Sockets and Binary data

oulevon · 04-05-2006, 11:43 PM

I have to write a program that sends binary files over a TCP socket in C. I've worked with sockets before but only with text. Is there anything I need to worry about before I start? For example, when one opens a file for reading, one needs to specify whether it's binary or not. Does this need to be done using TCP sockets too? Also, I normally use a char buffer to read data from the stream, what should one use with binary data?

Thanks for any help or pointers.

paulsm4 · 04-05-2006, 11:57 PM

Hi -

Sockets are intrinsically "binary" (for example, there's no such thing as "text mode" vs "binary mode" with a socket).

"char buff[BUFMAX]" is absolutely the right way to go, so you're covered there, too.

Byte order is definitely an issue:

Code:

  // BAD: potential read error on different platforms
  unsigned long u = 0xabcd;
  write (s, &u, sizeof (unsigned long));

  // BETTER (you'll need a corresponding "ntohl()" on the receiving end...)
  unsigned long u = htonl (0xabcd);
  write (s, &u, sizeof (unsigned long));

But basically, all of your reads and writes are just a binary byte stream. If you treat it as such, you should have no problem.

'Hope that helps .. PSM

oulevon · 04-06-2006, 12:14 AM

Thanks for your help. You answered all my questions.

Thanks again.

primo · 04-06-2006, 04:35 AM

Be aware that systems are not expected to agree on sizeof(unsigned long). Be aware of structures directly passed to socket, too, because the offsets that lead to the struct members may be different across platforms. The best way would be to strictly declare each member with types such as uint32_t, etc. and code a function to write the structure. You may use writev() to add each member and specify its size, which is better than manually parsing each one.