NAME
fscanf, scanf, sscanf, vfscanf – scan formatted input

SYNOPSIS
#include <u.h>
#include <stdio.h>

int fscanf(FILE *f, char *format, ...)

int scanf(char *format, ... )

int sscanf(char *s, char *format, ...)

int vfscanf(FILE *stream, char *format, char *args)

DESCRIPTION
Fscanf reads from the named input stream f (see fopen(2)) under control of the string pointed to by format that specifies the admissible input sequences and how they are to be converted for assignment, using subsequent arguments as pointers to the objects to receive the converted input. If there are insufficient arguments for the format, the behavior is undefined. If the format is exhausted while arguments remain, the excess arguments are evaluated (as always) but are otherwise ignored.

Scanf and sscanf are the same, but they read from stdin and the character string s, respectively. Vfscanf is like scanf, except the args argument is a pointer to an argument in an argument list of the calling function and the effect is as if the calling function's argument list from that point on is passed to the scanf routines.

The format is composed of zero or more directives: one or more white–space characters; an ordinary character (not %); or a conversion specification. Each conversion specification is introduced by the character %. After the %, the following appear in sequence:

An optional assignment–suppressing character *.

An optional decimal integer that specifies the maximum field width.

An optional h, l (ell) or L indicating the size of the receiving object. The conversion specifiers d, i, and n shall be preceded by h if the corresponding argument is a pointer to short rather than a pointer to int, or by l if it is a pointer to long. Similarly, the conversion specifiers o, u, and x shall be preceded by h if the corresponding argument is a pointer to unsigned short rather than a pointer to unsigned, or by l if it is a pointer to unsigned long. Finally, the conversion specifiers e, f, and g shall be preceded by l if the corresponding argument is a pointer to double rather than a pointer to float, or by L if it is a pointer to long double. If an h, l, or L appears with any other conversion specifier, the behavior is undefined.

A character that specifies the type of conversion to be applied. The valid conversion specifiers are described below.

Fscanf executes each directive of the format in turn. If a directive fails, as detailed below, fscanf returns. Failures are described as input failures (due to the unavailability of input), or matching failures (due to inappropriate input).

A directive composed of white space is executed by reading input up to the first non–white–space character (which remains unread), or until no more characters can be read.

A directive that is an ordinary character is executed by reading the next character from the stream. If if differs from the one comprising the directive, the directive fails, and the differing and subsequent characters remain unread.

A directive that is a conversion specification defines a set of matching input sequences, as described below for each specifier. A conversion specification is executed in the following steps:

Input white–space characters (as specified by isspace, see ctype(2)) are skipped, unless the specification includes a [, c, or n specifier.

An input item is read from the stream, unless the specification includes an n specifier. An input item is defined as the longest sequence of input characters (up to any specified maximum field width) which is an initial subsequence of a matching sequence. The first character, if any, after the input item remains unread. If the length of the input item is zero, the execution of the directive fails: this condition is a matching failure, unless an error prevented input from the stream, in which case it is an input failure.

Except in the case of a % specifier, the input item (or, in the case of a %n directive, the count of input characters) is converted to a type appropriate to the conversion specifier. If the input item is not a matching sequence, the execution of the directive fails: this condition is a matching failure. Unless assignment suppression was indicated by a *, the result of the conversion is placed in the object pointed to by the first argument following the format argument that has not already received a conversion result. If this object does not have an appropriate type, or if the result of the conversion cannot be represented in the space provided, the behavior is undefined.

The following conversion specifiers are valid:
d    Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the strtol (see atof(2)) function with 10 for the base argument. The corresponding argument shall be a pointer to int.
i    Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the strtol function with 0 for the base argument. The corresponding argument shall be a pointer to int.
o    Matches an optionally signed octal integer, whose format is the same as expected for the subject sequence of the strtoul (see atof(2)) function with 8 for the base argument. The corresponding argument shall be a pointer to unsigned int.
u    Matches an optionally signed decimal integer, whose format is the same as expected for the subject sequence of the strtoul function with 10 for the base argument. The corresponding argument shall be a pointer to unsigned int.
x    Matches an optionally signed hexadecimal integer, whose format is the same as expected for the subject sequence of the strtoul function with 16 for the base argument. The corresponding argument shall be a pointer to unsigned int.
e,f,g
Matches an optionally signed floating–point number, whose format is the same as expected for the subject string of the strtod (see atof(2)) function. The corresponding argument shall be a pointer to float.
s    Matches a sequence of non–white–space characters. The corresponding argument shall be a pointer to the initial character of an array large enough to accept the sequence and a terminating NUL (0) character, which will be added automatically.
[    Matches a nonempty sequence of characters from a set of expected characters (the scanset). The corresponding argument shall be a pointer to the initial character of an array large enough to accept the sequence and a terminating NUL character, which will be added automatically. The conversion specifier
includes all subsequent characters in the format string, up to and including the matching right brace (]). The characters between the brackets (the scanlist) comprise the scanset, unless the character after the left bracket is a circumflex (^), in which case the scanset contains all characters that do not appear in the scanlist between the circumflex and the right bracket. As a special case, if the conversion specifier begins with [] or [^], the right bracket character is in the scanlist and the next right bracket character is the matching right bracket that ends the specification. If a – character is in the scanlist and is not the first, nor the second where the first character is a ^, nor the last character, the behavior is implementation–defined (in Plan 9: the scanlist includes all characters in the ASCII (sic) range between the two characters on either side of the –).
c    Matches a sequence of characters of the number specified by the field width (1 if no field width is present in the directive). The corresponding argument shall be a pointer to the initial character of an array large enough to accept the sequence. No NUL character is added.
P    Matches an implementation–defined set of sequences, which should be the same as the set of sequences that may be produced by the %P conversion of the fprintf(2) function (in Plan 9, a hexadecimal number). The corresponding argument shall be a pointer to a pointer to void. The interpretation of the input
item is implementation defined; however, for any input item other than a value converted earlier during the same program execution, the behavior of the %P conversion is undefined.
n    No input is consumed. The corresponding argument shall be a pointer to integer into which is written the number of characters read from the input stream so far by this call to fscanf. Execution of a %n directive does not increment the assignment count returned at the completion of fscanf. %    Matches a single %; no conversion or assignment occurs. The complete conversion specification shall be %%.

If a conversion specification is invalid, the behavior is undefined.

The conversion specifiers E, G, and X are also valid and behave the same as, respectively, e, g, and x.

If end–of–file is encountered during input, conversion is terminated. If end–of–file occurs before any characters matching the current directive have been read (other than leading white space, where permitted), execution of the current directive terminates with an input failure; otherwise, unless execution of the current directive is terminated with a matching failure, execution of the following directive (if any) is terminated with an input failure.

If conversion terminates on a conflicting input character, the offending input character is left unread in the input stream. Trailing white space (including newline characters) is left unread unless matched by a directive. The success of literal matches and suppressed assignments is not directly determinable other than via the %n directive.

The return value from fscanf is the number of input items assigned, which can be fewer than provided for, or even zero, in the event of an early matching failure. However, if an input failure occurs before any conversion, EOF is returned.

SOURCE
/sys/src/libstdio

SEE ALSO
fopen(2), fgetc(2)

BUGS
Does not know about UTF.
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