BUFSIZ

BUFSIZ is a macro defined in the C standard library header stdio.h. It expands to an integer constant that represents the preferred size of the buffer used by standard I/O streams (FILE*) by default. The macro is implementation-defined and its value can vary across platforms and libraries; there is no requirement that all implementations use the same size.

Usage and purpose: BUFSIZ is commonly used to allocate buffers for I/O or to tune performance. For example, a program might declare char buffer[BUFSIZ]; to hold data read from or written to a FILE*. The macro also indicates the default buffer size used by the library for streams that are created without an explicit buffer via setbuf or setvbuf.

Notes on buffering behavior: The actual buffering of FILE streams depends on the library and the stream; some streams may be line-buffered or unbuffered in certain conditions. BUFSIZ is a guideline for the default buffer size and does not by itself guarantee any specific buffering mode.

History and origin: BUFSIZ originates from early Unix C libraries and stands for "buffer size." It has become a standard symbol in C's stdio.h and remains widely supported across modern implementations as a portable reference to the library’s recommended buffer size.

See also: stdio.h, setbuf, setvbuf, FILE.