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Format Specification Syntax: printf and wprintf Functions

Describes the syntax for format string arguments to printf, wprintf, and related functions. More secure versions of these functions are available; for more information, see Security Features in the CRT. For information about the individual functions, see the documentation for those specific functions. For a listing of these functions, see Stream I/O.

A format specification, which consists of optional and required fields, has the following form:

%[flags] [width] [.precision] [{h | l | ll | w | I | I32 | I64}] type

Each field of the format specification is a character or a number that signifies a particular format option or conversion specifier. The required type character specifies the kind of conversion to be applied to an argument. The optional flags, width, and precision fields control additional format aspects. A basic format specification contains only the percent sign and a type character—for example, %s, which specifies a string conversion. If a percent sign is followed by a character that has no meaning as a format field, the character is copied to the output unchanged. To print a percent-sign character, use %%.

The fields of the format specification control the following aspects of argument conversion and formatting:

type

Required conversion specifier character that determines whether the associated argument is interpreted as a character, a string, an integer, or a floating-point number. For more information, see printf Type Field Characters.

flags

Optional character or characters that control output justification and output of signs, blanks, leading zeros, decimal points, and octal and hexadecimal prefixes. For more information, see Flag Directives. More than one flag can appear in a format specification, and flags can appear in any order.

width

Optional decimal number that specifies the minimum number of characters that are output. For more information, see printf Width Specification.

precision

Optional decimal number that specifies the maximum number of characters that are printed for strings, the number of significant digits or the number of digits after the decimal-point character for floating-point values, or the minimum number of digits that are printed for integer values. For more information, see "How Precision Values Affect Type" in Precision Specification.

h | l | ll | w | I | I32 | I64

Optional prefixes to type that specify the size of the corresponding argument. For more information, see"Size Prefixes" in Size Specification.

Security note Security Note

Ensure that format specification strings are not user-defined. For example, consider a program that prompts the user to enter a name and stores the input in a string variable that's named name. To print name, do not do this:

printf( name ); /* Danger! If name contains "%s", program will crash */

Instead, do this:

printf( "%s", name );

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