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To: cs551@merlot.usc.edu
Subject: Re: Problem using memcpy and send function
Date: Sat, 06 Sep 2008 08:57:14 -0700
From: Bill Cheng <william@bourbon.usc.edu>

Someone wrote:

  > I have a doubt in using memcpy.
  > I have a structure
  > 
  > typedef struct tagReqMsg {
  >  unsigned short MsgType;
  > 
  >  unsigned int Offset;
  >  unsigned int DataLen;
  > 
  >  char *Data;
  > } ReqMsg;
  > 
  > 
  > ReqMsg *request;
  > char *msg_buf = (char*)malloc(10);  ...for eg.
  > 
  > memcpy(msg_buf,*request.MsgType,2);
  > memcpy(msg_buf[2],*request.Offset,4);
  > memcpy(msg_buf[6],*request.DataLen,4);
  > 
  > The memcpy statement gives me a segmentation fault.
  > How can I copy the contents of pointer to a structure into a char buffer?

Did you turn on compiler warning (-Wall) when you compile?
Please try to fix the warning and if you still don't know how
to fix your code, please ask me again.

  > Second question,
  > 
  > In the send function how can I send a char?
  > for eg.
  > while(msg_buf[index] != '\0'){
  >           send(sockfd, msg_buf[index], 1, 0) ;
  >           index++;
  > }.
  > 
  > The above code breaks at the send function giving some argument mismatch
  > error.

The 2nd argument for send() should be compatible with
(const char *) and it should be a memory location and
not a character.  msg_buf[index] is a character (or
unsigned char, depending on how you define msg_buf).
You need the address of this character as a starting
point of a memory buffer.  So, you should use put an
ampercent in front of it, i.e.:

     &msg_buf[index]
--
Bill Cheng // bill.cheng@usc.edu <URL:http://merlot.usc.edu/william/usc/>