max size_t value on send() in C

Question

I'm writing a tcp server in C but I'm facing problems on send. I read local file and send data back to the client, when the file is small I have no problems, but when it becomes bigger I have this strange situation:

server tcp:

 // create socket, bind, listen accept

// read file
fseek(fptr, 0, SEEK_SET);
// malloc for the sending buffer
ssize_t read = fread(sbuf, 1, file_size, fptr);

while(to_send>0) {
  sent = send(socket, sbuf, buf_size, 0);
  sbuf += sent;
  to_send -= sent;
}

On huge files sent becomes equals to the max value of size_t, I think that I have a buffer overflow. How can I prevent this? What is the best practice to read from a file and send it back?

Answer 1

The problem is that you send buf_size bytes every time, even if there aren't that many left.

For example, pretend buf_size is 8 and you are sending 10 bytes (So initially, to_send is also 10). The first send sends 8 bytes, so you need to send 2 more. The second time, you also send 8 bytes (Which probably reads out of bounds). Then, to_send will be will be -6, which is the same as SIZE_MAX - 5.

Simple fix is to send to_send if it is smaller:

sent = send(socket, sbuf, to_send < buf_size ? to_send : buf_size, 0);

Also, send returns -1 if it is unsuccessful. This is the same as SIZE_MAX when it is assigned to a size_t. You would need some error handling to fix this.