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The 'b' has no affect on the operation, but is included for clarity. It stands for binary. When a file is opened with update mode ( '+' as the second or third character in the mode argument), both input and output may be performed on the associated stream. However, writes cannot be followed by reads without an intervening call to fflush() or to a file positioning function ( fseek(), fsetpos(), or rewind()), and reads cannot be followed by writes without an intervening call to a file positioning function. [http://www.opengroup.org/onlinepubs/009695399/functions/fopen.html]
The C standard gives two kinds of files - text files and binary files — although operating systems may or may not distinguish between the two. A ''text file'' is a file consisting of text arranged in lines with some sort of distinguishing end-of-line character or sequence (in [[Unix]], a bare linefeed character; in the [[Apple Macintosh|Macintosh]] OS, a bare carriage return; on [[DOS]] and [[Microsoft Windows]], a carriage return followed by a linefeed). When bytes are read in from a text file, an end-of-line sequence is usually mapped to a linefeed for ease in processing. When a text file is written to, a bare linefeed is mapped to the OS-specific end-of-line character sequence before writing. A ''binary file'' is a file where bytes are read in "raw," and delivered "raw," without any kind of mapping.
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