Tribblix: manual page: scanf.3c

SCANF(3C) Standard C Library Functions SCANF(3C)

NAME

scanf, fscanf, sscanf, vscanf, vfscanf, vsscanf - convert formatted
input

SYNOPSIS

#include <stdio.h>

int scanf(const char *restrict format...);

int fscanf(FILE *restrict stream, const char *restrict format...);

int sscanf(const char *restrict s, const char *restrict format...);

#include <stdarg.h>
#include <stdio.h>

int vscanf(const char *format, va_list arg);

int vfscanf(FILE *stream, const char *format, va_list arg);

int vsscanf(const char *s, const char *format, va_list arg);

DESCRIPTION

The scanf() function reads from the standard input stream stdin.

The fscanf() function reads from the named input stream.

The sscanf() function reads from the string s.

The vscanf(), vfscanf(), and vsscanf() functions are equivalent to
the scanf(), fscanf(), and sscanf() functions, respectively, except
that instead of being called with a variable number of arguments,
they are called with an argument list as defined by the <stdarg.h>
header . These functions do not invoke the va_end() macro.
Applications using these functions should call va_end(ap) afterwards
to clean up.

Each function reads bytes, 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 result is
undefined if there are insufficient arguments for the format. If the
format is exhausted while arguments remain, the excess arguments are
evaluated but are otherwise ignored.

Conversions can be applied to the nth argument after the format in
the argument list, rather than to the next unused argument. In this
case, the conversion character % (see below) is replaced by the
sequence %n$, where n is a decimal integer in the range [1,
NL_ARGMAX]. This feature provides for the definition of format
strings that select arguments in an order appropriate to specific
languages. In format strings containing the %n$ form of conversion
specifications, it is unspecified whether numbered arguments in the
argument list can be referenced from the format string more than
once.

The format can contain either form of a conversion specification,
that is, % or %n$, but the two forms cannot normally be mixed within
a single format string. The only exception to this is that %% or %*
can be mixed with the %n$ form.

The scanf() function in all its forms allows for detection of a
language-dependent radix character in the input string. The radix
character is defined in the program's locale (category LC_NUMERIC).
In the POSIX locale, or in a locale where the radix character is not
defined, the radix character defaults to a period (.).

The format is a character string, beginning and ending in its initial
shift state, if any, composed of zero or more directives. Each
directive is composed of one of the following:

o one or more white-space characters (space, tab, newline,
vertical-tab or form-feed characters);

o an ordinary character (neither % nor a white-space
character); or

o a conversion specification.

Conversion Specifications

Each conversion specification is introduced by the character % or the
character sequence %n$, after which the following appear in sequence:

o An optional assignment-suppressing character *.

o An optional non-zero decimal integer that specifies the
maximum field width.

o An option length modifier that specifies the size of the
receiving object.

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

The scanf() functions execute each directive of the format in turn.
If a directive fails, as detailed below, the function returns.
Failures are described as input failures (due to the unavailability
of input bytes) or matching failures (due to inappropriate input).

A directive composed of one or more white-space characters is
executed by reading input until no more valid input can be read, or
up to the first byte which is not a white-space character which
remains unread.

A directive that is an ordinary character is executed as follows. The
next byte is read from the input and compared with the byte that
comprises the directive; if the comparison shows that they are not
equivalent, the directive fails, and the differing and subsequent
bytes remain unread.

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

Input white-space characters (as specified by isspace(3C)) are
skipped, unless the conversion specification includes a [, c, C, or n
conversion character.

An item is read from the input unless the conversion specification
includes an n conversion character. The length of the item read is
limited to any specified maximum field width, which is interpreted in
either characters or bytes depending on the conversion character. In
Solaris default mode, the input item is defined as the longest
sequence of input bytes that forms a matching sequence. In some
cases, scanf() might need to read several extra characters beyond the
end of the input item to find the end of a matching sequence. In
C99/SUSv3 mode, the input item is defined as the longest sequence of
input bytes that is, or is a prefix of, a matching sequence. With
this definition, scanf() need only read at most one character beyond
the end of the input item. Therefore, in C99/SUSv3 mode, some
sequences that are acceptable to strtod(3C), strtol(3C), and similar
functions are unacceptable to scanf(). In either mode, scanf()
attempts to push back any excess bytes read using ungetc(3C).
Assuming all such attempts succeed, the first byte, if any, after the
input item remains unread. If the length of the input item is 0, the
conversion fails. This condition is a matching failure unless end-of-
file, an encoding error, or a read error prevented input from the
stream, in which case it is an input failure.

Except in the case of a % conversion character, the input item (or,
in the case of a %n conversion specification, the count of input
bytes) is converted to a type appropriate to the conversion
character. If the input item is not a matching sequence, the
execution of the conversion specification 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 the conversion specification is
introduced by %, or in the nth argument if introduced by the
character sequence %n$. 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.

Length Modifiers

The length modifiers and their meanings are:

hh
Specifies that a following d, i, o, u, x, X, or n
conversion specifier applies to an argument with type
pointer to signed char or unsigned char.

h
Specifies that a following d, i, o, u, x, X, or n
conversion specifier applies to an argument with type
pointer to short or unsigned short.

l (ell)
Specifies that a following d, i, o, u, x, X, or n
conversion specifier applies to an argument with type
pointer to long or unsigned long; that a following a,
A, e, E, f, F, g, or G conversion specifier applies
to an argument with type pointer to double; or that a
following c, s, or [ conversion specifier applies to
an argument with type pointer to wchar_t.

ll (ell-ell)
Specifies that a following d, i, o, u, x, X, or n
conversion specifier applies to an argument with type
pointer to long long or unsigned long long.

j
Specifies that a following d, i, o, u, x, X, or n
conversion specifier applies to an argument with type
pointer to intmax_t or uintmax_t.

z
Specifies that a following d, i, o, u, x, X, or n
conversion specifier applies to an argument with type
pointer to size_t or the corresponding signed integer
type.

t
Specifies that a following d, i, o, u, x, X, or n
conversion specifier applies to an argument with type
pointer to ptrdiff_t or the corresponding unsigned
type.

L
Specifies that a following a, A, e, E, f, F, g, or G
conversion specifier applies to an argument with type
pointer to long double.

If a length modifier appears with any conversion specifier other than
as specified above, the behavior is undefined.

Conversion Characters

The following conversion characters are valid:

d
Matches an optionally signed decimal integer, whose format
is the same as expected for the subject sequence of
strtol(3C) with the value 10 for the base argument. In the
absence of a size modifier, the corresponding argument
must be a pointer to int.

i
Matches an optionally signed integer, whose format is the
same as expected for the subject sequence of strtol() with
0 for the base argument. In the absence of a size
modifier, the corresponding argument must be a pointer to
int.

o
Matches an optionally signed octal integer, whose format
is the same as expected for the subject sequence of
strtoul(3C) with the value 8 for the base argument. In the
absence of a size modifier, the corresponding argument
must 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
strtoul() with the value 10 for the base argument. In the
absence of a size modifier, the corresponding argument
must 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
strtoul() with the value 16 for the base argument. In the
absence of a size modifier, the corresponding argument
must be a pointer to unsigned int.

a,e,f,g
Matches an optionally signed floating-point number,
infinity, or NaN, whose format is the same as expected for
the subject sequence of strtod(3C). In the absence of a
size modifier, the corresponding argument must be a
pointer to float. The e, f, and g specifiers match
hexadecimal floating point values only in C99/SUSv3 (see
standards(7)) mode, but the a specifier always matches
hexadecimal floating point values.

These conversion specifiers match any subject sequence
accepted by strtod(3C), including the INF, INFINITY, NAN,
and NAN(n-char-sequence) forms. The result of the
conversion is the same as that of calling strtod() (or
strtof() or strtold()) with the matching sequence,
including the raising of floating point exceptions and the
setting of errno to ERANGE, if applicable.

s
Matches a sequence of bytes that are not white-space
characters. The corresponding argument must be a pointer
to the initial byte of an array of char, signed char, or
unsigned char large enough to accept the sequence and a
terminating null character code, which will be added
automatically.

If an l (ell) qualifier is present, the input is a
sequence of characters that begins in the initial shift
state. Each character is converted to a wide-character as
if by a call to the mbrtowc(3C) function, with the
conversion state described by an mbstate_t object
initialized to zero before the first character is
converted. The corresponding argument must be a pointer
to an array of wchar_t large enough to accept the sequence
and the terminating null wide-character, which will be
added automatically.

[
Matches a non-empty sequence of characters from a set of
expected characters (the scanset). The normal skip over
white-space characters is suppressed in this case. The
corresponding argument must be a pointer to the initial
byte of an array of char, signed char, or unsigned char
large enough to accept the sequence and a terminating null
byte, which will be added automatically.

If an l (ell) qualifier is present, the input is a
sequence of characters that begins in the initial shift
state. Each character in the sequence is converted to a
wide-character as if by a call to the mbrtowc() function,
with the conversion state described by an mbstate_t object
initialized to zero before the first character is
converted. The corresponding argument must be a pointer
to an array of wchar_t large enough to accept the sequence
and the terminating null wide-character, which will be
added automatically.

The conversion specification includes all subsequent
characters in the format string up to and including the
matching right square bracket (]). The characters between
the square brackets (the scanlist) comprise the scanset,
unless the character after the left square 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 square bracket. If the
conversion specification begins with [] or [^], the right
square bracket is included in the scanlist and the next
right square bracket is the matching right square bracket
that ends the conversion specification; otherwise the
first right square bracket is the one that ends the
conversion specification. If a - is in the scanlist and is
not the first character, nor the second where the first
character is a ^, nor the last character, it indicates a
range of characters to be matched.

c
Matches a sequence of characters of the number specified
by the field width (1 if no field width is present in the
conversion specification). The corresponding argument must
be a pointer to the initial byte of an array of char,
signed char, or unsigned char large enough to accept the
sequence. No null byte is added. The normal skip over
white-space characters is suppressed in this case.

If an l (ell) qualifier is present, the input is a
sequence of characters that begins in the initial shift
state. Each character in the sequence is converted to a
wide-character as if by a call to the mbrtowc() function,
with the conversion state described by an mbstate_t object
initialized to zero before the first character is
converted. The corresponding argument must be a pointer
to an array of wchar_t large enough to accept the
resulting sequence of wide-characters. No null wide-
character is added.

p
Matches the set of sequences that is the same as the set
of sequences that is produced by the %p conversion of the
corresponding printf(3C) functions. The corresponding
argument must be a pointer to a pointer to void. If the
input item is a value converted earlier during the same
program execution, the pointer that results will compare
equal to that value; otherwise the behavior of the %p
conversion is undefined.

n
No input is consumed. The corresponding argument must be a
pointer to the integer into which is to be written the
number of bytes read from the input so far by this call to
the scanf() functions. Execution of a %n conversion
specification does not increment the assignment count
returned at the completion of execution of the function.

C
Same as lc.

S
Same as ls.

%
Matches a single %; no conversion or assignment occurs.
The complete conversion specification must be %%.

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

The conversion characters A, E, F, G, and X are also valid and behave
the same as, respectively, a, e, f, g, and x.

If end-of-file is encountered during input, conversion is terminated.
If end-of-file occurs before any bytes matching the current
conversion specification (except for %n) have been read (other than
leading white-space characters, where permitted), execution of the
current conversion specification terminates with an input failure.
Otherwise, unless execution of the current conversion specification
is terminated with a matching failure, execution of the following
conversion specification (if any) is terminated with an input
failure.

Reaching the end of the string in sscanf() is equivalent to
encountering end-of-file for fscanf().

If conversion terminates on a conflicting input, the offending input
is left unread in the input. Any trailing white space (including
newline characters) is left unread unless matched by a conversion
specification. The success of literal matches and suppressed
assignments is only directly determinable via the %n conversion
specification.

The fscanf() and scanf() functions may mark the st_atime field of the
file associated with stream for update. The st_atime field will be
marked for update by the first successful execution of fgetc(3C),
fgets(3C), fread(3C), fscanf(), getc(3C), getchar(3C), gets(3C), or
scanf() using stream that returns data not supplied by a prior call
to ungetc(3C).

RETURN VALUES

Upon successful completion, these functions return the number of
successfully matched and assigned input items; this number can be 0
in the event of an early matching failure. If the input ends before
the first matching failure or conversion, EOF is returned. If a read
error occurs the error indicator for the stream is set, EOF is
returned, and errno is set to indicate the error.

ERRORS

For the conditions under which the scanf() functions will fail and
may fail, refer to fgetc(3C) or fgetwc(3C).

In addition, fscanf() may fail if:

EILSEQ
Input byte sequence does not form a valid character.

EINVAL
There are insufficient arguments.

USAGE

If the application calling the scanf() functions has any objects of
type wint_t or wchar_t, it must also include the header <wchar.h> to
have these objects defined.

EXAMPLES

Example 1: 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 Hamster

will assign to n the value 3, to i the value 25, to x the value
5.432, and name will contain the string Hamster.

Example 2: The call:

int i; float x; char name[50];
(void) scanf("%2d%f%*d %[0123456789]", &i, &x, name);

with input:

56789 0123 56a72

will assign 56 to i, 789.0 to x, skip 0123, and place the string 56\0
in name. The next call to getchar(3C) will return the character a.

ATTRIBUTES

NOTES

The behavior of the conversion specifier "%%" has changed for all of
the functions described on this manual page. Previously the "%%"
specifier accepted a "%" character from input only if there were no
preceding whitespace characters. The new behavior accepts "%" even
if there are preceding whitespace characters. This new behavior now
aligns with the description on this manual page and in various
standards. If the old behavior is desired, the conversion
specification "%*[%]" can be used.

July 10, 2008 SCANF(3C)