I've encountered characters being lost while writing large buffers to a pseudoterminal master on OpenBSD 5.2. The linked program ptycheck.c exercises this behavior on 5.2 i386/amd64 (as well as 4.7 and 4.8 i386):
https://gist.github.com/4377173 Briefly, the program opens a pty pair and generates a pseudorandom message of MESSAGE_LEN bytes. It then forks off a child process which writes this message to the master; meanwhile, the parent process attempts to read this message back through the slave. On OpenBSD 5.2, however, the parent blocks indefinitely after reading only 4088 out of 4096 expected bytes from the pty slave. Similar behavior is observed with other values of MESSAGE_LEN >= 1024. This seems to be a small issue with ptcwrite in sys/kern/tty_pty.c. When tp->t_rawq hits 1022 bytes awaiting read from the pty slave, control will jump to the "block" label from line 573. But in the case that uio->uio_resid is zero (all characters have been copied from uio to the 100-byte kernel space buf on ptcwrite's stack) but cc > 0 (more characters in buf still need to be sent), control will not reenter the while (uio->uio_resid > 0) { /* ... */ } loop upon jumping to "again" after tsleep, and these remaining characters are lost. i.e., up to BUFSIZ == 100 characters can be discarded when performing writes of greater than 1022 bytes. The following patch alters the loop condition so that ptycheck.c no longer fails... Mark Index: sys/kern/tty_pty.c =================================================================== RCS file: /cvs/src/sys/kern/tty_pty.c,v retrieving revision 1.58 diff -u sys/kern/tty_pty.c --- sys/kern/tty_pty.c 22 Apr 2012 05:43:14 -0000 1.58 +++ sys/kern/tty_pty.c 25 Dec 2012 18:06:42 -0000 @@ -550,7 +550,7 @@ wakeup(&tp->t_canq); goto done; } - while (uio->uio_resid > 0) { + while (uio->uio_resid > 0 || cc > 0) { if (cc == 0) { cc = MIN(uio->uio_resid, BUFSIZ); if (cc > bufcc)
