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Stream buffering

When you write characters to a stream, they are not usually stored in the file on a character-by-character basis as soon as they are written to the stream, but instead are accumulated in a buffer first, then written to the file in a block when certain conditions are met. (A buffer is an area of the computer's memory that acts as a temporary holding area for input or output.) Similarly, when you are reading characters from a stream, they are often buffered, or stored in a buffer first.

It's important to understand how buffering works, or you may find your programs behaving in an unexpected way, reading and writing characters when you do not expect them to. (You can bypass stream buffering entirely, however, by using low-level rather than high-level file routines. See Low-level file routines, for more information.)

There are three main kinds of buffering you should know about:

Most streams are fully buffered when you open them, and this is usually the best solution. However, streams connected to interactive devices such as terminals are line-buffered when you open them; yes, this means that stdin and stdout are line-buffered.

Having stdin and stdout be line-buffered is convenient, because most meaningful chunks of data you write to them are terminated with a newline character. In order to ensure that the data you read from or write to a fully-buffered stream shows up right away, use the fflush function. In the jargon, this is called flushing the stream. Flushing moves the characters from the buffer to the file, if they haven't already been moved. After the move, other functions can then work on the characters.1

To use fflush, simply pass the function the stream you want to flush. The fflush function returns 0 if successful, or the value EOF (which is a macro defined in the GNU C Library) if there was a write error.

Note that using fflush is not always necessary; output is flushed automatically when you try to write and the output buffer is already full, when the stream is closed, when the program exits, when an input operation on a stream actually reads data from the file, and of course, when a newline is written to a line-buffered stream. (See fputs, for a code example that uses fflush.)


  1. Strictly speaking, there are multiple levels of buffering on a GNU system. Even after flushing characters to a file, data from the file may remain in memory, unwritten to disk. On GNU systems, there is an independently-running system program, or daemon, that periodically commits relevant data still in memory to disk. Under GNU/Linux, this daemon is called bdflush.