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Manual page for SCANF(3V)

scanf, fscanf, sscanf - formatted input conversion

SYNOPSIS

#include <stdio.h>

int scanf(format  [ , pointer... ] )
char *format;

int fscanf(stream, format  [ , pointer... ] )
FILE *stream;
char *format;

int sscanf(s, format  [ , pointer... ] )
char *s, *format;

SYSTEM V SYNOPSIS

The following are provided for XPG2 compatibility:

#define	nl_scanf	scanf
#define	nl_fscanf	fscanf
#define	nl_sscanf	sscanf

DESCRIPTION

scanf() reads from the standard input stream stdin. fscanf() reads from the named input stream. sscanf() reads from the character string s. Each function reads characters, interprets them according to a format, and stores the results in its arguments. Each expects, as arguments, a control string format, described below, and a set of pointer arguments indicating where the converted input should be stored. The results are undefined in there are insufficient args for the format. If the format is exhausted while args remain, the excess args are simply ignored.

The control string usually contains conversion specifications, which are used to direct interpretation of input sequences. The control string may contain:

White-space characters (SPACE, TAB, or NEWLINE) which, except in two cases described below, cause input to be read up to the next non-white-space character.
An ordinary character (not `%'), which must match the next character of the input stream.
Conversion specifications, consisting of the character `%' or the character sequence %digit$, an optional assignment suppressing character `*', an optional numerical maximum field width, an optional l (ell) or h indicating the size of the receiving variable, and a conversion code.

Conversion specifications are introduced by the character % or the character sequence %digit$. A conversion specification directs the conversion of the next input field; the result is placed in the variable pointed to by the corresponding argument, unless assignment suppression was indicated by `*'. The suppression of assignment provides a way of describing an input field which is to be skipped. An input field is defined as a string of non-space characters; it extends to the next inappropriate character or until the field width, if specified, is exhausted. For all descriptors except ``['' and ``c'', white space leading an input field is ignored.

The conversion character indicates the interpretation of the input field; the corresponding pointer argument must usually be of a restricted type. For a suppressed field, no pointer argument is given. The following conversion characters are legal:

%: A single % is expected in the input at this point; no assignment is done.
d: A decimal integer is expected; the corresponding argument should be an integer pointer.
u: An unsigned decimal integer is expected; the corresponding argument should be an unsigned integer pointer.
o: An octal integer is expected; the corresponding argument should be an integer pointer.
x: A hexadecimal integer is expected; the corresponding argument should be an integer pointer.
i: An integer is expected; the corresponding argument should be an integer pointer. It will store the value of the next input item interpreted according to C conventions: a leading ``0'' implies octal; a leading ``0x'' implies hexadecimal; otherwise, decimal.
n: Stores in an integer argument the total number of characters (including white space) that have been scanned so far since the function call. No input is consumed.
e,f,g: A floating point number is expected; the next field is converted accordingly and stored through the corresponding argument, which should be a pointer to a float. The input format for floating point numbers is as described for string_to_decimal.3 with fortran_conventions zero.
s: A character string is expected; the corresponding argument should be a character pointer pointing to an array of characters large enough to accept the string and a terminating \0, which will be added automatically. The input field is terminated by a white space character.
c: A character is expected; the corresponding argument should be a character pointer. The normal skip over white space is suppressed in this case; to read the next non-space character, use %1s. If a field width is given, the corresponding argument should refer to a character array, and the indicated number of characters is read.
[: Indicates string data; the normal skip over leading white space is suppressed. The left bracket is followed by a set of characters, which we will call the scanset, and a right bracket; the input field is the maximal sequence of input characters consisting entirely of characters in the scanset. The circumflex (^), when it appears as the first character in the scanset, serves as a complement operator and redefines the scanset as the set of all characters not contained in the remainder of the scanset string. There are some conventions used in the construction of the scanset. A range of characters may be represented by the construct first-last, thus [0123456789] may be expressed [0-9]. Using this convention, first must be lexically less than or equal to last, or else the dash will stand for itself. The dash will also stand for itself whenever it is the first or the last character in the scanset. To include the right square bracket as an element of the scanset, it must appear as the first character (possibly preceded by a circumflex) of the scanset, and in this case it will not be syntactically interpreted as the closing bracket. The corresponding argument must point to a character array large enough to hold the data field and the terminating \0, which will be added automatically. At least one character must match for this conversion to be considered successful.

The conversion characters d, u, o, x, and i may be preceded by l or h to indicate that a pointer to long or to short rather than to int is in the argument list. Similarly, the conversion characters e, f, and g may be preceded by l to indicate that a pointer to double rather than to float is in the argument list. The l or h modifier is ignored for other conversion characters.

Avoid this common error: because printf.3v does not require that the lengths of conversion descriptors and actual parameters match, coders sometimes are careless with the scanf() functions. But converting %f to &double or %lf to &float does not work; the results are quite incorrect.

scanf() conversion terminates at EOF, at the end of the control string, or when an input character conflicts with the control string. In the latter case, the offending character is left unread in the input stream.

scanf() returns the number of successfully matched and assigned input items; this number can be zero in the event of an early conflict between an input character and the control string. The constant EOF is returned upon end of input. Note: this is different from 0, which means that no conversion was done; if conversion was intended, it was frustrated by an inappropriate character in the input.

If the input ends before the first conflict or conversion, EOF is returned. If the input ends after the first conflict or conversion, the number of successfully matched items is returned.

Conversions can be applied to the nth argument in the argument list, rather than the next unused argument. In this case, the conversion character % (see below) is replaced by the sequence %digit$, where digit is a decimal integer n in the range [1,9], giving the position of the argument in the argument list. This feature provides for the definition of format strings that select arguments in an order appropriate to specific languages.

The format string can contain either form of a conversion specification, that is % or %digit$, although the two forms cannot be mixed within a single format string.

All forms of the scanf() functions allow for the detection of a language dependent radix character in the input string. The radix character is defined by the program's locale (category LC_NUMERIC). In the "C" locale, or in a locale where the radix character is not defined, the radix character defaults to `.'.

SYSTEM V DESCRIPTION

FORMFEED is allowed as a white space character in control strings.

XPG2 requires that nl_scanf, nl_fscanf and nl_sscanf be defined as scanf, fscanf and sscanf, respectively for backward compatibility.

RETURN VALUES

If any items are converted, scanf(), fscanf() and sscanf() return the number of items converted successfully. This number may smaller than the number of items requested. If no items are converted, these functions return 0. scanf(), fscanf() and sscanf() return EOF on end of input.

EXAMPLES

The call:

int i, n; float x; char name[50];
n = scanf("%d%f%s", &i, &x, name);

with the input line:

: 25 54.32E-1 thompson

will assign to n the value 3, to i the value 25, to x the value 5.432, and name will contain thompson\0. Or:

int i, j; float x; char name[50];
(void) scanf("%i%2d%f%*d %[0-9]", &j, &i, &x, name);

with input:

: 011 56789 0123 56a72

will assign 9 to j, 56 to i, 789.0 to x, skip 0123, and place the string 56\0 in name. The next call to getchar() (see getc.3v will return a. Or:

int i, j, s, e; char name[50];
(void) scanf("%i %i %n%s%n", &i, &j, &s, name, &e);

with input:

: 0x11 0xy johnson

will assign 17 to i, 0 to j, 6 to s, will place the string xy\0 in name, and will assign 8 to e. Thus, the length of name is e - s = 2. The next call to getchar() (see getc.3v will return a SPACE.

WARNINGS

Trailing white space (including a NEWLINE) is left unread unless matched in the control string.

BUGS

The success of literal matches and suppressed assignments is not directly determinable.