FSCANF(3P) POSIX Programmer's Manual FSCANF(3P)
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NAME
fscanf, scanf, sscanf -- convert formatted input
SYNOPSIS
#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, ...);
DESCRIPTION
The functionality described on this reference page is aligned with the
ISO C standard. Any conflict between the requirements described here
and the ISO C standard is unintentional. This volume of POSIX.1-2008
defers to the ISO C standard.
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$"--but the two forms cannot be mixed within a single for-
mat string. The only exception to this is that %% or %* can be mixed
with the "%n$" form. When numbered argument specifications are used,
specifying 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 current 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>, <tab>, <newline>, <vertical-tab>, or <form-feed>); an
ordinary character (neither '%' nor a white-space character); or a con-
version specification. Each conversion specification is introduced by
the character '%' or the character sequence "%n$", after which the fol-
lowing appear in sequence:
* An optional assignment-suppressing character '*'.
* An optional non-zero decimal integer that specifies the maximum
field width.
* An optional assignment-allocation character 'm'.
* 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 argu-
ment following the format argument that has not already received a con-
version 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.
The %c, %s, and %[ conversion specifiers shall accept an optional
assignment-allocation character 'm', which shall cause a memory buffer
to be allocated to hold the string converted including a terminating
null character. In such a case, the argument corresponding to the con-
version specifier should be a reference to a pointer variable that will
receive a pointer to the allocated buffer. The system shall allocate a
buffer as if malloc() had been called. The application shall be respon-
sible for freeing the memory after usage. If there is insufficient mem-
ory to allocate a buffer, the function shall set errno to [ENOMEM] and
a conversion error shall result. If the function returns EOF, any mem-
ory successfully allocated for parameters using assignment-allocation
character 'm' by this call shall be freed before the function returns.
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 con-
version 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. If the
'm' assignment-allocation character is specified, the conver-
sion applies to an argument with the type pointer to a 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 dou-
ble.
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 corre-
sponding 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
representations 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 charac-
ters. If the 'm' assignment-allocation character is not speci-
fied, the application shall ensure that the corresponding argu-
ment 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 character code, which shall be
added automatically. Otherwise, the application shall ensure
that the corresponding argument is a pointer to a pointer to a
char.
If an l (ell) qualifier is present, the input is a sequence of
characters that begins in the initial shift state. Each charac-
ter 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 char-
acter is converted. If the 'm' assignment-allocation character
is not specified, the application shall ensure that the corre-
sponding argument is a pointer to an array of wchar_t large
enough to accept the sequence and the terminating null wide
character, which shall be added automatically. Otherwise, the
application shall ensure that the corresponding argument is a
pointer to a pointer to a wchar_t.
[ Matches a non-empty sequence of bytes from a set of expected
bytes (the scanset). The normal skip over white-space charac-
ters shall be suppressed in this case. If the 'm' assignment-
allocation character is not specified, the application shall
ensure that the corresponding argument is a pointer to the ini-
tial 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. Otherwise, the
application shall ensure that the corresponding argument is a
pointer to a pointer to a char.
If an l (ell) qualifier is present, the input is a sequence of
characters that begins in the initial shift state. Each charac-
ter 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. If the 'm' assign-
ment-allocation character is not specified, 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
terminating null wide character, which shall be added automati-
cally.
Otherwise, the application shall ensure that the corresponding
argument is a pointer to a pointer to a wchar_t.
The conversion specification includes all subsequent bytes in
the format 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 scan-
list 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, 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). No null byte is added. The normal skip over
white-space characters shall be suppressed in this case. If the
'm' assignment-allocation character is not specified, 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. Other-
wise, the application shall ensure that the corresponding argu-
ment is a pointer to a pointer to a char.
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. No null wide charac-
ter is added. If the 'm' assignment-allocation character is not
specified, 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. Otherwise,
the application shall ensure that the corresponding argument is
a pointer to a pointer to a wchar_t.
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
argument 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;
otherwise, the behavior of the %p conversion specification is
undefined.
n No input is consumed. The application shall ensure that the
corresponding 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 con-
version 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 con-
version specification (except for %n) have been read (other than lead-
ing white-space characters, where permitted), execution of the current
conversion specification shall terminate with an input failure. Other-
wise, unless execution of the current conversion specification is ter-
minated with a matching failure, execution of the following conversion
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 <newline>
characters) shall be left unread unless matched by a conversion speci-
fication. 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 last data access time-
stamp of the file associated with stream for update. The last data
access timestamp shall be marked for update by the first successful
execution of fgetc(), fgets(), fread(), getc(), getchar(), getdelim(),
getline(), gets(), fscanf(), or scanf() using stream that returns data
not supplied by a prior call to ungetc().
RETURN VALUE
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 conversion (if any) has completed, and without a matching failure
having occurred, EOF shall be returned. If an error occurs before the
first conversion (if any) has completed, and without a matching failure
having occurred, EOF shall be returned and errno shall be set to indi-
cate the error. If a read error occurs, the error indicator for the
stream shall be set.
ERRORS
For the conditions under which the fscanf() functions fail and may
fail, refer to fgetc() or fgetwc().
In addition, the fscanf() function shall fail if:
EILSEQ Input byte sequence does not form a valid character.
ENOMEM Insufficient storage space is available.
In addition, the fscanf() function may fail if:
EINVAL There are insufficient arguments.
The following sections are informative.
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 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);
APPLICATION USAGE
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.
For functions that allocate memory as if by malloc(), the application
should release such memory when it is no longer required by a call to
free(). For fscanf(), this is memory allocated via use of the 'm'
assignment-allocation character.
RATIONALE
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.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.5, Standard I/O Streams, fprintf(), getc(), setlocale(), str-
tod(), strtol(), strtoul(), wcrtomb()
The Base Definitions volume of POSIX.1-2008, Chapter 7, Locale, <lang-
info.h>, <stdio.h>, <wchar.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
cal and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 FSCANF(3P)
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