C++ fwprintf()

The fwprintf() function in C++ is used to write a formatted wide string to a file stream.

The fwprintf() function is defined in <cwchar> header file.

fwprintf() prototype

int fwprintf( FILE* stream, const wchar_t* format, ... );

The fwprintf() function writes the wide string pointed to by format to the file stream stream. The wide string format may contain format specifiers starting with % which are replaced by the values of variables that are passed to the fwprintf() function as additional arguments.


fwprintf() Parameters

  • stream: Pointer to an output file stream where the contents are written.
  • format: A pointer to a null terminated wide string that is written to stdout. It consists of wide characters along with optional format specifiers starting with %. The format specifiers are replaced by the values of respective variables that follows format.
    The format specifier has the following parts:
    • A leading % sign
    • Flags: Optional one or more flags that modifies the conversion behavior.
      • - : Left justify the result within the field. By default it is right justified.
      • + : The sign of the result is attached to the beginning of the value, even for positive results.
      • Space: If there is no sign, a space is attached to the beginning of the result.
      • # : An alternative form of the conversion is performed.
      • 0 : It is used for integer and floating point number. Leading zeros are used to pad the numbers instead of space.
    • Width: An optional * or integer value used to specify minimum width field.
    • Precision : An optional field consisting of a . followed by * or integer or nothing to specify the precision.
    • Length : An optional length modifier that specifies the size of the argument.
    • Specifier: A conversion format specifier. The available format specifiers are as follows:
    Format specifiers
    Format Specifier Description
    % Prints %
    c Writes a single character
    s Writes a character string
    d or i Converts a signed integer to decimal representation
    o Converts an unsigned integer to octal representation
    X or x Converts an unsigned integer to hexadecimal representation
    u Converts an unsigned integer to decimal representation
    F or f Converts floating-point number to the decimal representation
    E or e Converts floating-point number to the decimal exponent notation
    A or a Converts floating-point number to the hexadecimal exponent
    G or g Converts floating-point number to either decimal or decimal exponent notation
    n Returns the number of characters written so far by this call to the function. The result is written to the value pointed to by the argument
    p Writes an implementation defined character sequence defining a pointer.
    So the general format of format specifier is:
    %[flags][width][.precision][length]specifier
  • … : Other additional arguments specifying the data to be written. They occur in a sequence according to the format specifier.

fwprintf() Return value

  • On success, the fwprintf() function returns number of characters written.
  • On failure it returns a negative value.

Example: How fwprintf() function works

#include <cwchar>
#include <cstdio>
#include <clocale>

int main()
{
	wchar_t hebrew_str[] = L"\u05D0 \u05D1 \u05E1 \u05D3 \u05EA";
	FILE *fp = fopen("example.txt","w");

	setlocale(LC_ALL, "en_US.UTF-8");
	fwprintf(fp, L"Here are some Hebrew letters: %ls", hebrew_str);
	fclose(fp);

	return 0;
}

When you run the program, the following will be written to example.txt:

Here are some Hebrew letters: א ב ס ד ת