[C Linux sockets] recv() gives messy result

This is a discussion on [C Linux sockets] recv() gives messy result within the Networking/Device Communication forums, part of the General Programming Boards category; Hey, I want to use socket programming (bluetooth) to receive NMEA data from my GPS receiver. On channel 1 is ...

  1. #1
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    [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

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    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.

  3. #3
    CSharpener vart's Avatar
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    also buf could be not null-terminated so %s is not the best way to print it...
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  4. #4
    Registered User
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    Apr 2010
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    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

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