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A binary file is a computer file that is not a text file. The term "binary file" is often used as a term meaning "non-text file". Many binary contain parts that can be interpreted as text; for example, some computer document files containing formatted text, such as older Microsoft Word document files, contain the text of the document but also contain formatting information in binary form.
Some "text files" contain portions that are actually binary data, and many "binary files" contain portions that are encoded text; for instance, textual data may be stored as a field within the binary format, or arbitrary constants may have been chosen to correspond to ASCII letters as a mnemonic (this is common for file magic numbers). These mixed binary-and-text file are usually regarded as "binary", because an application that can only process text will not know what to do with them.
It is quite common for a text file to use its text to encode data that could be encoded in binary some other way; a text file that does this is still regarded as a text file, and not a binary file, so long as it remains within the previously-stated constraints about what the bits of the file represent. For example, in a binary file, the number 250 could be encoded in bits as the sequence 11111010. It could also be encoded in a text file (for example, a programming language source file) as the digits 2, 5, 0; which would in turn be encoded by the bits in the text file as (for example, using UTF-8) 00110010, 00110101, 00110000.
Some binary files contain headers, blocks of metadata used by a computer program to interpret the data in the file. The header often contains a signature or magic number which can identify the format. For example, a GIF file can contain multiple images, and headers are used to identify and describe each block of image data. The leading bytes of the header would contain text like * or GIF87a that can identify the binary as a GIF file. If a binary file does not contain any headers, it may be called a flat binary file.
A text file may consist partly or entirely of encoded binary information. When sending binary files over the network they may be encoded so that they use only printable characters. This is often necessary due to the limitations of network protocols used for internet browsing and e-mail communication. One such encoding is Base64. Also, files containing public-key and private-key information for use in systems employing asymmetric cryptography (such as website certificates) may also be stored with the binary information encoded in printable characters.
Microsoft Windows and its standard libraries for the C and C++ programming languages allow the programmer to specify a parameter indicating if a file is expected to be plain text or binary when opening a file; this affects the standard library calls to read and write from the file in that the system converts between the C/C++ "end of line" character (the ASCII linefeed character) and the end-of-line sequence Windows expects in files (the ASCII carriage return and linefeed characters in sequence). In Unix-like systems, the C and C++ standard libraries on those systems also allow the programmer to specify whether a file is expected to be text or binary, but the libraries can and do ignore that parameter, as the end-of-line sequence in Unix-like systems is just the C/C++ end-of-line character.
If a binary file is opened in a text editor, each group of eight bits will typically be translated as a single character, and the user will see a (probably unintelligible) display of textual characters. If the file is opened in some other application, that application will have its own use for each byte: maybe the application will treat each byte as a number and output a stream of numbers between 0 and 255—or maybe interpret the numbers in the bytes as colors and display the corresponding picture. Other type of viewers (called 'word extractors') simply replace the unprintable characters with spaces revealing only the human-readable text. This type of view is useful for a quick inspection of a binary file in order to find passwords in games, find hidden text in non-text files and recover corrupted documents. It can even be used to inspect suspicious files (software) for unwanted effects. For example, the user would see any URL/email to which the suspected software may attempt to connect in order to upload unapproved data (to steal). If the file is itself treated as an executable and run, then the operating system will attempt to interpret the file as a series of instructions in its machine language.
The term is used most commonly to state that data files produced by one application are exactly the same as data files produced by another application. For example, some software companies produce applications for Windows and the Macintosh that are binary compatible, which means that a file produced in a Windows environment is interchangeable with a file produced on a Macintosh. This avoids many of the conversion problems caused by importing and exporting data.
One possible binary compatibility issue between different computers is the endianness of the computer. Some computers store the bytes in a file in a different order.
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