Linux non-canonical input mode works different in Linux and WSL - c

Question

The program below demonstrates the issue. When you build it with clang main.c -o main, run, and press a few keys, you'll see that the program always stops at "Calling poll()..." on Linux. Now if you do the same on an up-to-date WSL (Windows Subsystem for Linux), you will see that it stops at "Calling read()...". In other words, on Linux it's poll() that blocks, on WSL it's read().
The program basically sets the input to non-canonical mode, with VTIME and VMIN also set 0. If I'm reading the two references below right
https://www.gnu.org/software/libc/manual/html_node/Canonical-or-Not.html
https://www.gnu.org/software/libc/manual/html_node/Noncanonical-Input.html
then I think in this case read() should never block and I should never see the program wait after printing "Calling read()...". Yet on WSL this is the case.
Now if you pass -DNONBLOCK when compiling and run, both on Linux and WSL the program always blocks after printing "Calling poll()...", which is the expected behavior.
I'm wondering if this is a bug on WSL, or I'm misreading the non-canonical mode documentation.
Program:
// clang main.c -o main
#include <errno.h>
#include <fcntl.h>
#include <poll.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <termios.h>
#include <unistd.h>
void main_loop();
bool read_until_empty();
int main()
{
#if defined(NONBLOCK)
int old_stdin_flags = fcntl(STDIN_FILENO, F_GETFL);
fcntl(STDIN_FILENO, F_SETFL, old_stdin_flags | O_NONBLOCK);
printf("Enabled non-blocking mode\n");
#endif
bool fail = false;
int tty = open("/dev/tty", O_RDWR);
if (tty == -1) {
printf("Unable to open /dev/tty\n");
fail = true;
goto cleanup;
}
struct termios old_termios;
if (tcgetattr(STDIN_FILENO, &old_termios) == -1) {
printf("tcgetattr failed\n");
fail = true;
goto cleanup;
}
struct termios new_termios = old_termios;
cfmakeraw(&new_termios);
// These really need to be after cfmakeraw() !!!!
new_termios.c_cc[VMIN] = 0;
new_termios.c_cc[VTIME] = 0;
if (tcsetattr(STDIN_FILENO, TCSAFLUSH, &new_termios) == -1) {
printf("tcsetattr failed\n");
fail = true;
goto cleanup;
}
printf("Type 'q' to quit.\n");
main_loop();
cleanup:
tcsetattr(STDIN_FILENO, TCSAFLUSH, &old_termios);
#if defined(NONBLOCK)
fcntl(STDIN_FILENO, F_SETFL, old_stdin_flags);
printf("Restored stdin flags\n");
#endif
if (fail) {
return 1;
}
}
void main_loop()
{
printf("main_loop\n");
struct pollfd fds[1] = { { .fd = STDIN_FILENO, .events = POLLIN } };
for (;;)
{
printf("Calling poll()...\n");
int poll_ret = poll(fds, 1, -1);
if (poll_ret > 0)
{
printf("stdin ready for reading\n");
if (read_until_empty())
{
return;
}
}
}
}
bool read_until_empty()
{
uint8_t buf[10000];
for (;;)
{
printf("Calling read()...\n");
ssize_t n_read = read(STDIN_FILENO, buf, 10000);
if (n_read == -1)
{
if (errno == EAGAIN || errno == EWOULDBLOCK)
{
printf("EAGAIN or EWOULDBLOCK\n");
return false;
}
else
{
printf("Error %d: %s\n", errno, strerror(errno));
return false;
}
}
else if (n_read == 0)
{
printf("stdin is empty\n");
return false;
}
else
{
printf("Read %zd bytes\n", n_read);
if (buf[0] == 3 || buf[0] == 113)
{
return true;
}
}
}
}

Answer 1

Shortly after asking this I found the bug report in WSL issue tracker: https://github.com/microsoft/WSL/issues/3507
So it seems like this is a bug in WSL.