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12.12.2 Output Conversion Syntax

This section provides details about the precise syntax of conversion specifications that can appear in a printf template string.

Characters in the template string that are not part of a conversion specification are printed as-is to the output stream. Multibyte character sequences (see Character Set Handling) are permitted in a template string.

The conversion specifications in a printf template string have the general form:

% [ param-no $] flags width [ . precision ] type conversion


% [ param-no $] flags width . * [ param-no $] type conversion

For example, in the conversion specifier ‘%-10.8ld’, the ‘-’ is a flag, ‘10’ specifies the field width, the precision is ‘8’, the letter ‘l’ is a type modifier, and ‘d’ specifies the conversion style. (This particular type specifier says to print a long int argument in decimal notation, with a minimum of 8 digits left-justified in a field at least 10 characters wide.)

In more detail, output conversion specifications consist of an initial ‘%’ character followed in sequence by:

The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they use.

With the ‘-Wformat’ option, the GNU C compiler checks calls to printf and related functions. It examines the format string and verifies that the correct number and types of arguments are supplied. There is also a GNU C syntax to tell the compiler that a function you write uses a printf-style format string. See Declaring Attributes of Functions in Using GNU CC, for more information.

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