C++ fprintf()

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

fprintf() prototype

int fprintf(FILE* stream, const char* format, ...);

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

It is defined in <cstdio> header file.

fprintf() Parameters

  • stream: Pointer to an output file stream where the contents are written.
  • 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

  • … : Other additional arguments specifying the data to be printed. They occur in a sequence according to the format specifier.

fprintf() Return value

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

Example: How fprintf() function works

#include <cstdio>

int main()
{
    FILE *fp;
    fp = fopen("example.txt","w");
    char lang[5][20] = {"C","C++","Java","Python","PHP"};

    fprintf(fp,"Top 5 programming language\n");
    for (int i=0; i<5; i++)
        fprintf(fp, "%d. %s\n", i+1, lang[i]);

    fclose(fp);
    return 0;
}

When you run the program, a file "example.txt" will be created (if does not exist already) and it contains the following string:

Top 5 programming language
1. C
2. C++
3. Java
4. Python
5. PHP