tcpsocket send function 3rd argument

[lorence30]

I noticed the documentation of sf::TcpSocket in SFML 2.3.1, theres a
send (const void *data, std::size_t size, std::size_t &sent) function, tcpsocket guarantees all data it send will be sent successfully, so whats the sense of the 3rd parameter of this function?

[zsbzsb]

TCP guarantees that all data that is sent will arrive and in order without duplicates. However, if you try to send too much data all at once the OS won't be able to buffer all of it and will tell you to try to send it again - hence the 3rd parameter that tells you how much was actually sent.