scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf — input format conversion
#include <stdio.h>
int
scanf( |
const char * | format, |
...) ; |
int
fscanf( |
FILE * | stream, |
const char * | format, | |
...) ; |
int
sscanf( |
const char * | str, |
const char * | format, | |
...) ; |
#include <stdarg.h>
int
vscanf( |
const char * | format, |
va_list | ap) ; |
int
vsscanf( |
const char * | str, |
const char * | format, | |
va_list | ap) ; |
int
vfscanf( |
FILE * | stream, |
const char * | format, | |
va_list | ap) ; |
The scanf
() family of
functions scans input according to format
as described below. This
format may contain conversion
specifications; the results from such
conversions, if any, are stored in the locations pointed to
by the pointer
arguments that follow format
. Each pointer
argument must be of a
type that is appropriate for the value returned by the
corresponding conversion specification.
If the number of conversion specifications in format
exceeds the number of
pointer
arguments,
the results are undefined. If the number of pointer
arguments exceeds the
number of conversion specifications, then the excess
pointer
arguments
are evaluated, but are otherwise ignored.
The scanf
() function reads
input from the standard input stream stdin
, fscanf
() reads input from the stream
pointer stream
, and
sscanf
() reads its input from
the character string pointed to by str
.
The vfscanf
() function is
analogous to vfprintf(3) and reads input
from the stream pointer stream
using a variable
argument list of pointers (see stdarg(3). The vscanf
() function scans a variable argument
list from the standard input and the vsscanf
() function scans it from a string;
these are analogous to the vprintf(3) and vsprintf(3) functions
respectively.
The format
string
consists of a sequence of directives
which describe how
to process the sequence of input characters. If processing of
a directive fails, no further input is read, and scanf
() returns. A "failure" can be either
of the following: input
failure, meaning that input characters were
unavailable, or matching
failure, meaning that the input was inappropriate
(see below).
A directive is one of the following:
A sequence of white-space characters (space, tab, newline, etc; see isspace(3)). This directive matches any amount of white space, including none, in the input.
An ordinary character (i.e., one other than white space or '%'). This character must exactly match the next character of input.
A conversion specification, which commences with a
'%' (percent) character. A sequence of characters from
the input is converted according to this specification,
and the result is placed in the corresponding
pointer
argument. If the next item of input does not match the
conversion specification, the conversion fails —
this is a matching
failure.
Each conversion
specification in format
begins with either the
character '%' or the character sequence "%
n
$
" (see below for the
distinction) followed by:
An optional '*' assignment-suppression character:
scanf
() reads input as
directed by the conversion specification, but discards
the input. No corresponding pointer
argument is
required, and this specification is not included in the
count of successful assignments returned by
scanf
().
An optional 'a' character. This is used with string
conversions, and relieves the caller of the need to
allocate a corresponding buffer to hold the input:
instead, scanf
()
allocates a buffer of sufficient size, and assigns the
address of this buffer to the corresponding pointer
argument, which
should be a pointer to a char * variable (this
variable does not need to be initialized before the
call). The caller should subsequently free(3) this buffer
when it is no longer required. This is a GNU extension;
C99 employs the 'a' character as a conversion specifier
(and it can also be used as such in the GNU
implementation).
An optional decimal integer which specifies the maximum field width. Reading of characters stops either when this maximum is reached or when a non-matching character is found, whichever happens first. Most conversions discard initial whitespace characters (the exceptions are noted below), and these discarded characters don't count towards the maximum field width. String input conversions store a null terminator ('\0') to mark the end of the input; the maximum field width does not include this terminator.
An optional type modifier
character. For example, the l
type modifier is used
with integer conversions such as %d
to specify that the
corresponding pointer
argument refers
to a long int
rather than a pointer to an int
.
A conversion specifier that specifies the type of input conversion to be performed.
The conversion specifications in format
are of two forms, either
beginning with '%' or beginning with "%
n
$
". The two forms should not
be mixed in the same format
string, except that a
string containing "%
n
$
" specifications can include
%%
and %*
. If format
contains '%'
specifications then these correspond in order with successive
pointer
arguments.
In the "%
n
$
" form (which is specified
in POSIX.1-2001, but not C99), n
is a decimal integer that
specifies that the converted input should be placed in the
location referred to by the n
-th pointer
argument following
format
.
The following type modifier characters can appear in a conversion specification:
Indicates that the conversion will be one of
diouxX
or
n
and the
next pointer is a pointer to a short int or unsigned short int (rather
than int
).
As for h
, but the next
pointer is a pointer to a signed char or
unsigned
char.
As for h
, but the next
pointer is a pointer to a intmax_t
or
uintmax_t
.
This modifier was introduced in C99.
Indicates either that the conversion will be one
of diouxX
or n
and
the next pointer is a pointer to a long int or unsigned long int (rather
than int
),
or that the conversion will be one of efg
and the next
pointer is a pointer to double
(rather than
float
).
Specifying two l
characters is
equivalent to L
. If
used with %c
or %s
the corresponding
parameter is considered as a pointer to a wide
character or wide-character string respectively.
L
Indicates that the conversion will be either
efg
and the
next pointer is a pointer to long double or the
conversion will be dioux
and the next
pointer is a pointer to long long.
equivalent to L
.
This specifier does not exist in ANSI C.
As for h
, but the next
pointer is a pointer to a ptrdiff_t
. This
modifier was introduced in C99.
As for h
, but the next
pointer is a pointer to a size_t
. This modifier
was introduced in C99.
The following conversion specifiers are available:
Matches a literal '%'. That is, %%
in the format
string matches a single input '%' character. No
conversion is done, and assignment does not
occur.
Matches an optionally signed decimal integer; the
next pointer must be a pointer to int
.
D
Equivalent to ld
; this exists only
for backwards compatibility.
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Note |
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Thus only in libc4. In libc5 and glibc the
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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.
Matches an unsigned octal integer; the next pointer must be a pointer to unsigned int.
Matches an unsigned decimal integer; the next pointer must be a pointer to unsigned int.
Matches an unsigned hexadecimal integer; the next pointer must be a pointer to unsigned int.
X
Equivalent to x
.
Matches an optionally signed floating-point
number; the next pointer must be a pointer to
float
.
Equivalent to f
.
Equivalent to f
.
E
Equivalent to f
.
(C99) Equivalent to f
.
Matches a sequence of non-white-space characters; the next pointer must be a pointer to character array that is long enough to hold the input sequence and the terminating null character ('\0'), which is added automatically. The input string stops at white space or at the maximum field width, whichever occurs first.
Matches a sequence of characters whose length is
specified by the maximum
field width (default 1); the next pointer
must be a pointer to char
, and there must
be enough room for all the characters (no terminating
null byte 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 null byte. 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 "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.
Matches a pointer value (as printed by %p
in printf(3); the next
pointer must be a pointer to a pointer to void
.
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
*
assignment-suppression character. The C standard
says: "Execution of a %n
directive does not
increment the assignment count returned at the
completion of execution" but the Corrigendum seems to
contradict this. Probably it is wise not to make any
assumptions on the effect of %n
conversions on the
return value.
These functions return the number of input items successfully matched and assigned, which can be fewer than provided for, or even zero in the event of an early matching failure.
The value EOF
is returned if
the end of input is reached before either the first
successful conversion or a matching failure occurs.
EOF
is also returned if a read
error occurs, in which case the error indicator for the
stream (see ferror(3)) is set, and
errno
is set indicate the
error.
The functions fscanf
(),
scanf
(), and sscanf
() conform to C89 and C99.
The q
specifier
is the 4.4BSD notation for long
long, while ll
or the usage of
L
in integer conversions is the
GNU notation.
The Linux version of these functions is based on the
GNU
libio
library. Take a look at
the info
documentation of GNU
libc (glibc-1.08) for
a more concise description.
All functions are fully C89 conformant, but provide the
additional specifiers q
and a
as well as an additional
behavior of the L
and
l
specifiers. The
latter may be considered to be a bug, as it changes the
behavior of specifiers defined in C89.
Some combinations of the type modifiers and conversion
specifiers defined by ANSI C do not make sense (e.g.
%Ld
). While they
may have a well-defined behavior on Linux, this need not to
be so on other architectures. Therefore it usually is better
to use modifiers that are not defined by ANSI C at all, that
is, use q
instead
of L
in combination with
diouxX
conversions
or ll
.
The usage of q
is not the same as on 4.4BSD, as it may be used in float
conversions equivalently to L
.
getc(3), printf(3), setlocale(3), strtod(3), strtol(3), strtoul(3)
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