ioctl `TCGETS2` and friends

[marvin2k]

Hi,

using the TCGETS2 ioctl on serial ports yields Inappropriate ioctl for device in Windows Version 10.0.16299.15. Attaching this small demo program here:

#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

#include <asm-generic/termbits.h>
#include <sys/ioctl.h>

int main(int argc, char *argv[])
{
    struct termios2 old;

    int fd = open(argv[1], O_RDWR | O_NOCTTY | O_NDELAY);
    int rc = ioctl(fd, TCGETS2, &old);
    if (rc<0) {
        perror("fail");
        return rc;
    }

    close(fd);
    printf("fine\n");
    
    return 0;
}

Which, when compiling with g++ main.cpp and running with sudo strace ./a.out /dev/ttyS7, produces the following output:

execve("./a.out", ["./a.out", "/dev/ttyS7"], [/* 15 vars */]) = 0
brk(0)                                  = 0xc2d000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fffd6e80000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=62066, ...}) = 0
mmap(NULL, 62066, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fffd6e70000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0P \2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1857312, ...}) = 0
mmap(NULL, 3965632, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fffd6830000
mprotect(0x7fffd69ee000, 2097152, PROT_NONE) = 0
mmap(0x7fffd6bee000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1be000) = 0x7fffd6bee000
mmap(0x7fffd6bf4000, 17088, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fffd6bf4000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fffd6e60000
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fffd6e50000
arch_prctl(ARCH_SET_FS, 0x7fffd6e50740) = 0
mprotect(0x7fffd6bee000, 16384, PROT_READ) = 0
mprotect(0x600000, 4096, PROT_READ)     = 0
mprotect(0x7fffd6e22000, 4096, PROT_READ) = 0
munmap(0x7fffd6e70000, 62066)           = 0
open("/dev/ttyS7", O_RDWR|O_NOCTTY|O_NONBLOCK) = 3
ioctl(3, TCGETS2, 0x7fffdd40e0c0)       = -1 ENOTTY (Inappropriate ioctl for device)
dup(2)                                  = 4
fcntl(4, F_GETFL)                       = 0x1 (flags O_WRONLY)
close(4)                                = 0
write(2, "fail: Inappropriate ioctl for de"..., 37fail: Inappropriate ioctl for device
) = 37
exit_group(-1)                          = ?
+++ exited with 255 +++

Any plans on supporting this ioctl in later builds?

Greetings

[tomlogic]

Just ran into this issue 7+ years later on Windows 11 ARM with WSL v2.5.9.0, so it's still a problem. I'm opening a serial port at 3,686,400bps from Python (pyserial). When I run the program in Python 3.9 for Windows and open COMxx, it sets the baud rate just fine.

When I try to run from WSL2 with Python 3.8.10, I get:

ValueError: Failed to set custom baud rate (3686400): [Errno 25] Inappropriate ioctl for device

[wangqr]

glibc 2.42 now switches to termios2. Shell now behaves strange because isatty no longer works

[Marc-Aldorasi-Imprivata]

As a temporary workaround, I've written an override that's a copy of the old glibc implementation: https://gist.github.com/Marc-Aldorasi-Imprivata/f2eae16d8ce5fbf4a9298886dda78705
Compile it with g++ -O1 tty-fixup.cpp -shared -o tty-fixup.so, add the path of tty-fixup.so to /etc/ld.so.preload, and it should be inserted into all of your processes.

[g3nsvrv]

glibc 2.42 now switches to termios2. Shell now behaves strange because isatty no longer works

@wangqr indeed #13378

[rtiangha]

As a temporary workaround, I've written an override that's a copy of the old glibc implementation: https://gist.github.com/Marc-Aldorasi-Imprivata/f2eae16d8ce5fbf4a9298886dda78705 Compile it with g++ -O1 tty-fixup.cpp -shared -o tty-fixup.so, add the path of tty-fixup.so to /etc/ld.so.preload, and it should be inserted into all of your processes.

This fixed the issue I had with glibc 2.42 and Arch on WSL1. The terminal still worked without it, but this made it usable again. Thanks!