C vsprintf() function

C library function - vsprintf()

The vsprintf() function is used to format and store a series of characters and values in the buffer.


int vsprintf(char *target-string, const char *format, va_list arg_ptr);


Name Description
target-string Pointer to a buffer where the resulting string is stored.
format The format argument is a string containing C language conversion specifications. The conversion specification specifies the notation, alignment, significant digits, field width, and other aspects of the output format. To represent non-printing characters, such as newlines and tabs, the format string may contain escape characters. Below are the details.
arg A value identifying the variable arguments list. This should be initialized by the va_start macro defined in <stdarg>.

Format strings specify notation, alignment, significant digits, field width, and other aspects of output formats. It can contain ordinary alphanumeric characters; along with escape characters, conversion specifiers, and other characters. Format specifications are made up of a the percent sign (%) followed by one of the following conversion operators, which determine what printf does with its arguments.

Conversion Specifiers:

Specifier Description
%% Print a single % character
%c Convert an int to an unsigned character and print the resulting character
%d or %i Print an int as a signed decimal number
%u Print an unsigned as an unsigned decimal number
%o Print an unsigned as an unsigned octal number
%x Hexadecimal notation (using lowercase letters a-f)
%X Hexadecimal notation (using uppercase letters A-F)
%e Exponential notation (using a lowercase e as in 3.1415e+00)
%E Exponential notation (using an uppercase E as in 3.1415E+00)
%f Print a double using a decimal format like [-]ddd.ddd
%g The more compact of %e or %f, insignificant zeros do not print.
%G Same as g, but using an uppercase E
%s Print the string pointed to by a char *
%p Print a void * argument in hexadecimal (ANSI C only)
%n Store the number of characters printed at this point in the integer pointed to by the int * argument. Nothing is printed. (ANSI C only).

Conversion Flags:

You can control the alignment of the output using any of these optional flags.

Flag Format Example
+ (plus) Always prints a sign character (+ or -). %+7.2d
- (minus) Left-justifies the converted argument in its field. %-7.2d
0 (Zero) Pad with zeros rather than spaces. %07.2d
# (hash)

Use an alternate form for the output. The effect differs depending on the conversion specifier.

  • For o, the precision (described below) is increased so that the first digit printed is a 0
  • For x or X, a non-zero value has a 0x prepended (or 0X for the X specification)
  • For eEfg or G, the result will always contain a decimal point, even if no digits follow it. Additionally, trailing zeros are not removed from numbers formatted with g or G

Conversion width and precision:

By including these options in the format string, you can control the width and precision of the output:

Character Description Example
Field width A digit string specifying the minimum number of digits to be printed. %5f
Precision A digit string including a period (.) specifying the number of digits to be printed to the right of the decimal point. %5.2f

Some conversion operators may also be preceded by a size specification:

  • h indicates that the argument associated with a dioux or X operator is a short or unsigned short.
  • l indicates that the argument associated with a dioux or X operator is a long or unsigned long.
  • L indicates that the argument associated with a eEfg or G operator is a long double (ANSI C only)

Escape sequences in C:

An escape sequence is a series of characters that represents a special character. It begins with a backslash character (\), which indicates that the character(s) that follow the backslash character should be treated in a special way. C uses escape sequences within a format string to print certain special characters. For example \n moves the output position to the beginning of the next line. The following is a list of escape sequences.

Escape sequence Action
\n prints a new line
\b backs up one character
\t moves the output position to the next tab stop
\\ prints a backslash
\" prints a double quote
\' prints a single quote

Return value

  • Upon successful completion, the vsprintf() function returns the number of bytes written to target-string.
  • If an error occurs, the vsprintf() function returns a negative value.

Example: vsprintf() function

The following example shows the usage of vprintf() function:

#include <stdio.h>
#include <stdarg.h>
void vout(char *string, char *fmt, ...);
char fmt1 [] = "%d %s  %s  %s  %s\n";

void vout(char *string, char *fmt, ...)
   va_list arg_ptr;
   va_start(arg_ptr, fmt);
   vsprintf(string, fmt, arg_ptr);

int main(void)
   char string[100];
   vout(string, fmt1, 3, "colors -", "Red,", "Green,", "White.");
   printf("String:  %s\n", string);


String:  3 colors -  Red,  Green,  White.

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