C library function - vfprintf()

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Description

The C library function int vfprintf(FILE *stream, const char *format, va_list arg) sends formatted output to a stream using an argument list passed to it.

Declaration

Following is the declaration for vfprintf() function.

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

Parameters

  • stream -- This is the pointer to a FILE object that identifies the stream.

  • format -- This is the C string that contains the text to be written to the stream.It can optionally contain embedded format tags that are replaced by the values specified in subsequent additional arguments and formatted as requested. Format tags prototype: %[flags][width][.precision][length]specifier, as explained below:

specifierOutput
cCharacter.
d or iSigned decimal integer
eScientific notation (mantissa/exponent) using e character
EScientific notation (mantissa/exponent) using E character
fDecimal floating point
gUse the shorter of %e or %f.
GUse the shorter of %E or %f
oSigned octal
sString of characters
uUnsigned decimal integer
xUnsigned hexadecimal integer
XUnsigned hexadecimal integer (capital letters)
pPointer address
nNothing printed.
%Character.
flagsDescription
-Left-justify within the given field width; Right justification is the default (see width sub-specifier).
+Forces to preceed the result with a plus or minus sign (+ or -) even for positive numbers. By default, only negative numbers are preceded with a - sign..
(space)If no sign is going to be written, a blank space is inserted before the value.
#Used with o, x or X specifiers the value is preceeded with 0, 0x or 0X respectively for values different than zero. Used with e, E and f, it forces the written output to contain a decimal point even if no digits would follow. By default, if no digits follow, no decimal point is written. Used with g or G the result is the same as with e or E but trailing zeros are not removed.
0Left-pads the number with zeroes (0) instead of spaces, where padding is specified (see width sub-specifier).
widthDescription
(number)Minimum number of characters to be printed. If the value to be printed is shorter than this number, the result is padded with blank spaces. The value is not truncated even if the result is larger.
*The width is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.
.precision Description
.numberFor integer specifiers (d, i, o, u, x, X): precision specifies the minimum number of digits to be written. If the value to be written is shorter than this number, the result is padded with leading zeros. The value is not truncated even if the result is longer. A precision of 0 means that no character is written for the value 0. For e, E and f specifiers: this is the number of digits to be printed after de decimal point. For g and G specifiers: This is the maximum number of significant digits to be printed. For s: this is the maximum number of characters to be printed. By default all characters are printed until the ending null character is encountered. For c type: it has no effect. When no precision is specified, the default is 1. If the period is specified without an explicit value for precision, 0 is assumed.
.*The precision is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted.
lengthDescription
hThe argument is interpreted as a short int or unsigned short int (only applies to integer specifiers: i, d, o, u, x and X).
lThe argument is interpreted as a long int or unsigned long int for integer specifiers (i, d, o, u, x and X), and as a wide character or wide character string for specifiers c and s.
LThe argument is interpreted as a long double (only applies to floating point specifiers: e, E, f, g and G).
  • arg -- An object representing the variable arguments list. This should be initialized by the va_start macro defined in <stdarg>.

Return Value

If successful, the total number of characters written is returned otherwise, a negative number is returned.

Example

The following example shows the usage of vfprintf() function.

#include <stdio.h>
#include <stdarg.h>

void WriteFrmtd(FILE *stream, char *format, ...)
{
   va_list args;

   va_start(args, format);
   vfprintf(stream, format, args);
   va_end(args);
}

int main ()
{
   FILE *fp;

   fp = fopen("file.txt","w");

   WriteFrmtd(fp, "This is just one argument %d \n", 10);

   fclose(fp);
   
   return(0);
}

Let us compile and run the above program, this will open a file file.txt for writing in the current directory and will write the following content:

This is just one argument 10

Now let's the content of the above file using the following program:

#include <stdio.h>

int main ()
{
   FILE *fp;
   int c;

   fp = fopen("file.txt","r");
   while(1)
   {
      c = fgetc(fp);
      if( feof(fp) )
      {
          break ;
      }
      printf("%c", c);
   }
   fclose(fp);
   return(0);
}


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