C++ vsscanf()

The vsscanf() function in C++ is used to read the data from a string buffer.

The vsscanf() function is defined in <cstdio> header file.

vsscanf() prototype

int vsscanf(const char* buffer, const char* format, va_list vlist );

The vsscanf() function reads the data from the character string buffer buffer and stores the values into the respective locations as defined by vlist.


vsscanf() Parameters

  • format: Pointer to a null-terminated character string that specifies how to read the input. It consists of format specifiers starting with %.
    The format string has the following parts:
    • Non whitespace characters except % each of which consumes one identical character from the input stream. It can cause the function to fail if the next character on the stream does not compare equal.
    • Whitespace character: All the consecutive whitespace characters are treated as single whitespace character. Further, '\n', '\t' and ' ' are considered same.
    • Conversion specification: It follows the following format:
      • Initial % character that specifies the beginning
      • An optional * called assignment-suppressing character. If this character is present, vsscanf() does not assign the result to any receiving argument.
      • An optional positive integer number that specifies maximum field width. It specifies the maximum number of characters that vsscanf() is allowed to consume when doing the conversion specified by the current conversion specification.
      • An optional length modifier specifying the size of the receiving argument.
      • A conversion format specifier.
    Format specifiers
    Format Specifier Description
    % Matches the literal %
    c Matches a single character or multiple characters. If width is defined, matches exactly width characters.
    s Matches consecutive non whitespace characters. If width is defined, matches exactly width characters or until first whitespace is found.
    [set] Matches a non empty sequence of character from the given set of characters. If ^ is present at the beginning of set, then all the characters not in set are matched.
    d Matches a decimal integer.
    i Matches an integer.
    o Matches an unsigned octal integer.
    X or x Matches an unsigned hexadecimal integer.
    u Matches an unsigned decimal integer.
    A or a, E or e, F or f, G or g Matches a floating-point number.
    n Returns the number of characters read so far.
    p Matches an implementation defined character sequence defining a pointer.

    So the general format of format specifier is:
    %[*][width][length]specifier
  • vlist: A list of arguments for receiving the inputs.

vsscanf() Return value

  • If successful, the vsscanf() function returns the number of arguments successfully read.
  • On failure, EOF is returned.

Example: How vsscanf() function works?

#include <cstdio>
#include <cstdarg>

void read(const char* buf, const char * format, ... )
{
	va_list args;
	va_start (args, format);
	vsscanf (buf, format, args);
	va_end (args);
}

int main ()
{
	char buffer[100] = "Bruce Wayne Batman";
	char fname[20], lname[20], superhero[20];

	read(buffer, "%s %s %s", fname, lname, superhero);
	printf("%s %s is %s.\n", fname, lname, superhero);

	return 0;
}

When you run the program, a possible output will be:

Bruce Wayne is Batman.