C stdio.h fprintf() Function

fprintf() Function

The fprintf() function in C writes formatted data to a specified output stream. It allows you to combine static text with variable data by using format specifiers within a format string. This function is extremely useful for logging and file output operations where the formatted presentation of data is essential.

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Syntax of fprintf()

int fprintf(FILE *stream, const char *format, ...);

Parameters

Parameter	Description
stream	A pointer to a FILE object that identifies the output stream.
format	A C string containing the text to be written to the stream. It can include embedded format specifiers that will be replaced by the additional arguments.

The function processes the format string and replaces any format specifiers with the corresponding additional arguments, formatting the output accordingly. It supports a variety of specifiers such as %d, %f, %s, and more, allowing for flexible output formatting.

Return Value

On success, fprintf() returns the total number of characters written. If a writing error occurs, it returns a negative number, and the error indicator for the stream is set accordingly.

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Examples for fprintf()

Example 1: Writing Simple Formatted Text to a File

This example demonstrates how to use fprintf() to write a simple formatted message to a file.

Program

#include <stdio.h>

int main() {
    FILE *fp = fopen("example1.txt", "w");
    if (fp == NULL) {
        perror("Error opening file");
        return 1;
    }

    // Write a simple message to the file
    fprintf(fp, "Hello, fprintf!\n");

    fclose(fp);
    return 0;
}

Explanation:

1. The file example1.txt is opened in write mode.
2. fprintf() writes the formatted string “Hello, fprintf!\n” to the file.
3. The file is closed after writing.

Program Output:

Example 2: Writing Formatted Numeric Data

This example shows how to format and write numeric data to a file using various format specifiers.

Program

#include <stdio.h>

int main() {
    FILE *fp = fopen("example2.txt", "w");
    if (fp == NULL) {
        perror("Error opening file");
        return 1;
    }

    int num = 42;
    double pi = 3.14159;
    fprintf(fp, "Integer: %d\n", num);
    fprintf(fp, "Floating-point: %.2f\n", pi);

    fclose(fp);
    return 0;
}

Explanation:

1. The file example2.txt is opened in write mode.
2. An integer and a floating-point number are defined.
3. fprintf() writes the integer using the %d specifier and the floating-point number using the %.2f specifier to format the output with two decimal places.
4. The file is closed after writing.

Program Output:

Example 3: Writing Mixed Data Types

This example illustrates how to combine different data types into a single formatted output to a file.

Program

#include <stdio.h>

int main() {
    FILE *fp = fopen("example3.txt", "w");
    if (fp == NULL) {
        perror("Error opening file");
        return 1;
    }

    char name[] = "Arjun";
    int age = 30;
    double salary = 55000.75;

    fprintf(fp, "Name: %s\n", name);
    fprintf(fp, "Age: %d years\n", age);
    fprintf(fp, "Salary: $%.2f per year\n", salary);

    fclose(fp);
    return 0;
}

Explanation:

1. The file example3.txt is opened in write mode.
2. A string, an integer, and a floating-point number are defined.
3. fprintf() is used to write each variable with appropriate format specifiers: %s for the string, %d for the integer, and %.2f for the floating-point number formatted to two decimal places.
4. The file is closed after writing.

Output:

Example 4: Handling File Opening Errors

This example demonstrates how to check for errors when opening a file and using fprintf() to report the error.

Program

#include <stdio.h>

int main() {
    FILE *fp = fopen("nonexistent_directory/example4.txt", "w");
    if (fp == NULL) {
        fprintf(stderr, "Error: Unable to open file for writing.\n");
        return 1;
    }

    fprintf(fp, "This will not be printed if file open fails.\n");
    fclose(fp);
    return 0;
}

Explanation:

1. The program attempts to open a file in a directory that does not exist.
2. If fopen() fails, it returns NULL and an error message is printed to the standard error stream using fprintf() with stderr.
3. The program exits with an error code.

Program Output:

Error: Unable to open file for writing.