Magic number (programming)

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For other uses of the term, see magic number.

In computer programming, a magic number is a special constant used for some specific purpose. It is called magic because it is hardcoded in and its value or presence is inexplicable without some additional knowledge.

Magic numbers are often chosen based on (among other factors):

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Magic numbers in files

An early convention in the Unix operating system was that (binary) files started with two bytes containing a "magic number" identifying the type of the file. These were originally used by the Unix linker and loader. The concept has been expanded on over time, and is now in current use by many other programs across many operating systems. It is a form of in-band signaling.

In a quick hack, the very earliest magic numbers were PDP-11 branch instructions. The concept of magic numbers can be generalized to all files, since any un-encoded binary data is essentially a number; most file formats can thus be identified by some signature that occurs somewhere in the file. Detecting such sequences is therefore an effective way of distinguishing between file formats - and can often yield further information at the same time.

Some examples:

  • Compiled Java class files (bytecode) start with 0xCAFEBABE on big-endian systems.
  • GIF image files have the ASCII code for 'GIF89a' (0x474946383961) or 'GIF87a' (0x474946383761)
  • JPEG image files begin with 0xFFD8FF, and JPEG/JFIF files contain the ASCII code for 'JFIF' (0x4A464946) or JPEG/EXIF files contain the ASCII code for 'Exif' (0x45786966) beginning at byte 6 in the file, followed by more metadata about the file.
  • PNG image files begin with an 8-byte signature which identifies the file as a PNG file and allows immediate detection of some common file-transfer problems: \211 P N G \r \n \032 \n (0x89504e470d0a1a0a)
  • Standard MIDI music files have the ASCII code for 'MThd' (0x4D546864) followed by more metadata about the file.
  • Unix script files usually start with a shebang, '#!' (0x2321, or 0x2123 on little-endian processors) followed by the path to an interpreter.
  • Old MS-DOS .exe files and the newer Microsoft Windows PE (Portable Executable) .exe files start with the ASCII string 'MZ' (0x4D5A), the initials of the designer of the file format, Mark Zbikowski.
  • The Berkeley Fast File System superblock format is identified as either 0x19540119 or 0x011954 depending on version; both represent the birthday of author Marshall Kirk McKusick.
  • Executables for the Game Boy and Game Boy Advance handheld video game systems have a 48-byte or 156-byte magic number, respectively, at a fixed spot in the header. This magic number encodes a bitmap of the Nintendo logo.
  • Old Fat binaries (containing code for both 68K processors and PowerPC processors) on Mac OS 9 contained the ASCII code for 'Joy!' (0x4A6F7921) as a prefix.
  • TIFF files begin with either "II" or "MM" depending on the byte order (II for Intel, or little endian, MM for Motorola, or big endian), followed by 0x2A00 or 0x002A (decimal 42 as a 2-byte integer in Intel or Motorola byte ordering).

The Unix program file can read and interpret magic numbers from files.

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Magic numbers in code

The term magic number also refers to the bad programming practice of using numbers directly in source code without explanation. In most cases this makes programs harder to read, understand, and maintain. Although most guides make an exception for the numbers zero and one, it is a good idea to define all other numbers in code as named constants.

For example, to shuffle the values in an array randomly, this pseudocode will do the job: 

   for i from 1 to 52
       j := i + randomInt(53 - i) - 1
       swapEntries(i, j)

where the function randomInt(x) chooses a random integer between 1 to x, inclusive, and swapEntries(i, j) swaps the ith and jth entries in the array. In this example, 52 is a magic number. It is considered better programming style to write: 

   constant int deckSize := 52
   for i from 1 to deckSize
       j := i + randomInt(deckSize + 1 - i) - 1
       swapEntries(i, j)

This is preferable for several reasons:

  • It is easier to read and understand. A programmer reading the first example might wonder, What does the number 52 mean here? Why 52? The programmer might infer the meaning after reading the code carefully, but it's not obvious. Magic numbers become particularly confusing when the same number is used for different purposes in one section of code.
  • It is easier to maintain. Changing the value of a magic number is error-prone, because the same value is often used several times in different places within a program. To modify the first example to shuffle a Tarot deck, which has 78 cards, a programmer might naively replace every instance of 52 in the program with 78. This would cause two problems. First, it would miss the value 53 on the second line of the example, which would cause the algorithm to fail in a subtle way. Second, it would likely replace the characters 52 everywhere, regardless of whether they refer to the deck size or to something else entirely, which could introduce bugs. By contrast, changing the value of the deckSize variable in the second example would be a simple, one-line change.
  • It facilitates parameterization. For example, to generalize the above example into a procedure that shuffles a deck of any number of cards, it would be sufficient to turn deckSize into a parameter of that procedure. The first example would require several changes.
  • It helps detect typos. Using a variable (instead of a literal) takes advantage of a compiler's checking (if any). Accidentally typing "62" instead of "52" would go undetected, whereas typing "dekSize" instead of "deckSize" would result in the compiler's warning that dekSize is undeclared.
  • It can reduce typing in some IDEs. If an IDE supports code completion, it will fill in most of the variable's name from the first few letters.

Disadvantages are:

  • It can increase the line length of the source code, forcing lines to be broken up if many constants are used on the same line.
  • It can make debugging more difficult, especially on systems where the debugger doesn't display the values of constants.
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Magic debug values

Magic debug values are specific values written to memory during allocation or deallocation, so that it will later be possible to tell whether or not they have become corrupted and to make it obvious when values taken from uninitialized memory are being used.

Memory is usually viewed in hexadecimal, so common values used are often repeated digits or hexspeak.

Famous and common examples include:

Note that most of these are each 8 nibbles (32 bits) long, as most modern computers are designed to manipulate 32 bits at a time.

The prevalence of these values in Microsoft technology is no coincidence; they are discussed in detail in Steve Maguire's well-known book Writing Solid Code from Microsoft Press. He gives a variety of criteria for these values, such as:

  • They should not be useful; that is, most algorithms that operate on them should be expected to do something unusual. Numbers like zero don't fit this criterion.
  • They should be easily recognized by the programmer as invalid values in the debugger.
  • On machines that don't have byte alignment, they should be odd numbers, so that dereferencing them as addresses causes an exception.
  • They should cause an exception, or perhaps even a debugger break, if executed as code.

Since they were often used to mark areas of memory that were essentially empty, some of these terms came to be used in phrases meaning "gone, aborted, flushed from memory"; e.g. "Your program is DEADBEEF".

Pietr Brandehörst's ZUG programming language initialized memory to either 0x0000, 0xDEAD or 0xFFFF in development environment and to 0x0000 in the live environment, on the basis that uninitialised variables should be encouraged to misbehave under development to trap them, but encouraged to behave in a live environment to reduce errors.

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See also

Magic number section in 'File format' article

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References

ja:マジックナンバー (プログラム)

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