NAME¶

std::scanf,std::fscanf,std::sscanf - std::scanf,std::fscanf,std::sscanf

Synopsis¶

Defined in header <cstdio>
int scanf( const char* format, ... ); (1)
int fscanf( std::FILE* stream, const char* format, ... ); (2)
int sscanf( const char* buffer, const char* format, ... ); (3)

Reads data from a variety of sources, interprets it according to format and stores
the results into given locations.

1) Reads the data from stdin
2) Reads the data from file stream stream
3) Reads the data from null-terminated character string buffer

Parameters¶

stream - input file stream to read from
buffer - pointer to a null-terminated character string to read from
format - pointer to a null-terminated character string specifying how to read the
input
... - receiving arguments.

The format string consists of

* non-whitespace multibyte characters except %: each such character in the format
string consumes exactly one identical character from the input stream, or causes
the function to fail if the next character on the stream does not compare equal.
* whitespace characters: any single whitespace character in the format string
consumes all available consecutive whitespace characters from the input
(determined as if by calling isspace in a loop). Note that there is no
difference between "\n", " ", "\t\t", or other whitespace in the format string.
* conversion specifications. Each conversion specification has the following
format:

* introductory % character

* (optional) assignment-suppressing character *. If this option is
present, the function does not assign the result of the conversion to
any receiving argument.

* (optional) integer number (greater than zero) that specifies maximum
field width, that is, the maximum number of characters that the
function is allowed to consume when doing the conversion specified by
the current conversion specification. Note that %s and %[ may lead to
buffer overflow if the width is not provided.

* (optional) length modifier that specifies the size of the receiving
argument, that is, the actual destination type. This affects the
conversion accuracy and overflow rules. The default destination type is
different for each conversion type (see table below).

* conversion format specifier

The following format specifiers are available:

Conversion Explanation Argument type specifier
hh ll j z t
Length modifier → h (none) l L
(C++11) (C++11) (C++11) (C++11) (C++11)
% matches literal % N/A N/A N/A N/A N/A N/A N/A N/A N/A
matches a character
or a sequence of
characters

If a width specifier
is used, matches
exactly width
c characters (the
argument must be a
pointer to an array
with sufficient room).
Unlike %s and %[, does
not append the null
character to the
array.
matches a sequence
of non-whitespace
characters (a
string)

If width specifier is
used, matches up to
width or until the
s first whitespace
character, whichever
appears first. Always
stores a null
character in addition
to the characters
matched (so the
argument array must
have room for at least N/A N/A char* wchar_t* N/A N/A N/A N/A N/A
width+1 characters)
matches a non-empty
sequence of
character from set
of characters.

If the first character
of the set is ^, then
all characters not in
the set are matched.
If the set begins with
] or ^] then the ]
character is also
included into the set.
It is
implementation-defined
[set] whether the character
- in the non-initial
position in the
scanset may be
indicating a range, as
in [0-9]. If width
specifier is used,
matches only up to
width. Always stores a
null character in
addition to the
characters matched (so
the argument array
must have room for at
least width+1
characters)
matches a decimal
integer.

d The format of the
number is the same as
expected by strtol
with the value 10 for
the base argument
matches an integer.

The format of the
number is the same as
i expected by strtol
with the value 0
for the base argument
(base is determined by
the first characters
parsed)
matches an unsigned
decimal integer.

u The format of the
number is the same as
expected by strtoul
with the value 10 for signed signed
the base argument. signed short* signed signed long intmax_t*
matches an unsigned char* or or int* or long* or long* or or size_t* ptrdiff_t* N/A
octal integer. unsigned unsigned unsigned unsigned unsigned uintmax_t*
char* short* int* long* long
o The format of the long*
number is the same as
expected by strtoul
with the value 8 for
the base argument
matches an unsigned
hexadecimal
integer.

x, X The format of the
number is the same as
expected by strtoul
with the value 16 for
the base argument
returns the number
of characters read
so far.

No input is consumed.
n Does not increment the
assignment count. If
the specifier has
assignment-suppressing
operator defined, the
behavior is undefined
matches a
a, floating-point
A(C++11) number. long
e, E N/A N/A float* double* N/A N/A N/A N/A double*
f, F The format of the
g, G number is the same as
expected by strtof
matches
implementation
defined character
sequence defining a
pointer.
p N/A N/A void** N/A N/A N/A N/A N/A N/A
printf family of
functions should
produce the same
sequence using %p
format specifier

For every conversion specifier other than n, the longest sequence of input
characters which does not exceed any specified ﬁeld width and which either is
exactly what the conversion specifier expects or is a prefix of a sequence it would
expect, is what's consumed from the stream. The ﬁrst character, if any, after this
consumed sequence remains unread. If the consumed sequence has length zero or if the
consumed sequence cannot be converted as specified above, the matching failure
occurs unless end-of-ﬁle, an encoding error, or a read error prevented input from
the stream, in which case it is an input failure.

All conversion specifiers other than [, c, and n consume and discard all leading
whitespace characters (determined as if by calling isspace) before attempting to
parse the input. These consumed characters do not count towards the specified
maximum field width.

The conversion specifiers lc, ls, and l[ perform multibyte-to-wide character
conversion as if by calling mbrtowc with an mbstate_t object initialized to zero
before the first character is converted.

The conversion specifiers s and [ always store the null terminator in addition to
the matched characters. The size of the destination array must be at least one
greater than the specified field width. The use of %s or %[, without specifying the
destination array size, is as unsafe as std::gets.

The correct conversion specifications for the fixed-width integer types (int8_t,
etc) are defined in the header <cinttypes> (although SCNdMAX, SCNuMAX, etc is
synonymous with %jd, %ju, etc).

There is a sequence point after the action of each conversion specifier; this
permits storing multiple fields in the same "sink" variable.

When parsing an incomplete floating-point value that ends in the exponent with no
digits, such as parsing "100er" with the conversion specifier %f, the sequence
"100e" (the longest prefix of a possibly valid floating-point number) is consumed,
resulting in a matching error (the consumed sequence cannot be converted to a
floating-point number), with "r" remaining. Some existing implementations do not
follow this rule and roll back to consume only "100", leaving "er", e.g. glibc bug
1765

Return value¶

Number of receiving arguments successfully assigned (which may be zero in case a
matching failure occurred before the first receiving argument was assigned), or EOF
if input failure occurs before the first receiving argument was assigned.

Complexity¶

Not guaranteed. Notably, some implementations of std::sscanf are O(N), where N =
std::strlen(buffer) [1]. For performant string parsing, see std::from_chars.

Notes¶

Because most conversion specifiers first consume all consecutive whitespace, code
such as

std::scanf("%d", &a);
std::scanf("%d", &b);

will read two integers that are entered on different lines (second %d will consume
the newline left over by the first) or on the same line, separated by spaces or tabs
(second %d will consume the spaces or tabs).

The conversion specifiers that do not consume leading whitespace, such as %c, can be
made to do so by using a whitespace character in the format string:

std::scanf("%d", &a);
std::scanf(" %c", &c); // ignore the endline after %d, then read a char

Note that some implementations of std::sscanf involve a call to std::strlen, which
makes their runtime linear on the length of the entire string. This means that if
std::sscanf is called in a loop to repeatedly parse values from the front of a
string, your code might run in quadratic time (example).

Example¶

// Run this code

#include <iostream>
#include <clocale>
#include <cstdio>

int main()
{
int i, j;
float x, y;
char str1[10], str2[4];
wchar_t warr[2];
std::setlocale(LC_ALL, "en_US.utf8");

char input[] = "25 54.32E-1 Thompson 56789 0123 56ß水";
// parse as follows:
// %d: an integer
// %f: a floating-point value
// %9s: a string of at most 9 non-whitespace characters
// %2d: two-digit integer (digits 5 and 6)
// %f: a floating-point value (digits 7, 8, 9)
// %*d an integer which isn't stored anywhere
// ' ': all consecutive whitespace
// %3[0-9]: a string of at most 3 digits (digits 5 and 6)
// %2lc: two wide characters, using multibyte to wide conversion
const int ret = std::sscanf(input, "%d%f%9s%2d%f%*d %3[0-9]%2lc",
&i, &x, str1, &j, &y, str2, warr);

std::cout << "Converted " << ret << " fields:\n"
"i = " << i << "\n"
"x = " << x << "\n"
"str1 = " << str1 << "\n"
"j = " << j << "\n"
"y = " << y << "\n"
"str2 = " << str2 << std::hex << "\n"
"warr[0] = U+" << (int)warr[0] << "\n"
"warr[1] = U+" << (int)warr[1] << '\n';
}

Output:¶

Converted 7 fields:
i = 25
x = 5.432
str1 = Thompson
j = 56
y = 789
str2 = 56
warr[0] = U+df
warr[1] = U+6c34

Source file:	std::scanf,std::fscanf,std::sscanf.3.en.gz (from stdman 2022.07.30-1.4)
Source last updated:	2022-08-10T20:44:46Z
Converted to HTML:	2024-05-04T01:37:46Z