How many datas when I send datas each time?

Sorry ,maybe my englis is very poor.
I create a socket using SOCK_STREAM,and I
want to transfer a big file,and I know I
must divide the big file,but I don't know
how many datas to divide it?and what is
the reason?

Well send() doesn't guarantee to send a whole block anyway (for a stream), so you need to deal with fragmentation on sending.

Likewise for reading from a file, you may as well read in fixed sized blocks, rather than to try to allocate some xMB buffer you're only going to use once.

Code:

---

  char buff[BUFSIZ];
  while ( (n=fread(buff,1,BUFSIZ,fp)) != 0 ) {
    char *p = buff;
    size_t  remainder = n;
    while ( remainder != 0 ) {
      n = send(sock,p,remainder,0);
      if ( n > 0 ) {
        // advance by the amount actually sent
        p += n;
        remainder -= n;
      } else if ( n == 0 ) {
        // remote disconnected
      } else {
        // error
      }
    }
  }

---

One more thing, it's a good idea to send the size of the file first, so the receiver knows how much data to expect.