How to set timeout value when reading from a file descriptor.

George2 · 10-25-2003, 06:09 AM

Hello, everyone!

I want to set the reading timeout value to 5 second when using file descriptor reading from serial port. But I find my program never exits after 5 second idle time. Here is the source codes I am using under Red Hat Linux 8.0 environment.

The program never exits if I input nothing from serial port.

Source Code:

----------
#include <termios.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/signal.h>

#define BAUDRATE B38400
#define MODEMDEVICE "/dev/ttyS0"

#define FALSE 0
#define TRUE 1

int main (int argc, char** argv)
{
int fd,c;
struct termios oldtio,newtio,oldstdtio,newstdtio;
struct sigaction sa;

fd = open (MODEMDEVICE, O_RDWR | O_NOCTTY);
if (fd < 0)
{
perror (MODEMDEVICE);
exit (-1);
}

tcgetattr (fd, &oldtio);

newtio.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD;

newtio.c_iflag = IGNPAR;

newtio.c_oflag = 0;

newtio.c_lflag = 0;

newtio.c_cc [VMIN] = 1;
newtio.c_cc [VTIME] = 5;

tcflush (fd, TCIFLUSH);
tcsetattr (fd, TCSANOW, &newtio);

tcsetattr (1, TCSANOW, &newtio);

while (1)
{
if(!read (fd, &c, 1))
{
break;
}
write (1, &c, 1);
}

return 1;
}
----------

Can anyone help?

Thanks in advance,
Geo

dorian33 · 10-25-2003, 07:20 AM

Read this. Very good documentation.

George2 · 10-25-2003, 08:27 AM

Thanks, dorian33 buddy!

I have read this article, and I can not find the explanation of how to use newtio.c_cc [VMIN] and newtio.c_cc [VTIME] together. My purpose is, when a char is read I process this char and when 5 second serial input idle time is met with, the program is terminated.

I think I should use them together and I can not find how to use the two parameters together.

Can you help with my special case?

Best regards,
Geo

Quote:

Originally posted by dorian33
Read this. Very good documentation.

silatts · 10-25-2003, 09:34 AM

in general an alarm call might work with a signal catcher,. but I don't know for sure,

man alarm

?

Sil

George2 · 10-25-2003, 09:42 AM

Thanks, silatts buddy!

I can not image how to use a timer to do timeout enabled read task. I think select is better.

Can you show me your idea in detail or sample with signal alarm.

Best regards,
Geo

Quote:

Originally posted by silatts
in general an alarm call might work with a signal catcher,. but I don't know for sure,

man alarm

?

Sil

silatts · 10-25-2003, 09:45 AM

ah - you are right - select is better

Sil

infamous41md · 10-26-2003, 01:09 AM

hi, maybe this helps:

Code:

#include <stdio.h>
#include <unistd.h>
#include <sys/select.h>

#ifndef ERR_LIB
        void perr_quit(char *x){
                perror(x);
                exit(1);
        };
#endif

int main(void)
{
        fd_set  rset;
        int     fd = 0, ret = 0;
        char    buf[100];
        struct timeval tv;

        tv.tv_sec = 5;
        tv.tv_usec = 0;

        //for simplicity this references stdin
        if( (fd = dup(STDIN_FILENO)) < 0 )
                perr_quit("dup");

        //always need to zero it first, then add our new descripto
        FD_ZERO(&rset);
        FD_SET(fd, &rset);

        //wait for 5 seconds
        if( (ret = select(fd + 1, &rset, NULL, NULL, &tv)) < 0)
                perr_quit("select");
        else if(ret == 0){
                puts("timed out");
                return 1;
        }

        //ret must be positive
        if(FD_ISSET(fd, &rset)){
                read(fd, buf, 99);
                return 0;
        }

        puts("some error");
        return 1;
}

infamous41md · 10-26-2003, 01:15 AM

also, you i believe you can fix this much simpler now that i look closer. shouldn't this:
newtio.c_cc [VMIN] = 1;
newtio.c_cc [VTIME] = 5;
be
newtio.c_cc [VMIN] = 0;
newtio.c_cc [VTIME] = 5;

when VMIN = 1, you have to wait for at least 1 byte of data, then you wait up to 5 seconds
between future byte of data. when it is 0 you will wait at most 5 seconds for the first byte, and then
5 seconds between future bytes. this is covered in stevens(APUE) term i/o section.