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Format Specification Fields: scanf and wscanf Functions

The information here applies to the entire scanf family of functions, including the secure versions and describes the symbols used to tell the scanf functions how to parse the input stream, such as the input stream stdin for scanf, into values that are inserted into program variables.

A format specification has the following form:

%[*] [width] [{h | l | ll | I64 | L}]type

The format argument specifies the interpretation of the input and can contain one or more of the following:

  • White-space characters: blank (' '); tab ('\t'); or newline ('\n'). A white-space character causes scanf to read, but not store, all consecutive white-space characters in the input up to the next non–white-space character. One white-space character in the format matches any number (including 0) and combination of white-space characters in the input.

  • Non–white-space characters, except for the percent sign (%). A non–white-space character causes scanf to read, but not store, a matching non–white-space character. If the next character in the input stream does not match, scanf terminates.

  • Format specifications, introduced by the percent sign (%). A format specification causes scanf to read and convert characters in the input into values of a specified type. The value is assigned to an argument in the argument list.

The format is read from left to right. Characters outside format specifications are expected to match the sequence of characters in the input stream; the matching characters in the input stream are scanned but not stored. If a character in the input stream conflicts with the format specification, scanf terminates, and the character is left in the input stream as if it had not been read.

When the first format specification is encountered, the value of the first input field is converted according to this specification and stored in the location that is specified by the first argument. The second format specification causes the second input field to be converted and stored in the second argument, and so on through the end of the format string.

An input field is defined as all characters up to the first white-space character (space, tab, or newline), or up to the first character that cannot be converted according to the format specification, or until the field width (if specified) is reached. If there are too many arguments for the given specifications, the extra arguments are evaluated but ignored. The results are unpredictable if there are not enough arguments for the format specification.

Each field of the format specification is a single character or a number signifying a particular format option. The type character, which appears after the last optional format field, determines whether the input field is interpreted as a character, a string, or a number.

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-control character, that character and the following characters (up to the next percent sign) are treated as an ordinary sequence of characters, that is, a sequence of characters that must match the input. For example, to specify that a percent-sign character is to be input, use %%.

An asterisk (*) following the percent sign suppresses assignment of the next input field, which is interpreted as a field of the specified type. The field is scanned but not stored.

The secure versions (those with the _s suffix) of the scanf family of functions require that a buffer size parameter be passed immediately following each parameter of type c, C, s, S or [. For more information on the secure versions of the scanf family of functions, see scanf_s, _scanf_s_l, wscanf_s, _wscanf_s_l.

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