Thread: [C Linux sockets] recv() gives messy result

Hey,
I want to use socket programming (bluetooth) to receive NMEA data from my GPS receiver.

On channel 1 is the serial Port service.

When I use the rfcomm program available on linux and connect the bluetooth device to a specific device file /dev/rfcomm1 I receive clear data

Code:

rfcomm connect /dev/rfcomm1 <btaddr>
cat /dev/rfcomm1

gives

Code:

$GPGGA,1745.000,457.2558,N,00559.1555,E,1,5,0.55-9.5,M,55.1,M,,*75

$GPGSA,A,3,55,26,55,15,55,22,55,27,,,,,1.55,0.93,1.25*05

$GPGSV,3,1,55,15,55,248,19,58,58,064,55,57,47,556,15,26,30,302,31*55

$GPGSV,3,2,12,18,29,305,29,08,20,073,17,17,16,119,,05,13,189,22*73

$GPGSV,3,3,12,22,06,330,18,24,04,285,,21,01,281,,38,,,*4A

$GPRMC,555545.055,A,5557.5558,N,00559.1552,E,0.55,155.28,155510,,,A*65

$GPVTG,155.25,T,,M,0.25,N,0.53,K,A*3D

But when I use a simple recv() call in a while loop.

Code:

while(1)
{
		bytes_recv = recv(sock, &buf, sizeof(buf), 0);
		printf("%s", buf);
}

I get a more messy result

Code:

VTG,155.55,T,,M,0.55,N,0.21,K,A*35
555,E,0.15,555.46,15510,,,A*67
$GP
$GPGSA,A,5,18,26,27,15,55,28,58,,,,,,4.00,2.73,2.92*07
$GPGSV,3,1,55,15,80,243,27,55,48,064,35,25,45,255,25,55,30,355,25*75
$GPGSV,3,2,15,25,50,352,25,08,20,553,21,15,16,119,,05,12,189,*79
$GPGSV,3,3,12,25,07,359,25,24,03,554,18,21,51,281,,51,,,*4D
$GP

Does anyone knows how this is possible and how to fix it.
Of course it's possible because the rfcomm program can do it.

Martijn

You probably need to pay attention to bytes_recv -- any particular recv call may not get the entire message, for one reason or another, so if it (appears to be) incomplete, then you will need to paste the next message onto the end of the first one.

Yes yes,

the data certainly arrives in half messages and by using

Code:

printf("%.*s", bytes_recv, buf);

I get clear data now.

Now ofcourse... I have to add every recv() result to a string and then walk through that string to reveal every NMEA sentence.

Thanks