std::scanf, std::fscanf, std::

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

Parameters

The format string consists of

• non-whitespace multibyte characters except %
• 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"
• conversion specifications. Each conversion specification has the following format:

• introductory % character.

• (optional) assignment-suppressing character *

• (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 %[

• (optional) length modifier

• conversion format specifier.

The following format specifiers are available:

For every conversion specifier other than n

All conversion specifiers other than [, c, and n consume and discard all leading whitespace characters (determined as if by calling isspace

The conversion specifiers lc, ls, and l[ perform multibyte-to-wide character conversion as if by calling mbrtowc with an mbstate_t

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

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

A conversion specification must be valid. Otherwise, the behavior is undefined.

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

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

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

#include <clocale>
#include <cstdio>
#include <iostream>
 
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';
}

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

See also