How to set pseudo terminal non-blocking?

john.daker · 12-15-2008, 09:58 PM

Hi,all
I'm writing a little program using pseudo terminal and I want non-blocking read from pty. So I first using select:

Code:

fd_set rfs;
FD_ZERO(&rfs);
FD_SET(tty,&rfs);
struct timeval tv;
tv.tv_sec=0;
tv.tv_usec=0;
if((select(tty+1,&rfs,NULL,NULL,&tv)>0)){
     if(FD_ISSET(tty,&rfs)){
           n=read(tty,buf,MAX_BUFFER);
....

But my program will hang, and I use gdb to attach the process found that it block in read().
I did a little googling and found that 'select() may give a
"false indication" of readability ... a read() would complete at the time select() was done, but later (when you actually issue the read())
it might block. '
in http://fixunix.com/unix/84142-portab...nal-input.html

Then I using non-blocking by turning on NON_BLOCKING bit :

Code:

pid=forkpty(&tty,NULL,NULL,&ws);
flag=fcntl(tty,F_GETFL,0);
flag|=O_NONBLOCK;
fcntl(tty,F_SETFL,flag);
.....
fd_set rfs;
FD_ZERO(&rfs);
FD_SET(tty,&rfs);
struct timeval tv;
tv.tv_sec=0;
tv.tv_usec=0;
if((select(tty+1,&rfs,NULL,NULL,&tv)>0)){
     if(FD_ISSET(tty,&rfs)){
           n=read(tty,buf,MAX_BUFFER);
           if(n<=0&&errno==EAGAIN){
            ..........

But it will block anyway, so how to do a non-blocking read with pty?

Thanks.

dwhitney67 · 12-17-2008, 09:00 PM

See if this will work for you:

PHP Code:


			
#include <termios.h>

#include <string.h>

...



struct termios old;

struct termios tmp;



if (tcgetattr(tty, &old))

{

  perror("failed to obtain tty attributes");

}



memcpy(&tmp, &old, sizeof(old));



tmp.c_lflag &= ~ICANON;



if (tcsetattr(tty, TCSANOW, (const struct termios*) &tmp))

{

  perror("failed to configure tty descriptor");

}



// perform read()

...



// restore tty settings (optional?)

tcsetattr(tty, TCSANOW, (const struct termios*) &old);



...

mastemmer · 12-18-2008, 01:31 PM

I use the following command to set the tty descriptor as non-blocking:

Code:

fcntl(tty, F_SETFL, FNDELAY);

Then you may use select to read only when there is data available:

Code:

char buffer[80];
size_t n;
fd_set myset;
FD_ZERO(&myset);
FD_SET(tty,  &myset);
select(FD_SETSIZE, &myset, NULL, NULL, NULL);
if(n=read(tty,buffer, 80)<0) perror("reading");

You can restore normal blocking behavior with

Code:

fcntl(STDIN_FILENO, 0, FNDELAY);

If you are dealing with a console, (keyboard) maybe you want to change it to raw mode so that you can process every key as soon as you type it. Normally there is an input buffer that only sends data to the program when you press ENTER. To set standard input to raw mode use the following:

Code:

struct termios mytty, kbdios;
/* This function is called whenever the program terminates */
void back_to_normal(void)
{
/* Set console back to normal */
fcntl(STDIN_FILENO, 0, FNDELAY);
tcsetattr(STDIN_FILENO, TCSANOW, &kbdios);
}

void kbd_rawmode()
{
/* Set keyboard to raw mode */
tcgetattr(STDIN_FILENO, &kbdios);
atexit(back_to_normal);         /* Restore tty mode when exiting */
memcpy(&mytty, &kbdios, sizeof(struct termios));
mytty.c_lflag &= ~(ECHO|ECHONL|ICANON|ISIG|IEXTEN);
tcsetattr(STDIN_FILENO, TCSANOW, &mytty);
/* These calls make read to return imedately even when there is
   no data in the input stream */
fcntl(STDIN_FILENO, F_SETFL, FNDELAY);
}

See a RS232 terminal the i wrote:
http://www.ee.pucrs.br/~stemmer/linux/ltser.c
Se also a nice tutorial in
http://www.easysw.com/~mike/serial/

bytehunter · 12-22-2008, 09:31 PM

I did the following for a pseudo-term I did, similar to what has been posted:

int rc=0;
struct termios params;

// Get terminal atributes
rc = tcgetattr(serialDev, &params);

// Modify terminal attributes
cfmakeraw(&params);

rc = cfsetispeed(&params, B9600);

rc = cfsetospeed(&params, B9600);

// CREAD - Enable port to read data
// CLOCAL - Ignore modem control lines
params.c_cflag |= (CLOCAL|CREAD);

// Make Read Blocking
fcntl(serialDev, F_SETFL, 0);

// Set serial attributes
rc = tcsetattr(serialDev, TCSANOW, &params);

// Flush serial device of both non-transmitted
// output data and non-read input data....
tcflush(serialDev, TCIOFLUSH);