Standard Library Functions in C

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vfprintf() Function
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Description

The C library function, The vfprintf() function writes formatted output to stream using arg as the variable argument list.

Declaration

Following is the declaration for vfprintf() function.

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

Parameters

Stream - Pointer to a FILE object that identifies an output 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.

A format specifier follows this prototype:

%[flags][width][.precision][length]specifier

Where the specifier character at the end is the most significant component, since it defines the type and the interpretation of its corresponding argument:

specifier Output Example
d or iSigned decimal integer392
uUnsigned decimal integer7235
oUnsigned octal610
xUnsigned hexadecimal integer7fa
XUnsigned hexadecimal integer (uppercase)7FA
fDecimal floating point, lowercase392.65
FDecimal floating point, uppercase392.65
eScientific notation (mantissa/exponent), lowercase3.9265e+2
EScientific notation (mantissa/exponent), uppercase3.9265E+2
gUse the shortest representation: %e or %f392.65
GUse the shortest representation: %E or %F392.65
aHexadecimal floating point, lowercase-0xc.90fep-2
AHexadecimal floating point, uppercase-0XC.90FEP-2
cCharactera
sString of characterssample
pPointer addressb8000000
nNothing printed.The corresponding argument must be a pointer to a signed int. The number of characters written so far is stored in the pointed location.
%A % followed by another % character will write a single % to the stream%

The format specifier can also contain sub-specifiers: flags, width, .precision and modifiers (in that order), which are optional and follow these specifications:

flags description
-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 a, A, e, E, f, F, g or G it forces the written output to contain a decimal point even if no more digits follow. By default, if no digits follow, no decimal point is written.
0Left-pads the number with zeroes (0) instead of spaces when padding is specified (see width sub-specifier).
width Description
(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 the 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. length Description
length Description
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).

additional arguments - An object representing the variable arguments list. This should be initialized by the va_start macro defined in .

Return Value

The vfprintf function returns the number of characters that was written. If an error occurs, it will return a negative value.

Example

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

void WriteForm(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");

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

   fclose(fp);
   
   return(0);
}

Output

This is just one argument 10
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