C++ vfprintf()

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

vfprintf() prototype

int vfprintf( FILE* stream, const char* format, va_list vlist );

The vfprintf() function writes the string pointed to by format to the file stream stream. The string format may contain format specifiers starting with % which are replaced by the values of variables that are passed as a list vlist.

It is defined in <cstdio> header file.

vfprintf() Parameters

  • stream: An output file stream to write the result.
  • format: Pointer to a null terminated string that is written to the file stream. It consists of characters along with optional format specifiers starting with %.

    The format specifiers are replaced by the values of respective variables that follows the format string.

    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 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

  • vlist: A list of arguments containing the data to write.

vfprintf() Return value

If successful, the vfprintf() function returns number of characters written. On failure it returns a negative value.

Example: How vfprintf() function works

#include <cstdio>
#include <cstdarg>

void write(FILE* fp, const char *fmt, ...)
{
    va_list args;
    va_start(args, fmt);
    vfprintf(fp, fmt, args);
    va_end(args);
}

int main ()
{
    FILE *fp = fopen("data.csv","w");
    char name[5][50] = {"John","Harry","Kim","Yuan","Laxmi"};
    int age[5] = {13,41,26,21,32};

    write(fp, "%s,%s\n", "name", "age");
    for (int i=0; i<5; i++)
        write(fp, "%s,%d\n", name[i], age[i]);

    return 0;
}

When you run the program, the following will be written to data.csv file:

name,age
John,13
Harry,41
Kim,26
Yuan,21
Laxmi,32