A Complete Guide - C Programming Formatted File IO fprintf, fscanf

Last Updated: 03 Jul, 2025   
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C Programming Formatted File I/O: fprintf and fscanf

C programming offers robust support for file operations, including reading from and writing to files using formatted I/O functions such as fprintf for output and fscanf for input. These functions enable developers to read and write data to files in a structured format, similar to the way printf and scanf handle standard input and output on the console. Here, we will delve into the details of fprintf and fscanf and illustrate their usage with examples.

fprintf

fprintf is used to write formatted data to a file. It works similarly to the printf function, but instead of writing to the standard output (console), it writes to a specified file stream. The function prototype is as follows:

int fprintf(FILE *stream, const char *format, ...);
  • *FILE stream: A pointer to the file structure where data will be written. This file must be opened in write mode ("w", "a", "wb", "ab", etc.).
  • *const char format: A string that specifies the format of the data to be written. It can include both literal characters and format specifiers (e.g., %d, %f, %c).
  • ...: Variable number of arguments following the format string. These arguments are converted and formatted according to the format specifiers in the format string.

Format Specifiers:

  • %d for signed integers
  • %u for unsigned integers
  • %f for floating-point numbers
  • %e or %E for exponential representation of floating-point numbers
  • %c for characters
  • %s for strings
  • %p for pointers
  • %% for a literal percent sign

Example:

#include <stdio.h>

int main() {
    FILE *filePointer;
    int number = 10;
    float floatNumber = 10.76;
    char string[] = "Hello, World!";
    
    filePointer = fopen("example.txt", "w");  // Open file in write mode
    
    if (filePointer == NULL) {
        printf("Failed to open the file.\n");
        return 1;
    }
    
    // Using fprintf to write data to the file
    fprintf(filePointer, "Integer: %d\n", number);
    fprintf(filePointer, "Float: %f\n", floatNumber);
    fprintf(filePointer, "String: %s\n", string);
    
    fclose(filePointer);  // Close file
    
    return 0;
}

fscanf

fscanf is the counterpart to fprintf; it reads formatted data from a file. Similar to scanf, fscanf reads data from a file stream and stores it in variables according to the format specifiers provided. The function prototype is as follows:

int fscanf(FILE *stream, const char *format, ...);
  • *FILE stream: A pointer to the file structure from which data will be read. This file must be opened in read mode ("r", "rb", etc.).
  • *const char format: A string that specifies the format of the data to be read. It can include literal characters and format specifiers (e.g., %d, %f, %c).
  • ...: Variable number of pointers to variables where the read data will be stored. These pointers are associated with the format specifiers in the format string.

Example:

#include <stdio.h>

int main() {
    FILE *filePointer;
    int number;
    float floatNumber;
    char string[100];
    
    filePointer = fopen("example.txt", "r");  // Open file in read mode
    
    if (filePointer == NULL) {
        printf("Failed to open the file.\n");
        return 1;
    }
    
    // Using fscanf to read data from the file
    fscanf(filePointer, "Integer: %d\n", &number);
    fscanf(filePointer, "Float: %f\n", &floatNumber);
    fscanf(filePointer, "String: %s\n", string);
    
    printf("Data read from file:\nInteger: %d\nFloat: %f\nString: %s\n", number, floatNumber, string);
    
    fclose(filePointer);  // Close file
    
    return 0;
}

Important Considerations

  1. File Opening Modes:

    • r: Open file for reading. File must exist.
    • w: Open file for writing. If file exists, it is truncated (length set to zero). If file does not exist, it is created.
    • a: Open file for appending. Data is written to the end of the file.
    • rb, wb, ab: Binary versions of r, w, a.

  2. Error Handling:

    • Always check if the file was opened successfully using fopen. If fopen returns NULL, the file could not be opened.
    • After all file operations, close the file using fclose to free up resources.

  3. Buffering:

    • Writing operations with fprintf are buffered. To ensure data is written to the file immediately, use fflush to flush the buffer or fclose to close the file.

  4. Security:

    • Avoid using %s with fscanf without specifying a field width to prevent buffer overflow. Instead, use %Ns where N is the maximum number of characters to read.

  5. Multiple Line Reading:

    • fscanf stops reading a string when it encounters whitespace or a newline. For multi-line reading, consider using fgets or getline.

Understanding and effectively using fprintf and fscanf is crucial for handling file I/O operations in C programming. These functions provide a powerful way to write and read data to and from files in a structured and formatted manner.

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Step-by-Step Guide: How to Implement C Programming Formatted File IO fprintf, fscanf

Step-by-Step Example

Step 1: Understanding the Functions

  • fprintf: This function formats data and writes it to a file.

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

  • fscanf: This function reads and parses formatted data from a file.

    • Syntax: int fscanf(FILE *stream, const char *format, ...);

Step 2: Writing Data to a File using fprintf

Let's start by creating a program that writes some data into a file using fprintf.

#include <stdio.h>
#include <stdlib.h>

int main() {
    // Open a file in write mode
    FILE *file = fopen("example.txt", "w");
    if (file == NULL) {
        perror("Failed to open file for writing");
        return EXIT_FAILURE;
    }

    // Data to write
    int age = 25;
    double height = 5.9;
    char name[] = "Alice";

    // Write data to the file in a formatted manner
    fprintf(file, "Name: %s\n", name);
    fprintf(file, "Age: %d\n", age);
    fprintf(file, "Height: %.2f feet\n", height);

    // Close the file
    fclose(file);

    printf("Data written successfully to example.txt\n");
    return EXIT_SUCCESS;
}

Explanation:

  1. Opening the File: FILE *file = fopen("example.txt", "w"); opens a file named example.txt in write mode. If the file does not exist, it creates a new one.
  2. Writing Data: fprintf(file, "Name: %s\n", name); writes formatted data to the file. The %s format specifier tells fprintf to insert a string at that position.
  3. Closing the File: fclose(file); is used to close the file after writing.

Step 3: Reading Data from a File using fscanf

Now, let's create a program that reads the data we just wrote back from the file using fscanf.

#include <stdio.h>
#include <stdlib.h>

int main() {
    // Open a file in read mode
    FILE *file = fopen("example.txt", "r");
    if (file == NULL) {
        perror("Failed to open file for reading");
        return EXIT_FAILURE;
    }

    // Variables to store data read from the file
    int age;
    double height;
    char name[50];

    // Read data from the file in a formatted manner
    fscanf(file, "Name: %s", name);
    fscanf(file, "Age: %d", &age);
    fscanf(file, "Height: %lf feet", &height);

    // Close the file
    fclose(file);

    // Print the data read from the file
    printf("Name: %s\n", name);
    printf("Age: %d\n", age);
    printf("Height: %.2f feet\n", height);

    return EXIT_SUCCESS;
}

Explanation:

  1. Opening the File: FILE *file = fopen("example.txt", "r"); opens the file in read mode.
  2. Reading Data: fscanf(file, "Name: %s", name); reads the formatted data into the variables. The %s format specifier tells fscanf to read a string, %d for an integer, and %lf for a double.
  3. Closing the File: fclose(file); closes the file after reading.
  4. Printing Data: printf is used to display the data read from the file.

Step 4: Running the Programs

  1. Compile the first program that writes data to example.txt:

    gcc -o write_file write_file.c
./write_file

  2. Compile the second program that reads data from example.txt:

    gcc -o read_file read_file.c
./read_file

Summary

  1. Writing Data:

    • Use fprintf to write formatted data to a file.

  2. Reading Data:

    • Use fscanf to read and parse formatted data from a file.

  3. File Handling:

    • Always open a file using fopen().
    • Close the file using fclose() after completing file operations.
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Top 10 Interview Questions & Answers on C Programming Formatted File IO fprintf, fscanf

Top 10 Questions and Answers on C Programming: Formatted File I/O with fprintf and fscanf

1. What are fprintf and fscanf in C?

2. How do you use fprintf to write to a file?

You need a FILE* pointer to a file opened in write mode before using fprintf. The basic syntax is:

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

Example:

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "w");
    if (fp == NULL) {
        printf("Error opening file!\n");
        return 1;
    }
    fprintf(fp, "Hello, world!\nNumber: %d\n", 42);
    fclose(fp);
    return 0;
}

This code writes "Hello, world!" followed by "Number: 42" to "example.txt".

3. How do you use fscanf to read from a file?

You need a FILE* pointer to a file opened in read mode before using fscanf. The syntax is similar to scanf:

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

Example:

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "r");
    if (fp == NULL) {
        printf("Error opening file!\n");
        return 1;
    }
    char str[50];
    int num;
    fscanf(fp, "%s Number: %d", str, &num);
    printf("Read string: %s\nRead number: %d\n", str, num);
    fclose(fp);
    return 0;
}

This reads the string "Hello, world!" and the number 42 from "example.txt".

4. What are the common format specifiers for fprintf and fscanf?

Common format specifiers include:

  • %d, %i for integers.
  • %f for floating point numbers.
  • %s for strings.
  • %c for single characters.
  • %x for hexadecimal numbers.
  • %o for octal numbers.
  • %u for unsigned integers.
  • %p for pointers.

5. How does fprintf handle whitespace and newlines?

fprintf treats whitespace in the format string like any other character. To insert a newline, you can use \n. Extra spaces in the format string are output as a single space.

Example:

fprintf(fp, "Hello  %s  %d\n", "world", 123);

This outputs "Hello world 123" with extra spaces preserved and a newline at the end.

6. How does fscanf handle whitespace and newlines when reading a file?

fscanf ignores any whitespace (spaces, newlines, tabs) in the input file until it encounters a non-whitespace character that matches the specified format specifier. This behavior can vary depending on the format specifier used.

Example:

fscanf(fp, "%s %d", str, &num);

This would ignore any leading whitespace, then read a word into str, then any subsequent whitespace, and finally an integer into num.

7. How can you handle errors when using fprintf and fscanf?

After using fprintf or fscanf, always check for errors. fprintf returns the number of items successfully written, and fscanf returns the number of items successfully read. If the return value is less than the number of items expected, an error occurred.

Example:

int ret = fscanf(fp, "%d", &num);
if (ret != 1) {
    printf("Error reading from file!\n");
}

8. What are the differences between fprintf and sprintf?

fprintf writes formatted data to a file pointed to by a FILE* pointer, while sprintf writes formatted data to a string.

Syntax:

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

Example:

char str[100];
sprintf(str, "Hello, %s! Number: %d", "world", 55);

This stores "Hello, world! Number: 55" into the string str.

9. How can you append data to an already existing file using fprintf?

To append data to an existing file, open the file in append mode "a".

Example:

FILE *fp = fopen("example.txt", "a");
if (fp == NULL) {
    printf("Error opening file!\n");
    return 1;
}
fprintf(fp, "Appending line.\n");
fclose(fp);

This adds "Appending line." to the end of "example.txt" without overwriting its existing content.

10. How can you read a file line by line using fscanf or fgets?

While fscanf is not the most ideal for reading lines due to its whitespace handling, fgets is more suited for this task.

Using fgets:

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "r");
    if (fp == NULL) {
        printf("Error opening file!\n");
        return 1;
    }
    char buffer[100];
    while (fgets(buffer, sizeof(buffer), fp) != NULL) {
        printf("Line: %s", buffer);
    }
    fclose(fp);
    return 0;
}

fgets reads a line from the file into buffer up to sizeof(buffer) - 1 characters and includes the newline at the end of each line.

Using fscanf:

#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "r");
    if (fp == NULL) {
        printf("Error opening file!\n");
        return 1;
    }
    char str[100];
    while (fscanf(fp, "%99[^\n]%*c", str) == 1) {
        printf("Line: %s\n", str);
    }
    fclose(fp);
    return 0;
}

This uses fscanf with a format specifier that reads characters up to a newline and ignores the newline itself.

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