Problems reading\writing to devices for embedded Linux project - c

Question

This project consists of web server (using libwebsockets library) and a motor control plugin for the web server that writes out and reads in packets to a motor control board as well as communicating with the server.
I have two file descriptors (fd) to a COM port on an embedded Linux board, one for writing and one for reading. They are both working individually, but I can't get them to work at the same time.
To test the read(), I hook the receive line for the COM port to a computer and send a packet of 16 bytes repeatedly from a terminal program. The bytes are received to the byte array and passed to the GUI via the web server. For this test, write is not disabled and writes out the COM port back to the terminal. The motors are not connected to the COM port for this test.
To test the write(), read is disabled and the web server is run. I can control the motors using the GUI and motor positions are reported to the server.
When both read and write are enabled, something appears to block when the web server is run. The motor cannot be enabled, nor run. As soon as I press Ctrl-C to stop the server, one or more packets from the motor appear to be processed.
The motors start to move and motor packet data is transferred to the GUI.
Is the read blocking for some reason? It works alone.
I have gotten the following outputs at various times by checking the return of the the read() call:
EINTR - Interrupted system call
EIO - Input/Output error
EBADF - Bad file descriptor
After the last tests of the read and write (separately), the only error I get is the latter. After quitting out of the program, I get a string of about 50 bad reads (I am printing perror to the terminal).
The read/write code (below) is running in a separate thread, with the packets being processed in a continuous loop. I was trying to isolate the timing for writing from fluctuations in the scheduler. The write routine is driven by a Linux timer at 8.333ms (120Hz) and SIGALRM, thus the "gotAlarm" variable.
Is this a problem with processes? I do not fork any processes and I want the web server to be the main process.
Is it a threading problem? Both the read and write have been working within the spawned thread.
Thank you for your input! Code is below.
Call to initialize fd's:
mc_tx_fd = InitPort("/dev/ttyS1", "COM2", O_WRONLY | O_NOCTTY, B115200);
mc_rx_fd = InitPort("/dev/ttyS1", "COM2", O_RDONLY | O_NOCTTY, B115200);
InitPort function:
int InitPort( char *port, char *name, int oflags, speed_t baudRate )
{
int fd, rg, rs; // File descriptor
fd = open(port, oflags); // Open the port like a file
assert(fd > 0); // Open returns -1 on error
struct termios options; // Initialize a termios struct
rg = tcgetattr(fd, &options); // Populate with current attributes
if( rg < 0 ) {
printf("Failed to get attr: %d, %s\n", fd, strerror(errno));
}
cfsetospeed (&options, baudRate); // Set baud rate out
cfsetispeed (&options, baudRate); // Set baud rate in (same as baud rate out)
options.c_cflag &= ~CSIZE; // Clear bit-length flag so it can be set
//8N1 Serial Mode
options.c_cflag &= ~CSTOPB; // Set stop bit: 1
options.c_cflag &= ~CRTSCTS; // Set flow control: none
options.c_cc[VMIN] = 1;
options.c_cc[VTIME] = 2;
cfmakeraw(&options);
options.c_cflag |= (CLOCAL | CREAD); // Enable receiver, and set local mode
rs = tcsetattr(fd, TCSANOW, &options); // Set new attributes to hardware
if( rs < 0 ) {
printf("Failed to set attr: %d, %s\n", fd, strerror(errno));
}
return fd;
}
Here is the SendMotorPacket function called at the creation of a new thread:
void *SendMotorPacket( void *arg )
{
char timez, checksum;
static unsigned short rcvTimer = 0;
int flags, rcvAzPos, bytesRead, wc, wo, r, t;
while( 1 ) {
// receive byte from motor
r = read( mc_rx_fd, inByte, 1 );
if( r == -1 ) {
if( errno == EINTR ) {
continue;
} else {
perror("read");
}
}
inChar = inByte[0];
// FSM to receive packet
switch( receiveState ) {
case MOTOR_HEAD_1:
if( inChar == 0xA5 ) {
receiveState = MOTOR_HEAD_2;
checksum = 0;
}
break;
case MOTOR_HEAD_2:
if( inChar == 0x52 ) {
receiveState = MOTOR_DATA;
idx = 0;
checksum = 0xA5 + 0x52;
} else {
receiveState = MOTOR_HEAD_1;
}
break;
case MOTOR_DATA:
readPacket[idx++] = inChar;
checksum += inChar;
if( idx >= 16 ) {
receiveState = MOTOR_CHECKSUM;
}
break;
case MOTOR_CHECKSUM:
if( checksum == inChar ) {
// Check status bits and set global variables
mc.az_status = *(signed short *)&readPacket[2];
mc.el_status = *(signed short *)&readPacket[10];
// If motor is disabled, set global variable for feedback to GUI
if( !az.enable ) {
mc.az_position = *(signed int *)&readPacket[4];
}
if( !el.enable ) {
mc.el_position = *(signed int *)&readPacket[12];
}
}
receiveState = MOTOR_HEAD_1;
break;
default:
break;
}
// TRANSMIT
if( gotAlarm ) {
mc.checkSum = 0;
gotAlarm = 0;
// Call both motor position functions to update position
AzMotorPos();
ElMotorPos();
if ( az.enable ) {
mc.az_control |= 0x0001;
} else {
mc.az_control &= ~0x0001;
}
if ( el.enable ) {
mc.el_control |= 0x0001;
} else {
mc.el_control &= ~0x0001;
}
*( unsigned short * )&writePacket = mc.header;
*( signed short * )&writePacket[2] = mc.az_control;
*( signed short * )&writePacket[4] = mc.az_status;
*( signed int * )&writePacket[6] = mc.az_position;
*( signed short * )&writePacket[10] = mc.el_control;
*( signed short * )&writePacket[12] = mc.el_status;
*( signed int * )&writePacket[14] = mc.el_position;
// Calculate checksum for all bytes in packet
for( int i = 0; i < 18; i++ ) {
mc.checkSum += writePacket[i];
}
*(unsigned char * )&writePacket[18] = mc.checkSum;
write( mc_tx_fd, writePacket, MC_PACKET_SIZE );
}
}
}
Here is thread creation:
void CreatePacketThread ( void )
{
int err = -1;
err = pthread_create( &packetThread, NULL, &SendMotorPacket, NULL );
if (err != 0)
printf("\ncan't create MC SEND thread :[%s]\n\n", strerror(err));
else
printf("\nMC SEND THREAD created successfully\n\n");
}
UPDATE:
Running the web server (lwsws) and then quitting results in one EIO error:
read: Input/output error
and multiple EBADF errors:
read: Bad file descriptor
read: Bad file descriptor
read: Bad file descriptor
read: Bad file descriptor
read: Bad file descriptor
read: Bad file descriptor
read: Bad file descriptor
read: Bad file descriptor
read: Bad file descriptor
read: Bad file descriptor
About 50 of them.

Answer 1

I solved the problem by moving the receive routine to a separate thread. To recap, the web server (LWSWS program from libwebsockets library) is running in headless (auto start and run) mode on a Linux single board computer(SBC). The motor control code is a plugin of LWSWS and it now spawns two threads: one for sending packets out to the motor control board, and one for receiving packets.
Originally, I started with this two-thread design, but apparently had other problems at that point. Before asking this question, I had moved the receive routine into the send thread to try to get read() and write() both working.
The working combination seems to be two threads with each having its own file descriptor to the COM port. I am surprised that a mutex is not needed, as #thekamilz suggested in the comments, but pleased that it works. If I need to expand the project, I am hopeful that this design will work; with each peripheral operating on its own thread for communication.