Problem Conditions:
1) linux 2.6.11.7, but is probably otherplaces too
1) realtime thread
2) serial port open()ed *either* with NON_BLOCKING or not
3) ioctl( FIONREAD ) always returns zero
int ncharin;
ioctl( fd, FIONREAD, &ncharin )
The above lines of code operate incorrectly from a real time thread on a
normal PC serial port. They ncharin is always set to zero if called
from a loop. The work around is to block the calling realtime thread
with a sleep of some kind. Said workaround is ugly. Need this to work
from a realtime thread.
The program below illustrates the problem, but requires two computers.
On one computer run minicom and connect an RS-232 cross over cable to
the test box (at COM2, per the test source). Compiler and run the
program below on the test box. Set minicom to 38400 kbaud to match the
baudrate in the program below. Compile test program. Run the test
program as root so it can escalate the priority to realtime.
It will only run for 20 seconds, while it's running, you should send
characters from minicom to the test box and you will see the buffered
count of characters increment by 1. That is how it is supposed to
work. Then comment out the usleep() call and recompile and do the same
test again. Here you see the test program does not operate properly.
The buffered character count remains at 0. What causes this? Where
do I patch?
-----------------------------------
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <linux/serial.h>
#include <sched.h>
#include <time.h>
int fd;
int openPort();
void setRealtimePriority();
int main( int argc, char** argv )
{
int ncharin;
int last;
time_t start;
if( openPort() != 0 )
{
fprintf( stderr, "unable to open port\n" );
exit(1);
}
setRealtimePriority();
// poll the FIONREAD and print the number of chars in the recv buffers
start = time(NULL);
// run only for 20 seconds, since realtime priority steals cpu from
consoles.
while( time(NULL) - start < 20 )
{
if( -1 == ioctl( fd, FIONREAD, &ncharin ) )
{
printf( "error from ioctl\n" );
ncharin = 0;
}
// if this line is commented out, then ioctl FIONREAD does not work.
usleep(1);
if( last != ncharin )
{
// this should show a monotonically increasing number of
characters.
printf(" %d", ncharin );
fflush(stdout);
last = ncharin;
}
}
}
int openPort()
{
struct termios t;
/* The O_NDELAY flag tells UNIX that this program doesn't care what
state the DCD signal line is in - whether the other end of the port
is up and running. If you do not specify this flag, your process
will
be put to sleep until the DCD signal line is the space voltage.
*/
fd = open( "/dev/ttyS1", O_RDWR
| O_NDELAY
| O_NOCTTY );
if( fd == -1 )
return -1;
memset( &t, 0, sizeof(t) );
cfsetispeed( &t, B38400 );
cfsetospeed( &t, B38400 );
// Enable the receiver and set local mode...
// CLOCAL : local connection, no modem contol
// CREAD : enable receiving characters
t.c_cflag |= (CLOCAL | CREAD);
t.c_cflag &= ~CSTOPB;
t.c_cflag &= ~CSIZE; // Mask the character size bits
t.c_cflag |= CS8; // Select 8 data bits
t.c_cflag &= ~PARENB;
t.c_cflag &= ~CSTOPB;
// Raw input is unprocessed.
// Input characters are passed through exactly as they are received,
// when they are received. Generally you'll deselect the ICANON, ECHO,
// ECHOE, and ISIG options when using raw input:
t.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
// Raw output is selected by resetting the OPOST option in the
c_oflag member:
t.c_oflag &= ~OPOST;
t.c_cc[VMIN] = 1;
t.c_cc[VTIME] = 0;
// Set the new options for the port...
tcsetattr( fd, TCSANOW, &t );
return 0;
}
void setRealtimePriority()
{
int ec;
printf("setRealtimePriority()\n");
struct sched_param p;
memset( &p, 0, sizeof(p) );
p.sched_priority = 2;
ec = sched_setscheduler( 0, SCHED_RR, &p );
if( ec != 0 )
{
fprintf(stderr, "pthread_setschedparam ec=%d, %s\n", ec,
strerror(ec) );
fflush(stderr);
exit(2);
}
}