4-stable termios diff behaviour lc/lc_r

Julian Elischer julian at vicor.com
Tue Aug 31 11:36:11 PDT 2004

this should have been sent to -threads..
 >Hi list,
 >> I think I've found a possible bug in 4-stable, though I'm not that
 >> kind of an expert so I'll leave that decision up to you.
 >> Attached is a little test program that opens /dev/cuaa0 and tries to
 >> read a couple bytes.
 >> When compiled using "gcc vtime.c -o vtime" there's no problem. For
 >> example, when you run vtime without anything attached to cuaa0,
 >> it'll wait about 2 seconds and timeout, since VMIN=0 and VTIME=20.
 >> When compiled using "gcc -pthread vtime.c -o vtime" however, the
 >> read function returns immediately, which as far as I can understand,
 >> is not what it's supposed to do.
 >> Does anyone know if this intended or not, or how I can get the right
 >> behaviour in a threaded program?

I've done some more testing. It appears that when linked with lc_r,
VTIME is completely ignored.

As explained in termios(4), when VMIN>0, the read call will wait
indefinitely for the first byte and then use VTIME as an interbyte
timer. When using lc_r however, it'll return immediately after the
first couple bytes read, even if that's less than VMIN.

The case where VTIME==0 works as it should both with lc and lc_r.

I dont have access to a box with -current, so I can't say if the same
applies to it.

I hope somebody understands this better or could at least give me a
few pointers as to where to look in the source code.

Thanks, tijl
-------------- next part --------------
#include <fcntl.h>
#include <stdio.h>
#include <sys/types.h>
#include <termios.h>
#include <unistd.h>

extern int errno;

int main(void) {
	int fd, len;
	struct termios termset;
	uint8_t data[256];

	fd = open("/dev/cuaa0", O_RDWR | O_FSYNC);
	printf("%d %d\n", fd, errno);
	if (fd != -1) {
		tcgetattr(fd, &termset);
		cfsetspeed(&termset, B9600);
		termset.c_cc[VMIN] = 1;
		termset.c_cc[VTIME] = 0;
		tcsetattr(fd, TCSANOW, &termset);

		len = read(fd, (void *) data, 3);
		printf("%d\n", len);

	return 0;

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