Format Specification Fields: printf and wprintf Functions
A format specification, which consists of optional and required fields, has the following form:
Each field of the format specification is a single character or a number signifying a particular format option. The simplest format specification contains only the percent sign and a type character (for example,
%s). If a percent sign is followed by a character that has no meaning as a format field, the character is copied to stdout. For example, to print a percent-sign character, use
The optional fields, which appear before the type character, control other aspects of the formatting, as follows:
- Required character that determines whether the associated argument is interpreted as a character, a string, or a number (see the printf Type Field Characters table.
- Optional character or characters that control justification of output and printing of signs, blanks, decimal points, and octal and hexadecimal prefixes (see the Flag Characters table). More than one flag can appear in a format specification.
- Optional number that specifies the minimum number of characters output (see printf Width Specification).
- Optional number that specifies the maximum number of characters printed for all or part of the output field, or the minimum number of digits printed for integer values (see the How Precision Values Affect Type table).
- h | l | I | I32 | I64
- Optional prefixes to type-that specify the size of argument (see the Size Prefixes for printf and wprintf Format-Type Specifiers table).
Security Note Ensure that format specification strings are not user-defined. For example, consider a program that prompts the user to enter his name and stores the input in a string variable called name. To print name, do not do this:
printf( name ); // Danger! If name contains "%s", program will crash
Instead, do this:
printf( "%s", name );