How recv() function works when looping?

Question

I read in MSDN about the send() and recv() function, and there is one thing that I'm not sure I understand.

If I send a buffer of size 256 for example, and receive first 5 bytes, so the next time I call the recv() function, it will point to the 6th byte and get the data from there?

for example :

char buff[256];
memcpy(buff,"hello world",12);
send(sockfd, buffer, 100) //sending 100 bytes

//server side:
char buff[256];
recv(sockfd, buff, 5) // now buffer contains : "Hello"?
recv(socfd, buff,5) // now I ovveride the data and the buffer contains "World"?

thanks!

recv() doesn't change where your buffer points.. but it tells you how much it read so you can continue from where it left off. You need to adjust the pointer and size you pass it yourself, though. — Dmitri

user207421 user207421 · Accepted Answer · 2014-11-30T00:37:09

The correct way to receive into a buffer in a loop from TCP in C is as follows:

char buffer[8192]; // or whatever you like, but best to keep it large
int count = 0;
int total = 0;

while ((count = recv(socket, &buffer[total], sizeof buffer - total, 0)) > 0)
{
    total += count;
    // At this point the buffer is valid from 0..total-1, if that's enough then process it and break, otherwise continue
}
if (count == -1)
{
    perror("recv");
}
else if (count == 0)
{
    // EOS on the socket: close it, exit the thread, etc.
}

How recv() function works when looping?

2 Answers