Format string

Printf format string (of which "printf" stands for "print formatted") refers to a control parameter used by a class of functions in the string-processing libraries of various programming languages. The format string is written in a simple template language, and specifies a method for rendering an arbitrary number of varied data type parameters into a string. This string is then by default printed on the standard output stream, but variants exist that perform other tasks with the result, such as returning it as the value of the function. Characters in the format string are usually copied literally into the function's output, as is usual for templates, with the other parameters being rendered into the resulting text in place of certain placeholders – points marked by format specifiers, which are typically introduced by a % character, though syntax varies. The format string itself is very often a string literal, which allows static analysis of the function call. However, it can also be the value of a variable, which allows for dynamic formatting but also a security vulnerability known as an uncontrolled format string exploit.

The term "printf" is due to the C language, which popularized this type of function, but these functions predate C, and other names are used, notably "format". Printf format strings, which provide formatted output (templating), are complementary to scanf format strings, which provide formatted input (parsing). In both cases these provide simple functionality and fixed format compared to more sophisticated and flexible template engines or parsers, but are sufficient for many purposes.

Many programming languages implement a printf function to output a formatted string. It originated from the C programming language, where it has a prototype similar to the following:

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