Thread: read string from socket

  1. #1
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    read string from socket

    I hardly ever touch C so apologies if this is totally basic!

    After reading data from a socket all I want to do is print the string to the screen.

    I have this code, as part of a larger script:

    Code:
    void read_socket(int s) {
       int status;
       char result[1024];
       
       status = read(s,result,sizeof(result));
       
       if ( status < 0 ) {
          fprintf(stderr,"Error reading from socket.");
          exit(0);
       }
       
       printf("string: %s\nbytes: %d\n", result, status);
    
    }
    The output of this is:

    string:
    bytes: 948

    As far as I can see 'result' should be holding the string which is 948 bytes in size but when trying to print it to the screen I am getting nothing, any ideas please?

    Thanks, Geester

  2. #2
    Hurry Slowly vart's Avatar
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    And what is the contents of the string? maybe there is just 0 as the first character?
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

  3. #3
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    Quote Originally Posted by vart View Post
    And what is the contents of the string? maybe there is just 0 as the first character?
    It is a small string of xml, I have it working fine in PHP, looking at it with that script I just noticed that the beginning of the string contains some strange characters:



    I assume this may be causing the problem? Any idea how to get around this please?

    Thanks, Grant

  4. #4
    Hurry Slowly vart's Avatar
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    Try fo the start to print several chars at the beginnign of the string in hex to see if you receive something
    All problems in computer science can be solved by another level of indirection,
    except for the problem of too many layers of indirection.
    David J. Wheeler

  5. #5
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    Hmm, not sure if I did this correctly but I tried:

    Code:
    printf("%x\n",result);

    And this is the output : a297b80

    ????


    Thanks, Grant

  6. #6
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    read does not put a null terminator on the end of the string.

  7. #7
    and the Hat of Guessing tabstop's Avatar
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    Quote Originally Posted by geester View Post
    Hmm, not sure if I did this correctly but I tried:

    Code:
    printf("%x\n",result);

    And this is the output : a297b80

    ????


    Thanks, Grant
    result is an array, and when used by itself it almost always decays down to a pointer to the first element, and %x means "print this integer in hexadecimal format". So, you printed the address of your buffer.

    Maybe
    Code:
    printf("%c%c%c%c", result[0], result[1], result[2], result[3]);
    to see the first characters of your result? And if you wanted more you could loop.

  8. #8
    Frequently Quite Prolix dwks's Avatar
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    Note that you could use something like this to limit printf() to printing the right number of characters:
    Code:
    printf("&#37;.*s", length, string);
    where length is status and string is result in your case.

    Of course, that only works if there is actually something in the string, and would stop immediately upon finding a NULL in the string. Perhaps fwrite() or write() would be better.
    dwk

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
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  9. #9
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    Thanks for all your help, Im only testing something at the moment and just need the first part of the string so looping this, as provided above:

    Code:
    printf("%c", result[x]);
    does the job.

    Thanks, Geester

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