undefined reference to `ioctl_tty'
Hi Everyone, I'm working from http://man7.org/linux/man-pages/man4/tty_ioctl.4.html . According to the man page: TIOCEXCL void Put the terminal into exclusive mode. No further open(2) operations on the terminal are permitted. (They fail with EBUSY, except for a process with the CAP_SYS_ADMIN capability.) The page goes on to say in the colophon it is part of the 4.16 kernel. I am running on the 4.20 kernel: $ uname -a Linux silo 4.20.13-200.fc29.x86_64 #1 ... GNU/Linux However: gcc -D_GNU_SOURCE -g2 -std=gnu99 test.c -o tttt -lrt test.c: In function ‘main’: test.c:11:7: warning: implicit declaration of function ‘ioctl_tty’; did you mean ‘ioctl’? [-Wimplicit-function-declaration] if (ioctl_tty(1 /*STDOUT_FILENO*/, TIOCEXCL, NULL) == -1) { ^~~~~~~~~ ioctl /usr/bin/ld: /tmp/ccCNNyG3.o: in function `main': /home/test/test.c:11: undefined reference to `ioctl_tty' collect2: error: ld returned 1 exit status Search is producing a lot of non-relevant hits. Any ideas why I can't find ioctl_tty during link when using a 4.20 kernel? Thanks in advance. ---------- $ cat test.c #include <stdio.h> #include <string.h> #include <errno.h> #include <termios.h> #include <fcntl.h> #include <sys/ioctl.h> int main(int argc, char* argv[]) { if (ioctl_tty(1 /*STDOUT_FILENO*/, TIOCEXCL, NULL) == -1) { fprintf(stderr, "%s\n", strerror(errno)); return 1; } return 0; }
On Wed, 06 Mar 2019 01:58:41 -0500, Jeffrey Walton said:
TIOCEXCL void Put the terminal into exclusive mode. No further open(2) operations on the terminal are permitted. (They fail with EBUSY, except for a process with the CAP_SYS_ADMIN capability.)
Note that this is still slightly racy - if two processes both try to do an open and an ioctl, this can happen: Process 1 open() Process 2 open() Process 1 ioctl(TIOEXCL) Process 2 ioctl() But at this point, the second process has already succeeded in opening the device, so "no further opens" doesn't save you. Usually, this sort of race would be really hard to hit, especially if the ioctl() happens right after the open() - except that many systems will use cron or similar to launch stuff "once per hour" or similar. And they end up batching together two processes that each try to do this. Suddenly, it's a lot easier to hit that hole when cron launches two processes at 12:54:17 PM..... If there's one thing I've learned from almost 4 decades of sysadmin'ing on all sorts of systems, saying "Oh that timing hole is so tiny it won't ever get hit in production" guarantees that the Clock Gods will smite you for your hubris. :)
On Wed, Mar 6, 2019 at 3:36 AM <valdis.kletnieks@vt.edu> wrote:
On Wed, 06 Mar 2019 01:58:41 -0500, Jeffrey Walton said:
TIOCEXCL void Put the terminal into exclusive mode. No further open(2) operations on the terminal are permitted. (They fail with EBUSY, except for a process with the CAP_SYS_ADMIN capability.)
Note that this is still slightly racy - if two processes both try to do an open and an ioctl, this can happen:
Process 1 open() Process 2 open() Process 1 ioctl(TIOEXCL) Process 2 ioctl()
But at this point, the second process has already succeeded in opening the device, so "no further opens" doesn't save you.
Usually, this sort of race would be really hard to hit, especially if the ioctl() happens right after the open() - except that many systems will use cron or similar to launch stuff "once per hour" or similar. And they end up batching together two processes that each try to do this. Suddenly, it's a lot easier to hit that hole when cron launches two processes at 12:54:17 PM.....
If there's one thing I've learned from almost 4 decades of sysadmin'ing on all sorts of systems, saying "Oh that timing hole is so tiny it won't ever get hit in production" guarantees that the Clock Gods will smite you for your hubris.
Yeah, the race seems to be the downside to ioctl and TIOEXCL. It is too bad Linux does not honor O_EXCL for the device, or provide a similar open flag like SOCK_NONBLOCK was added to avoid the race in sockets. It would avoid a lot of problems. Jeff
On Wed, 06 Mar 2019 03:50:10 -0500, Jeffrey Walton said:
Yeah, the race seems to be the downside to ioctl and TIOEXCL.
It is too bad Linux does not honor O_EXCL for the device, or provide a similar open flag like SOCK_NONBLOCK was added to avoid the race in sockets. It would avoid a lot of problems.
-ENOPATCH :)
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Jeffrey Walton -
valdis.kletnieks＠vt.edu