FSCANF(P) FSCANF(P)
fscanf, scanf, sscanf - convert formatted input
#include <stdio.h>
int fscanf(FILE *restrict stream, const char *restrict format, ... );
int scanf(const char *restrict format, ... );
int sscanf(const char *restrict s, const char *restrict format, ... );
The fscanf() function shall read from the named input stream. The
scanf() function shall read from the standard input stream stdin. The
sscanf() function shall read from the string s. 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 shall be evaluated but
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 specifier character % (see below) is replaced by the
sequence "%n$", where n is a decimal integer in the range
[1,{NL_ARGMAX}]. This feature provides for the definition of format
strings that select arguments in an order appropriate to specific lan‐
guages. In format strings containing the "%n$" form of conversion spec‐
ifications, it is unspecified whether numbered arguments in the argu‐
ment 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$"-but the two forms cannot be mixed within a single format
string. The only exception to this is that %% or %* can be mixed with
the "%n$" form. When numbered argument specifications are used, speci‐
fying the Nth argument requires that all the leading arguments, from
the first to the ( N-1)th, are pointers.
The fscanf() function in all its forms shall allow detection of a lan‐
guage-dependent radix character in the input string. The radix charac‐
ter 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 shall default 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 direc‐
tive is composed of one of the following: one or more white-space char‐
acters ( <space>s, <tab>s, <newline>s, <vertical-tab>s, or <form-
feed>s); an ordinary character (neither ’%’ nor a white-space charac‐
ter); or a conversion specification. Each conversion specification is
introduced by the character ’%’ or the character sequence "%n$",
after which the following appear in sequence:
* An optional assignment-suppressing character ’*’ .
* An optional non-zero decimal integer that specifies the maximum
field width.
* An option length modifier that specifies the size of the receiving
object.
* A conversion specifier character that specifies the type of conver‐
sion to be applied. The valid conversion specifiers are described
below.
The fscanf() functions shall execute each directive of the format in
turn. If a directive fails, as detailed below, the function shall
return. Failures are described as input failures (due to the unavail‐
ability of input bytes) or matching failures (due to inappropriate
input).
A directive composed of one or more white-space characters shall be
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 shall be executed as follows:
the next byte shall be read from the input and compared with the byte
that comprises the directive; if the comparison shows that they are not
equivalent, the directive shall fail, and the differing and subsequent
bytes shall remain unread. Similarly, if end-of-file, an encoding
error, or a read error prevents a character from being read, the direc‐
tive shall fail.
A directive that is a conversion specification defines a set of match‐
ing input sequences, as described below for each conversion character.
A conversion specification shall be executed in the following steps.
Input white-space characters (as specified by isspace() ) shall be
skipped, unless the conversion specification includes a [ , c , C , or
n conversion specifier.
An item shall be read from the input, unless the conversion specifica‐
tion includes an n conversion specifier. An input item shall be defined
as the longest sequence of input bytes (up to any specified maximum
field width, which may be measured in characters or bytes dependent on
the conversion specifier) which is an initial subsequence of a matching
sequence. The first byte, if any, after the input item shall remain
unread. If the length of the input item is 0, the execution of the con‐
version specification shall fail; 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 specifier, the input item (or, in
the case of a %n conversion specification, the count of input bytes)
shall be converted to a type appropriate to the conversion character.
If the input item is not a matching sequence, the execution of the con‐
version specification fails; this condition is a matching failure.
Unless assignment suppression was indicated by a ’*’ , the result of
the conversion shall be 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 behav‐
ior is undefined.
The length modifiers and their meanings are:
hh Specifies that a following d , i , o , u , x , X , or n conver‐
sion 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 conver‐
sion 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 conver‐
sion 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 conver‐
sion 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 conver‐
sion specifier applies to an argument with type pointer to int‐
max_t or uintmax_t.
z Specifies that a following d , i , o , u , x , X , or n conver‐
sion 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 conver‐
sion 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 con‐
version 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.
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 strtol() with
the value 10 for the base argument. In the absence of a size
modifier, the application shall ensure that the corresponding
argument is 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 applica‐
tion shall ensure that the corresponding argument is a pointer
to int.
o Matches an optionally signed octal integer, whose format is the
same as expected for the subject sequence of strtoul() with the
value 8 for the base argument. In the absence of a size modi‐
fier, the application shall ensure that the corresponding argu‐
ment is a pointer to unsigned.
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 application shall ensure that the corresponding
argument is a pointer to unsigned.
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 application shall ensure that the correspond‐
ing argument is a pointer to unsigned.
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(). In the absence of a size modifier, the
application shall ensure that the corresponding argument is a
pointer to float.
If the fprintf() family of functions generates character string repre‐
sentations for infinity and NaN (a symbolic entity encoded in floating-
point format) to support IEEE Std 754-1985, the fscanf() family of
functions shall recognize them as input.
s Matches a sequence of bytes that are not white-space characters.
The application shall ensure that the corresponding argument is
a pointer to the initial byte of an array of char, signed char,
or unsigned char large enough to accept the sequence and a ter‐
minating null character code, which shall be added automati‐
cally.
If an l (ell) qualifier is present, the input is a sequence of charac‐
ters that begins in the initial shift state. Each character shall be
converted to a wide character as if by a call to the mbrtowc() func‐
tion, with the conversion state described by an mbstate_t object ini‐
tialized to zero before the first character is converted. The applica‐
tion shall ensure that the corresponding argument is a pointer to an
array of wchar_t large enough to accept the sequence and the terminat‐
ing null wide character, which shall be added automatically.
[ Matches a non-empty sequence of bytes from a set of expected
bytes (the scanset). The normal skip over white-space characters
shall be suppressed in this case. The application shall ensure
that the corresponding argument is 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 shall
be added automatically.
If an l (ell) qualifier is present, the input is a sequence of charac‐
ters that begins in the initial shift state. Each character in the
sequence shall be 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
application shall ensure that the corresponding argument is a pointer
to an array of wchar_t large enough to accept the sequence and the ter‐
minating null wide character, which shall be added automatically.
The conversion specification includes all subsequent bytes in the for‐
mat string up to and including the matching right square bracket ( ’]’
). The bytes between the square brackets (the scanlist) comprise the
scanset, unless the byte after the left square bracket is a circumflex
( ’^’ ), in which case the scanset contains all bytes 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 con‐
version 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 charac‐
ter is a ’^’ , nor the last character, the behavior is implementation-
defined.
c Matches a sequence of bytes of the number specified by the field
width (1 if no field width is present in the conversion
specification). The application shall ensure that the corre‐
sponding argument is 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 shall be suppressed in this case.
If an l (ell) qualifier is present, the input shall be 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
application shall ensure that the corresponding argument is 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 an implementation-defined set of sequences, which shall
be the same as the set of sequences that is produced by the %p
conversion specification of the corresponding fprintf() func‐
tions. The application shall ensure that the corresponding argu‐
ment is a pointer to a pointer to void. The interpretation of
the input item is implementation-defined. If the input item is a
value converted earlier during the same program execution, the
pointer that results shall compare equal to that value; other‐
wise, the behavior of the %p conversion specification is unde‐
fined.
n No input is consumed. The application shall ensure that the cor‐
responding argument is a pointer to the integer into which shall
be written the number of bytes read from the input so far by
this call to the fscanf() functions. Execution of a %n conver‐
sion specification shall not increment the assignment count
returned at the completion of execution of the function. No
argument shall be converted, but one shall be consumed. If the
conversion specification includes an assignment-suppressing
character or a field width, the behavior is undefined.
C Equivalent to lc .
S Equivalent to ls .
% Matches a single ’%’ character; no conversion or assignment
occurs. The complete conversion specification shall be %% .
If a conversion specification is invalid, the behavior is undefined.
The conversion specifiers A , E , F , G , and X are also valid and
shall be equivalent to a , e , f , g , and x , respectively.
If end-of-file is encountered during input, conversion shall be termi‐
nated. 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 cur‐
rent conversion specification shall terminate with an input failure.
Otherwise, unless execution of the current conversion specification is
terminated with a matching failure, execution of the following conver‐
sion specification (if any) shall be terminated with an input failure.
Reaching the end of the string in sscanf() shall be 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 <new‐
line>s) shall be left unread unless matched by a conversion specifica‐
tion. 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 shall be
marked for update by the first successful execution of fgetc(),
fgets(), fread(), getc(), getchar(), gets(), fscanf(), or fscanf()
using stream that returns data not supplied by a prior call to
ungetc().
Upon successful completion, these functions shall return the number of
successfully matched and assigned input items; this number can be zero
in the event of an early matching failure. If the input ends before the
first matching failure or conversion, EOF shall be returned. If a read
error occurs, the error indicator for the stream is set, EOF shall be
returned, and errno shall be set to indicate the error.
For the conditions under which the fscanf() functions fail and may
fail, refer to fgetc() or fgetwc() .
In addition, fscanf() may fail if:
EILSEQ Input byte sequence does not form a valid character.
EINVAL There are insufficient arguments.
The following sections are informative.
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
assigns to n the value 3, to i the value 25, to x the value 5.432, and
name contains the string "Hamster" .
The call:
int i; float x; char name[50];
(void) scanf("%2d%f%*d %[0123456789]", &i, &x, name);
with input:
56789 0123 56a72
assigns 56 to i, 789.0 to x, skips 0123, and places the string "56\0"
in name. The next call to getchar() shall return the character ’a’ .
Reading Data into an Array
The following call uses fscanf() to read three floating-point numbers
from standard input into the input array.
float input[3]; fscanf (stdin, "%f %f %f", input, input+1, input+2);
If the application calling fscanf() has any objects of type wint_t or
wchar_t, it must also include the <wchar.h> header to have these
objects defined.
This function is aligned with the ISO/IEC 9899:1999 standard, and in
doing so a few "obvious" things were not included. Specifically, the
set of characters allowed in a scanset is limited to single-byte char‐
acters. In other similar places, multi-byte characters have been per‐
mitted, but for alignment with the ISO/IEC 9899:1999 standard, it has
not been done here. Applications needing this could use the correspond‐
ing wide-character functions to achieve the desired results.
None.
getc() , printf() , setlocale() , strtod() , strtol() , strtoul() ,
wcrtomb() , the Base Definitions volume of IEEE Std 1003.1-2001, Chap‐
ter 7, Locale, <langinfo.h>, <stdio.h>, <wchar.h>
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
POSIX 2003 FSCANF(P)