What Is Sprintf Function in C?

The sprintf() function in C is used to format and store a string in a buffer (array of characters) according to a specified format. It works similarly to printf(), but instead of printing the output to the console, it stores the formatted result in a string.

Syntax:

int sprintf(char *str, const char *format, ...);

• str: A pointer to the character array where the formatted string will be stored.
• format: A format string that specifies how subsequent arguments (if any) should be formatted.
• ...: A list of variables or values to be formatted according to the format string.

Return Value:

• sprintf() returns the number of characters written (excluding the null terminator) to the string str. If an error occurs, it returns a negative value.

Example:

#include <stdio.h>

int main() {
    char buffer[100];
    int x = 42;
    float y = 3.14159;

    // Use sprintf to format and store the output in buffer
    int length = sprintf(buffer, "The value of x is %d and the value of y is %.2f", x, y);

    // Print the formatted string
    printf("Formatted string: %s\n", buffer);
    printf("Number of characters written: %d\n", length);

    return 0;
}

Explanation:

• The sprintf() function formats the string "The value of x is 42 and the value of y is 3.14", storing it in the buffer array.
• %d is used to format the integer x, and %.2f is used to format the float y with two decimal places.
• The return value, length, contains the number of characters written into the buffer (excluding the null-terminator).

Output:

Formatted string: The value of x is 42 and the value of y is 3.14
Number of characters written: 53

Notes:

• Buffer Size: It’s important to ensure the buffer is large enough to hold the formatted string to avoid buffer overflows.
• Security Warning: sprintf() does not check the size of the destination buffer. For safer string formatting, consider using snprintf(), which allows you to specify the buffer size to prevent overflows.

Example with snprintf for safety:

#include <stdio.h>

int main() {
    char buffer[100];
    int x = 42;
    float y = 3.14159;

    // Use snprintf to format safely with buffer size limit
    int length = snprintf(buffer, sizeof(buffer), "The value of x is %d and the value of y is %.2f", x, y);

    printf("Formatted string: %s\n", buffer);
    printf("Number of characters written: %d\n", length);

    return 0;
}

This ensures that the string fits within the specified buffer size, avoiding potential overflows.