Question

In C (UNIX), how can I transfer and receive a file in multiple blocks using a socket?

For example, if I had a file of 1234 bytes, and a block size of 500, I would transfer:

• 500 bytes,
• then 500 bytes,
• then 234 bytes

I have attempted this using fseek, read, write, but I just cannot get the logic right. Even a good reference would be much appreciated.

My socket routines are:

int readn(sd, chunk, bytesToRead);

int writen(sd, chunk, bytesToWrite);

Answer 1

If you’re using TCP then all you need to do is send your data block (I assume you have some kind of protocol which tells you how many bytes are in the block, such as a header?) and when you get your block at the other end simply write it to the file that you are writing to. TCP will deal with making sure everything is arriving in the expected order so you should just be able to walk your way through the file reading in X bytes at a time and sending them and then on the recv side you simply recv your data and write it to the file… Just remember that every read you issue on your socket can return anywhere between 1 and “block size” bytes and that your protocol should be able to tell you how many to expect and that you should then loop until you have actually got as many bytes as you’d expect…

If you’re using UDP then things get a little more fun as you need to track which block a particular datagram represents…

Homework question?