Writev

Writev is a system call in POSIX-compatible operating systems that performs vectorized output by writing data from multiple user-space buffers described by an array of iovec structures to a file descriptor in a single call. The function signature is ssize_t writev(int fd, const struct iovec *iov, int iovcnt).

Each element of the iovec describes a memory region with a base pointer and a length: struct iovec { void *iov_base; size_t iov_len; }; The kernel copies at most iov_len bytes from each iov_base in sequence and writes them to the target file descriptor.

Return value and errors: On success, writev returns the number of bytes written, which may be less than the sum of all iov_len if fewer bytes are accepted (for example, due to a full socket or interrupt). If the call fails, it returns -1 and sets errno. Applications must handle partial writes by retrying with an adjusted iov array until all data is written or an unrecoverable error occurs.

Semantics: writev is designed to minimize system calls when writing data that is already split across multiple buffers, such as a protocol header and payload. It does not guarantee that all buffers are written; partial completion can occur within a single system call, or across calls if interrupted.

Portability: writev is defined by POSIX.1-2001 and is implemented on Linux, BSDs, macOS, and other Unix-like systems. Windows does not provide a direct equivalent; similar functionality exists via vectored I/O APIs such as WriteFileGather and socket-based WSASend.

Related: readv (the counterpart for reading into multiple buffers) and the iovec structure.