sendfile

Sendfile is a system call and API used by operating systems to transfer data from a file descriptor that refers to a regular file to a network socket without copying data into user space. The primary goal is to reduce CPU usage and memory bandwidth by performing zero-copy data transfer in the kernel.

Origins and availability: The concept originated in Unix-like systems, and today Linux provides a sendfile interface while BSD-derived systems have their own variants with different prototypes and semantics. Windows and other platforms offer related functionality under different names or through different APIs.

Operation and usage: On Linux, the call is typically invoked as sendfile(outfd, in_fd, offset, count). If offset is non-null, the file’s offset is updated after the transfer; otherwise it uses the file’s current offset. The outfd usually must be a socket. The kernel can move data directly from file pages to the network stack, reducing copies between kernel and user space and improving throughput for large transfers.

Limitations and considerations: The benefits vary with file type, network stack configuration, and kernel implementation. Not all environments support zero-copy for every scenario, and portability requires handling platform-specific variants and edge cases such as headers or trailers and offset management. Security considerations include ensuring proper permissions and avoiding inadvertent disclosure of unintended file ranges when offsets are manipulated by callers.

Impact and usage notes: Sendfile is commonly used by web servers and file servers to serve static content efficiently, particularly for large files. It is typically complemented by other I/O strategies (buffered, asynchronous) and may require platform-specific wrappers to maintain cross-platform compatibility in multi-platform applications.