select function does not return immediately

cricketlong · 09-12-2011, 11:12 AM

I am writing a server and a client which communicate via USB-TTL devices. Here are the example codes of server and client.
server:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <memory.h>

int main()
{
    int fd_ttl;
    int read_len;
    fd_set readfds;
    char buf[64];

    fd_ttl = open("/dev/ttyUSB0", O_RDWR);

    if(fd_ttl < 0)
        return -1; 

    while(1)
    {   
        FD_ZERO(&readfds);
        FD_SET(fd_ttl, &readfds);

        if(select(fd_ttl + 1, &readfds, NULL, NULL, NULL) < 0)
            continue;

        if(FD_ISSET(fd_ttl, &readfds))
        {   
            bzero(buf, sizeof(buf));
            read_len = read(fd_ttl, buf, 1); 
            printf("%c", buf[0]);
            if(buf[0] == 0x00)
                printf("\n");
        }
    }

    close(fd_ttl);

    return 0;
}

client:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <fcntl.h>

int main(int argc, char **argv)
{
    int fd_ttl;
    int wr_len;
    char buf[] = "Cricket";

    fd_ttl = open("/dev/ttyUSB0", O_RDWR);

    wr_len = write(fd_ttl, buf, sizeof(buf));
    printf("%d byte(s) written for %d\n", wr_len, fd_ttl);

    close(fd_ttl);

    return 0;
}

The server never print the content the client sends to it, instead, the data are buffered for the corresponding file descriptor. When I use the command cat "hallo" > /dev/ttyUSB0, all the data include that are buffered and sent with cat command are printed.

Does the redirect(>) do anything extra to signal select?

ta0kira · 09-12-2011, 11:56 AM

Does it work properly if you block on read and skip select? How do you know the block is on select and not on read? Redirection just opens the output file and uses dup2 to replace standard output before executing the program whose output is being redirected.
Kevin Barry

cricketlong · 09-12-2011, 01:10 PM

thanks, I am sure it blocks on select not on read. I use the code below to test:

Code:

......
if(select(fd_ttl + 1, &readfds, NULL, NULL, NULL) < 0)
     continue;

printf(FD_ISSET);//this line is for test, it is printed only when I use the command cat "hallo" > /dev/ttyUSB0
if(FD_ISSET(fd_ttl, &readfds))
{
......
}

ta0kira · 09-12-2011, 01:52 PM

Have you tried read without select?
Kevin Barry

cricketlong · 09-13-2011, 02:43 AM

As your suggestion, I tried read without select. But another problem blocked me: the program blocks on open function. Then I tried to figure it out. First I entered cat /dev/ttyUSB0 in one terminal and entered echo "Cricket" > /dev/ttyUSB0 in another, they both hang, no any output and not move on. CTRL+C to any of them, the other ends with "/dev/ttyUSB0: Input/output error" prompted.

I know it is going to be more and more troublesome......

cricketlong · 09-15-2011, 01:52 AM

Thanks, ta0kira. My programs are working well now. When my server hang on open function, modprobe -r pl2303 then modprobe pl2303 makes it running smoothly. I still don't understand what is the problem about both problems, maybe they solved by time.

TimothyEBaldwin · 09-16-2011, 05:57 PM

By default tty devices use line buffering with echo, so the data will not be sent your server program until a newline is received, and deadlock will quickly result.

Read the Serial Programming Guide for POSIX Operating Systems - Michael R Sweet

ta0kira · 09-16-2011, 10:04 PM

Quote:

Originally Posted by TimothyEBaldwin

By default tty devices use line buffering with echo, so the data will not be sent your server program until a newline is received, and deadlock will quickly result.

Read the Serial Programming Guide for POSIX Operating Systems - Michael R Sweet

Are you talking about canonical input mode like terminals have?
Kevin Barry

TimothyEBaldwin · 09-17-2011, 09:41 AM

cricketlong · 09-19-2011, 11:00 PM

Danke schön, guys.