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scanf(3c)
Standard C Library Functions scanf(3C)
NAME
scanf, fscanf, sscanf, vscanf, vfscanf, vsscanf - convert formatted
input
scanf_s, fscanf_s, sscanf_s, vscanf_s, vfscanf_s, vsscanf_s - convert
formatted input with additional safety checks
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);
#define __STDC_WANT_LIB_EXT1__ 1
#include <stdio.h>
int scanf_s(const char *restrict format, ...);
int fscanf_s(FILE *restrict stream, const char *restrict format, ...);
int sscanf_s(const char *restrict s,
const char *restrict format, ...);
#define __STDC_WANT_LIB_EXT1__1
#include <stdarg.h>
#include <stdio.h>
int vscanf_s(const char *restrict format, va_list arg);
int vfscanf_s(FILE *restrict stream, const char *restrict format,
va_list arg);
int vsscanf_s(const char *restrict s, const char *restrict 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 similar 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) afterward 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 con‐
trol 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$,
where n is a decimal integer in the range [1, NL_ARGMAX]. This feature
provides for the definition of format strings that select arguments in
an order appropriate to specific languages. In format strings contain‐
ing the %n$ 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$, but the two forms cannot normally be mixed within a sin‐
gle format string. The only exception to this is that %% or %* can be
mixed with the %n$ form.
The scanf() function in all its forms allows for detection of a 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 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 direc‐
tive 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 charac‐
ter); 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 max‐
imum 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 com‐
prises the directive; if the comparison shows that they are not equiva‐
lent, the directive fails, and the differing and subsequent bytes
remain unread.
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 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 lim‐
ited 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 pre‐
vented 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 conver‐
sion 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 argu‐
ment 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 con‐
version 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 con‐
version 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 con‐
version 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 fol‐
lowing 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 con‐
version 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 con‐
version 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 con‐
version 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 con‐
version 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 str‐
tol(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 str‐
toul() 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 for‐
mat is the same as expected for the subject sequence of str‐
toul() 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, infin‐
ity, 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 float‐
ing-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 str‐
told()) 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 charac‐
ters. 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 cor‐
responding 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 conver‐
sion state described by an mbstate_t object initialized to
zero before the first character is converted. The corre‐
sponding 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 charac‐
ters 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 con‐
version 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 conver‐
sion state described by an mbstate_t object initialized to
zero before the first character is converted. The corre‐
sponding argument must be a pointer to an array of wchar_t
large enough to accept the resulting sequence of wide-char‐
acters. 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 cor‐
responding 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 num‐
ber of bytes read from the input so far by this call to the
scanf() functions. Execution of a %n conversion specifica‐
tion 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.
The <inttypes.h> header defines some macros for convenient portability
between different architectures and platforms. For scanf() specifiers,
the names of the macros is "SCN", followed by the conversion specifier
letter and then the datatype size. For example, SCNd32 is the macro for
the scanf d conversion specifier with the flags for a 32 bit datatype.
For more information and a full list of macros, see inttypes.h(3HEAD).
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 exe‐
cution 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 encounter‐
ing 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 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 will be
marked for update by the first successful execution of fgetc(3C),
fgets(3C), fread(3C), fscanf(), getc(3C), getdelim(3C), getline(3C),
getchar(3C), gets(3C), or scanf() using stream that returns data not
supplied by a prior call to ungetc(3C).
C11 Bounds Checking Interfaces
The scanf_s(), fscanf_s(), sscanf_s(), vscanf_s(), vfscanf_s(), and
vsscanf_s() functions are part of the C11 bounds checking interfaces
specified in the C11 standard, Annex K. Each provide similar function‐
ality to the scanf(), fscanf(), sscanf(), vscanf(), vfscanf(), and vss‐
canf() functions respectively, except for additional checks on the
parameters passed and explicit runtime-constraints as defined in the
C11 standard. See runtime_constraint_handler(3C) and INCITS/ISO/IEC
9899:2011.
RETURN VALUES
Upon successful completion, these functions return the number of suc‐
cessfully 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, or if a runtime-constraint violation
occurs in fscanf_s(), scanf_s(), sscanf_s(), vscanf_s(), vfscanf_s(),
or vsscanf_s(), EOF is returned. If a read error occurs the error indi‐
cator for the stream is set, EOF is returned, and errno is set to indi‐
cate 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() and fscanf_s() may fail if:
EILSEQ Input byte sequence does not form a valid character.
EINVAL There are insufficient arguments.
The functions fscanf_s(), scanf_s(), sscnaf_s(), vscanf_s(), vfs‐
canf_s(), and vsscanf_s() may fail if:
EINVAL Null pointer is passed.
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%49s", &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 %49[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
See attributes(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ CSIEnabled _ Interface StabilityCommitted _ MT-LevelMT-
Safe _ StandardSee standards(7).
The scanf(), fscanf(), sscanf(), vscanf(), vfscanf(), and vsscanf()
functions can be used safely in multithreaded applications.
The scanf_s(), fscanf_s(), sscanf_s(), vscanf_s(), vfscanf_s(), and
vsscanf_s() functions cannot be used safely in a multithreaded applica‐
tion due to the runtime constraint handler. For more information, see
the runtime_constraint_handler(3C) man page.
SEE ALSO
fgetc(3C), fgets(3C), fgetwc(3C), fread(3C), getdelim(3C), getline(3C),
isspace(3C), printf(3C), printf_s(3C), setlocale(3C), strtod(3C), str‐
tol(3C), strtoul(3C), wcrtomb(3C), ungetc(3C), inttypes.h(3HEAD),
attributes(7), standards(7), runtime_constraint_handler(3C)
WARNINGS
The use of %s or %[ without length modifiers will result in buffer
overflows if the input string contains a buffer longer than the memory
allocation provided by the user.
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 white
space characters. The new behavior accepts % even if there are preced‐
ing white space 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.
Oracle Solaris 11.4 11 May 2021 scanf(3C)