C vprintf() Function – stdio.h

vprintf() Function

The vprintf() function in C stdio.h is used to print formatted data to the standard output using a variable argument list. It functions similarly to printf() but retrieves its arguments from a list initialized with va_start(), making it ideal for cases where the number and types of arguments are not fixed at compile time.

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Syntax of vprintf()

int vprintf(const char *format, va_list arg);

Parameters

Parameter	Description
format	A C string that contains the format to be used, following the same rules as printf().
arg	A variable argument list (of type va_list) that has been initialized with va_start() and contains the values to be printed.

Before calling vprintf(), ensure that the variable argument list is properly initialized using va_start() and released using va_end() after the call.

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Return Value

On success, vprintf() returns the total number of characters written to the standard output. In case of an error, such as a writing error or a multibyte character encoding error, a negative number is returned, with the error indicator set accordingly and errno possibly set to EILSEQ.

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Examples for vprintf()

Example 1: Basic Usage of vprintf() for Formatted Output

This example demonstrates how to use vprintf() to print a formatted string by passing a variable argument list.

Program

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

void printFormatted(const char *format, ...) {
    va_list args;
    va_start(args, format);
    vprintf(format, args);
    va_end(args);
}

int main() {
    printFormatted("Name: %s, Age: %d\n", "Alice", 30);
    return 0;
}

Explanation:

1. The printFormatted() function accepts a format string followed by a variable number of arguments.
2. The variable argument list is initialized using va_start() and then passed to vprintf() for printing.
3. The variable argument list is properly cleaned up using va_end().
4. The main() function calls printFormatted() with a format string and corresponding arguments, resulting in formatted output.

Program Output:

Name: Alice, Age: 30

Example 2: Using vprintf() in a Custom Logging Function

This example illustrates how vprintf() can be utilized within a custom logging function to handle formatted messages with variable arguments.

Program

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

void logMessage(const char *format, ...) {
    va_list args;
    va_start(args, format);
    printf("LOG: ");
    vprintf(format, args);
    va_end(args);
}

int main() {
    logMessage("Error code %d occurred at %s\n", 404, "main()");
    return 0;
}

Explanation:

1. The logMessage() function is designed as a custom logger that prefixes messages with “LOG: “.
2. The variable argument list is initialized using va_start() and passed to vprintf() for formatted output.
3. After printing, the variable argument list is cleaned up with va_end().
4. The main() function demonstrates logging an error message with a specific error code and location.

Program Output:

LOG: Error code 404 occurred at main()

Example 3: Handling Multiple Data Types with vprintf()

This example demonstrates using vprintf() to print a formatted string containing various data types, showcasing its versatility.

Program

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

void printDetails(const char *format, ...) {
    va_list args;
    va_start(args, format);
    vprintf(format, args);
    va_end(args);
}

int main() {
    printDetails("Integer: %d, Floating: %.2f, String: %s\n", 42, 3.14, "Test");
    return 0;
}

Explanation:

1. The printDetails() function accepts a format string with placeholders for an integer, a floating-point number, and a string.
2. The variable argument list is initialized with va_start() and processed by vprintf() to print the formatted output.
3. The main() function calls printDetails() with appropriate arguments, resulting in the display of various data types.

Program Output:

Integer: 42, Floating: 3.14, String: Test