygwin AT cygwin DOT com>
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Cc:	Christian Franke <Christian DOT Franke AT t-online DOT de>
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Unexpected behavior of an uncommon use case:

$ uname -r
3.7.0-0.98.gb39b510c1ce6.x86_64

$ cygstart mintty - # Start second terminal

In second terminal:

$ echo $$; tty
1219
/dev/pty1

Back to first terminal:

$ dd of=/proc/1219/fd/1 <<<"Hello" # errno=EBADF
dd: error writing '/proc/1219/fd/1': Bad file descriptor
0+1 records in
0+0 records out
0 bytes copied, 8.22e-05 s, 0.0 kB/s

$ readlink /proc/1219/fd/1
/dev/pty1

$ dd of=/dev/pty1 <<<"Hello" # OK, text appears in 2nd terminal
0+1 records in
0+1 records out
6 bytes copied, 5.41e-05 s, 111 kB/s

$ dd of=/proc/$$/fd/1 <<<"Hello" # OK
Hello
0+1 records in
0+1 records out
6 bytes copied, 4.77e-05 s, 126 kB/s

The problem apparently only occurs if /proc/PID/fd/N points to a foreign 
pty. It works for own pty or regular files.

-- 
Regards,
Christian


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