1. A system, comprising: a processor; and a memory coupled to the processor and configured to store program instructions executable by the processor to implement: an operating system comprising a system call interface for sending one or more data files to another system over a network via a connectionless network protocol; an application configured to invoke said system call interface to initiate sending of one or more data files to another system over the network, wherein each of the one or more data files is stored on a storage device as pre-formatted data divided into multiple segments by datagram boundaries, and wherein contents of the one or more data files are not in an address space of the application; wherein, in response to an invocation of said system call interface by the application, the operating system is configured to send the one or more data files including the contents to the another system over the network without the application copying the contents of the one or more data files into the address space of the application, wherein the operating system reads the one or more files from the storage device and prepares a plurality of datagrams for sending the contents of the one or more files, each of the datagrams corresponds to a respective one of the multiple segments and comprises a respective connectionless-protocol header and body; wherein, in response to the same invocation of the system call interface, the operating system is configured to: generate and prepare the connectionless-protocol header for each datagram of the plurality of datagrams without the application generating content for the header; and send the plurality of datagrams to the another system over the network without the application copying the contents of the one or more data files into the address space of the application.