std::scanf,std::fscanf,std::sscanf(3)

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

See also
   vscanf
   vfscanf reads formatted input from stdin, a file stream or a buffer
   vsscanf using variable argument list
   (C++11) (function)
   (C++11)
   (C++11)
   fgets gets a character string from a file stream
              (function)
   printf
   fprintf prints formatted output to stdout, a file stream or a buffer
   sprintf (function)
   snprintf
   (C++11)
   from_chars converts a character sequence to an integer or floating-point value
   (C++17) (function)