Serial communication problems in ARM Linux

[tepeyolotl]

Hi, i'm porting an application in C to ARM Linux, the problem is that i found some weird code and i'm thinking in a workaround, but i would like some advices since i'm not an expert in Linux so here is the piece of code:

while(read(hRS232Port,&resp[i],1)>0)
{	
	if(resp!="\x0")
	{		
		copy[j]= resp[i];
		j++;
		printf("%d,", resp[i]);			
                msleep(60); //sleep in milliseconds			
	}
}

The problem is that this particular piece of code blocks the whole application and nothing can take'em out of this loop. I understand what is trying to do, but i got some doubts:

a) the read() is a blocking call?
b) i was thinking of a possible workaround using just the read function but increasing the number of bytes to read; this could work?
c) As far as i know the condition of returning zero is in succesful reading the amount of bytes indicated and reaching the EOF. but in serial how can i reach this condition?

Thanks in advance.

[Rubberman]

Please show the entire function this is being used in. There are a number of potential bugs here that can cause a busy loop which can suck up all your processor cycles.

[tepeyolotl]

Hi, i've founded a partial workaround changing the while loop with a do-while (the application does not block anymore), but still persists the problem of breaking the loop, since i don't know the lenght of the string received.

void vdgetInfo()
{
	char cmd[6];
	char resp[100+1];   	
        char copy[100];
	int rcvd = 0;
	char dummy;
	
	//DATA COMMAND
	memset(cmd,'\x0',sizeof(cmd));
	memset(resp,'\x0',sizeof(resp));
	memset(copy,'\x0',sizeof(copy));
	strcpy(cmd,"\x87");
	strcat(cmd,"482");   
	strcat(cmd,"\x88");
	
	
	//hRS232Port = inInitializeSerialPort("/dev/ttySAC0");
	
	if (hRS232Port > 0)//Already initialized
	{
		
		rcvd = write(hRS232Port,cmd,strlen(cmd));
		printf("Bytes written: %d\n", rcvd);
		read(hRS232Port,&dummy,1);//Read ACK
		printf("Response: %d", dummy);
	        msleep(50);
                rcvd = read(hRS232Port, resp, sizeof(resp));
                do
		{
			rcvd= read(hRS232Port,&resp[i],1);
			if(resp != "\x0")
			{
				copy[j]= resp[i];
				j++;
				printf("%d,", resp[i]);
				msleep(60);
			}
		}while ( rcvd != 0);
		printf("\n%s\n", resp);		
		msleep(50);
	}
	else//Error on the port handler
	{
		printf("ERROR\n");
		msleep(5000);
	}
}

Best regards.

[Rubberman]

I presume you didn't write this code?

[theNbomr]

The usual method to deal with checking for asynchronously received data in a loop is to use the select() function. When correctly configured, it will allow you to test a group of input streams, and only call read() on each one when it has data to provide.
---- rod.

[Rubberman]

Please show the code where the port is opened.

[Rubberman]

The usual method to deal with checking for asynchronously received data in a loop is to use the select() function. When correctly configured, it will allow you to test a group of input streams, and only call read() on each one when it has data to provide.
---- rod.

Generally true, but there are times when you don't want to use select(). I'm not making any judgements until I get a better idea of what is going on here.

[tepeyolotl]

That's right this isn't my code, this code gave me a lot of headaches
The structures showed up here are device specific and privative functions/libraries, that's the reason i can't use termios.h

int inInitializeSerialPort(char* szPort)
{
	if((hRS232Port = open(szPort, O_RDWR)) < 0)
		{
			printf("Error cannot open %s errno: %d\n", szPort, errno);	
			return -1;
		}

	stConfig.rate = 19200;
	stConfig.format = cfg_A8N1 | cfg_2stp | cfg_DTR;
	stConfig.protocol = Char_mode;
	if(SetOpenBlock(hRS232Port,&stConfig) < 0) 
			return -1;
	printf("RS232 Port opened with no errors, handle: %d\n", hRS232Port);
	return(hRS232Port);
}

All Rx/Tx in the serial port, when the amount of expected data is known works great. Also i tried to do a Flush function changing to non block with fcntl function but it doesn't work i'm investigating if it's mandatory that sleep for 60ms because the given documentation doesn't specify that .

Best Regards

[Rubberman]

Do you know if there is anywhere you are setting the port for non-blocking access?

[tepeyolotl]

AFAIK the default is blocking, and as i said i tried to change it with the fcntl function but it doesn't work, the fcntl gave no errors but the function was still blocking