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C Library - vsprintf() function

The C library vsprintf(char *str, const char *format, va_list arg) function sends formatted output to a string using an argument list passed to it.This function is useful when you need to format a string using a list of arguments that is dynamically created at runtime.

Following is the C library syntax of the vsprintf() function −

int vsprintf(char *str, const char *format, va_list arg);

This function accepts three parameters −

• *char str: A pointer to the buffer where the resulting C-string is stored. The buffer must be large enough to contain the resulting string including the null-terminator.
• *const char format: A C-string that contains the text to be written to the string str. It can optionally contain embedded format specifiers that are replaced by the values specified in the subsequent argument list.
• va_list arg: A value that represents a variable argument list. This is typically initialized using the va_start macro and should be passed to vsprintf.

c

Character

d or i

Signed decimal integer

e

Scientific notation (mantissa/exponent) using e character

E

Scientific notation (mantissa/exponent) using E character

f

Decimal floating point

g

Uses the shorter of %e or %f

G

Uses the shorter of %E or %f

o

Signed octal

s

String of characters

u

Unsigned decimal integer

x

Unsigned hexadecimal integer

X

Unsigned hexadecimal integer (capital letters)

p

Pointer address

n

Nothing printed

%

Character

-

Left-justify within the given field width; Right justification is the default (see width sub-specifier).

+

Forces to precede the result with a plus or minus sign (+ or -) even for positive numbers. By default, only negative numbers are preceded with a -ve 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 preceded 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.

0

Left-pads the number with zeroes (0) instead of spaces, where padding is specified (see width sub-specifier).

(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.

.number

For 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.

h

The argument is interpreted as a short int or unsigned short int (only applies to integer specifiers i, d, o, u, x and X).

l

The 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.

L

The argument is interpreted as a long double (only applies to floating point specifiers e, E, f, g and G).

The function returns the total number of characters written to the string str, excluding the null-terminator. If an error occurs, the function returns a negative value.

The below code demonstrates simple string formatting where a single string argument is formatted into the placeholder %s.

Below is the illustration of C library vsprintf() function.

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

void formatString(char *buffer, const char *format, ...) {
    eva_list args;
    va_start(args, format);
    vsprintf(buffer, format, args);
    va_end(args);
}

int main() {
    char buffer[100];
    formatString(buffer, "Hello, %s!", "World");
    printf("%s\n", buffer);
    return 0;
}

The above code produces following result−

Hello, World!

The below code shows formatting multiple data types (string, integer, and float) into the buffer using appropriate format specifiers.

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

void formatString(char *buffer, const char *format, ...) {
   va_list args;
   va_start(args, format);
   vsprintf(buffer, format, args);
   va_end(args);
}

int main() {
   char buffer[100];
   formatString(buffer, "Name: %s, Age: %d, Score: %.2f", "Alice", 30, 95.5);
   printf("%s\n", buffer);
   return 0;
}

After execution of above code, we get the following result

Name: Alice, Age: 30, Score: 95.50

The below code demonstrates handling and formatting a long string, ensuring the buffer can accommodate the entire formatted string without truncation.

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

void formatString(char *buffer, const char *format, ...) {
   va_list args;
   va_start(args, format);
   vsprintf(buffer, format, args);
   va_end(args);
}

int main() {
   char buffer[200];
   const char *longString = "This is a very long string that needs to be formatted properly into the buffer.";
   formatString(buffer, "Formatted String: %s", longString);
   printf("%s\n", buffer);
   return 0;
}

The output of the above code is as follows −

Formatted String: This is a very long string that needs to be formatted properly into the buffer.

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