int vfscanf(
	FILE * __stream,
	const char * __fmt,
	va_list __ap)

Formatted input. This function is the heart of the scanf family of functions.

Characters are read from stream and processed in a way described by fmt. Conversion results will be assigned to the parameters passed via ap.

The format string fmt is scanned for conversion specifications. Anything that doesn't comprise a conversion specification is taken as text that is matched literally against the input. White space in the format string will match any white space in the data (including none), all other characters match only itself. Processing is aborted as soon as the data and format string no longer match, or there is an error or end-of-file condition on stream.

Most conversions skip leading white space before starting the actual conversion.

Conversions are introduced with the character %. Possible options can follow the %:

  • a * indicating that the conversion should be performed but the conversion result is to be discarded; no parameters will be processed from ap,

  • the character h indicating that the argument is a pointer to short int (rather than int),

  • the 2 characters hh indicating that the argument is a pointer to char (rather than int).

  • the character l indicating that the argument is a pointer to long int (rather than int, for integer type conversions), or a pointer to double (for floating point conversions),

In addition, a maximal field width may be specified as a nonzero positive decimal integer, which will restrict the conversion to at most this many characters from the input stream. This field width is limited to at most 255 characters which is also the default value (except for the c conversion that defaults to 1).

The following conversion flags are supported:

  • % Matches a literal % character. This is not a conversion.

  • d Matches an optionally signed decimal integer; the next pointer must be a pointer to int.

  • i Matches an optionally signed integer; the next pointer must be a pointer to int. The integer is read in base 16 if it begins with 0x or 0X, in base 8 if it begins with 0, and in base 10 otherwise. Only characters that correspond to the base are used.

  • o Matches an octal integer; the next pointer must be a pointer to unsigned int.

  • u Matches an optionally signed decimal integer; the next pointer must be a pointer to unsigned int.

  • x Matches an optionally signed hexadecimal integer; the next pointer must be a pointer to unsigned int.

  • f Matches an optionally signed floating-point number; the next pointer must be a pointer to float.

  • e, g, F, E, G Equivalent to f.

  • s Matches a sequence of non-white-space characters; the next pointer must be a pointer to char, and the array must be large enough to accept all the sequence and the terminating NUL character. The input string stops at white space or at the maximum field width, whichever occurs first.

  • c Matches a sequence of width count characters (default 1); the next pointer must be a pointer to char, and there must be enough room for all the characters (no terminating NUL is added). The usual skip of leading white space is suppressed. To skip white space first, use an explicit space in the format.

  • [ Matches a nonempty sequence of characters from the specified set of accepted characters; the next pointer must be a pointer to char, and there must be enough room for all the characters in the string, plus a terminating NUL character. The usual skip of leading white space is suppressed. The string is to be made up of characters in (or not in) a particular set; the set is defined by the characters between the open bracket [ character and a close bracket ] character. The set excludes those characters if the first character after the open bracket is a circumflex ^. To include a close bracket in the set, make it the first character after the open bracket or the circumflex; any other position will end the set. The hyphen character - is also special; when placed between two other characters, it adds all intervening characters to the set. To include a hyphen, make it the last character before the final close bracket. For instance, [^]0-9-] means the set of everything except close bracket, zero through nine, and hyphen. The string ends with the appearance of a character not in the (or, with a circumflex, in) set or when the field width runs out. Note that usage of this conversion enlarges the stack expense.

  • p Matches a pointer value (as printed by p in printf()); the next pointer must be a pointer to void.

  • n Nothing is expected; instead, the number of characters consumed thus far from the input is stored through the next pointer, which must be a pointer to int. This is not a conversion, although it can be suppressed with the * flag.

These functions return the number of input items assigned, which can be fewer than provided for, or even zero, in the event of a matching failure. Zero indicates that, while there was input available, no conversions were assigned; typically this is due to an invalid input character, such as an alphabetic character for a d conversion. The value EOF is returned if an input failure occurs before any conversion such as an end-of-file occurs. If an error or end-of-file occurs after conversion has begun, the number of conversions which were successfully completed is returned.

By default, all the conversions described above are available except the floating-point conversions and the width is limited to 255 characters. The float-point conversion will be available in the extended version provided by the library libscanf_flt.a. Also in this case the width is not limited (exactly, it is limited to 65535 characters). To link a program against the extended version, use the following compiler flags in the link stage:

-Wl,-u,vfscanf -lscanf_flt -lm

A third version is available for environments that are tight on space. In addition to the restrictions of the standard one, this version implements no %[ specification. This version is provided in the library libscanf_min.a, and can be requested using the following options in the link stage:

-Wl,-u,vfscanf -lscanf_min -lm