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To: cs551@merlot.usc.edu
Subject: Re: Regarding strlen/numbytes
Date: Fri, 17 Oct 2008 06:24:48 -0700
From: Bill Cheng <william@bourbon.usc.edu>

Someone wrote:

  > We are having an error with the string length of UOID printed out.
  > 
  > For example:
  > 
  > The first receive for the 27 bytes header is:
  > 
  >           if ((bnumbytes=recv(sockfd, &brecvBuf[0],27,0)) == -1)
  >             {
  >               perror("recv");
  >             }
  > 
  > Then we do a memcpy for parsing header fields as follows:
  > 
  >           memcpy(&brecvmsgType,&brecvBuf[0],1);       //  Get the message
  > type
  >           memcpy(&brecvUOID[0],&brecvBuf[1],20);       // Get the UOID
  >            cout<<"************uoid3 = "<<strlen((char*)brecvUOID)<<endl;  //
  > This prints out UOID Length = 20
  > 
  >           memcpy(&brecvTTL,&brecvBuf[21],1);       // Get the TTL
  >              cout<<"************uoid3 =
  > "<<strlen((char*)brecvUOID)<<endl;    // THIS PRINTS OUT 21
  > 
  >           memcpy(&bresField,&brecvBuf[22],1);  // Get the reserved field
  >               cout<<"************uoid3 =
  > "<<strlen((char*)brecvUOID)<<endl;     // THIS ALSO PRINTS OUT 21
  > 
  >           memcpy(&brecvmsgLength,&brecvBuf[23],4);                 // Get
  > the msglength
  >             cout<<"************uoid3 = "<<strlen((char*)brecvUOID)<<endl;
  > // THIS ALSO PRINTS OUT 21
  > 
  > If we print out the actual UOID received , the 20 byte UOID is same as was
  > sent by the sender.
  > 
  > But the length of the UOID keeps changing due to some reason, and this
  > changes the total number of bytes sent henceforth, that results in an error
  > in all subsequent send/receives.
  > 
  > All we are doing is a memcpy like warmup#1 , are we printing the string
  > length incorrectly ?

UOID is the output of the SHA1() function.  It's always 20
bytes long and it's *binary*, which means that it can
contain 0x00.

strlen() is a function that will return the length of a
*null-terminated string*.  As soon as it sees a 0x00,
it returns the length of *what it thinks is a string*.
Therefore, strlen() *cannot* be used with binary data.

You also need to print binary data differently.  You can
use:

    for (i=0; i < 20; i++) {
        printf("%02x", (unsigned char)buf[i]);
    }
--
Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>