vfprintf() Function

The vfprintf() function in C stdio.h writes formatted data to a specified output stream using a variable argument list. It operates similarly to printf() by replacing format specifiers with corresponding values from the variable argument list, making it especially useful for functions that accept a variable number of arguments.

Syntax of vfprintf()

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

Parameters

ParameterDescription
streamPointer to a FILE object that identifies the output stream.
formatC string that contains the format string, following the same specifications as in printf().
argA va_list initialized with va_start that holds the variable arguments.

Note: The vfprintf() function retrieves arguments from the va_list as if va_arg were used, thereby altering its state. It is essential that the va_list is initialized with va_start before calling vfprintf() and properly released using va_end after the call.

Return Value

On success, vfprintf() returns the total number of characters written. On error, a negative number is returned, the error indicator for the stream is set, and if a multibyte character encoding error occurs while writing wide characters, errno is set to EILSEQ.

Examples for vfprintf()

Example 1: Using vfprintf() in a Custom Print Function

This example demonstrates how to create a custom print function that leverages vfprintf() to write formatted output to a stream.

Program

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

void my_print(FILE *stream, const char *format, ...) {
    va_list args;
    va_start(args, format);
    vfprintf(stream, format, args);
    va_end(args);
}

int main() {
    my_print(stdout, "Number: %d, String: %s\n", 42, "Hello");
    return 0;
}

Explanation:

  1. A custom function my_print is defined to accept a variable number of arguments.
  2. The variable argument list is initialized using va_start.
  3. vfprintf() is used to write the formatted data to the specified stream.
  4. The variable argument list is cleaned up with va_end after printing.

Program Output:

Number: 42, String: Hello

Example 2: Writing Formatted Output to a File Using vfprintf()

This example shows how to use vfprintf() to write formatted data to a file by wrapping the call in a helper function.

Program

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

void file_print(const char *filename, const char *format, ...) {
    FILE *fp = fopen(filename, "w");
    if (fp == NULL) {
        return;
    }
    va_list args;
    va_start(args, format);
    vfprintf(fp, format, args);
    va_end(args);
    fclose(fp);
}

int main() {
    file_print("output.txt", "Pi: %.2f\n", 3.14159);
    return 0;
}

Explanation:

  1. The file_print function opens a file for writing using the provided filename.
  2. A variable argument list is initialized and passed to vfprintf() to write formatted output to the file.
  3. The file is closed after writing to ensure data is saved properly.

Program Output:

Pi: 3.14

Example 3: Logging Error Messages to Standard Error with vfprintf()

This example demonstrates how to use vfprintf() to log formatted error messages to the standard error stream.

Program

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

void log_message(const char *format, ...) {
    va_list args;
    va_start(args, format);
    vfprintf(stderr, format, args);
    va_end(args);
}

int main() {
    log_message("Error: %s occurred at line %d\n", "Segmentation fault", 27);
    return 0;
}

Explanation:

  1. A helper function log_message is defined to log messages to the standard error stream.
  2. The function initializes a variable argument list and passes it to vfprintf() for formatted output.
  3. The error message is printed to stderr and the argument list is subsequently cleaned up with va_end.

Program Output:

Error: Segmentation fault occurred at line 27