fillbuf

Fillbuf is a term used in programming to describe a routine that fills a pre-allocated buffer with data from a source. It appears in low-level I/O code, data streaming libraries, and various language runtimes. Because it is not a formal standard, the exact interface and semantics differ between languages and frameworks; however, the core idea is to move data into a buffer so that subsequent processing can proceed without further source queries during the fill operation.

Common behavior: A fillbuf operation accepts a pointer (or reference) to a memory buffer and a target size, and returns the number of bytes actually filled. It may block until data becomes available or return immediately to indicate that no data was currently obtainable. If the underlying source can provide only a partial amount rather than the full request, fillbuf will either loop internally to complete the request or report the partial fill to the caller along with an indication of whether more data is expected. Robust implementations distinguish between end-of-file/end-of-stream and error conditions.

Context and usage: In file I/O, a fillbuf call may replenish an in-memory cache from a disk file. In network programming, it often refills the input buffer from a socket as data arrives. In asynchronous or non-blocking environments, fillbuf is usually integrated with event loops or futures to avoid blocking the main thread. Some languages expose similar concepts under different names, such as readinto or transfer-to-buffer.

See also: Read, Readinto, Buffer, Buffering, Refill.