C++ vfprintf()

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