File pointers in C - Mini Project: Build & Apply

Working with File Pointers in C

📖 Scenario: You are creating a simple program to write and read text from a file using file pointers in C. This is like writing a note on a piece of paper and then reading it back later.

🎯 Goal: Build a C program that opens a file, writes a message, moves the file pointer to the start, reads the message back, and prints it on the screen.

📋 What You'll Learn

Create a file pointer variable

Open a file named example.txt in write mode

Write the exact string "Hello, File Pointer!" to the file

Use fseek to move the file pointer to the beginning of the file

Read the string back from the file into a character array

Print the read string using printf

Close the file properly

💡 Why This Matters

🌍 Real World

File pointers let programs read and write files, which is useful for saving data, logs, or configuration settings.

💼 Career

Understanding file handling is important for software developers working with data storage, system programming, and application development.

1

Create a file pointer and open a file

Declare a file pointer variable called fp and open a file named example.txt in write mode using fopen.

C

#include 

int main() {
    // Declare file pointer and open file in write mode
    // Your code here
    return 0;
}

Hint

Use FILE *fp to declare the file pointer and fopen("example.txt", "w") to open the file for writing.

2

Write a string to the file

Use fprintf to write the exact string "Hello, File Pointer!" to the file using the file pointer fp.

C

#include 

int main() {
    FILE *fp = fopen("example.txt", "w");
    // Write "Hello, File Pointer!" to the file
    // Your code here
    return 0;
}

Hint

Use fprintf(fp, "Hello, File Pointer!") to write the string to the file.

3

Move file pointer and read the string

Close the file fp, then reopen example.txt in read mode. Use fseek to move the file pointer to the beginning of the file. Read the string into a character array called buffer of size 50 using fgets.

C

#include 

int main() {
    FILE *fp = fopen("example.txt", "w");
    fprintf(fp, "Hello, File Pointer!");
    // Close the file
    // Reopen the file in read mode
    // Move file pointer to start
    // Read string into buffer
    // Your code here
    return 0;
}

Hint

Remember to close the file before reopening it in read mode. Use fseek(fp, 0, SEEK_SET) to move to the start. Use fgets(buffer, 50, fp) to read the string.

4

Print the read string and close the file

Use printf to print the string stored in buffer. Then close the file pointer fp.

C

#include 

int main() {
    FILE *fp = fopen("example.txt", "w");
    fprintf(fp, "Hello, File Pointer!");
    fclose(fp);

    fp = fopen("example.txt", "r");
    fseek(fp, 0, SEEK_SET);

    char buffer[50];
    fgets(buffer, 50, fp);

    // Print the buffer
    // Close the file
    // Your code here
    return 0;
}

Hint

Use printf("%s", buffer) to print the string and fclose(fp) to close the file.

Practice

1. What does a file pointer in C represent?

easy

A. A variable that stores file contents

B. A reference to a file opened in the program

C. A function to read files

D. A type of file format

Solution

1. Step 1: Understand what a file pointer is

   A file pointer is a special pointer that points to a file stream opened by the program.

2. Step 2: Differentiate from other options

   It is not the file contents or a function, but a reference to the file for reading or writing.

3. Final Answer:

   A reference to a file opened in the program -> Option B

4. Quick Check:

   File pointer = Reference to opened file [OK]

Hint: File pointer points to file stream, not contents [OK]

Common Mistakes:

• Thinking file pointer stores file data
• Confusing file pointer with file reading function
• Assuming file pointer is a file format

2. Which of the following is the correct way to open a file for reading in C?

easy

A. FILE *fp = fopen("file.txt", "r");

B. FILE fp = fopen("file.txt", "r");

C. fopen FILE *fp = "file.txt", "r";

D. FILE *fp = open("file.txt", "r");

Solution

1. Step 1: Recall fopen syntax

   The fopen function returns a pointer to FILE, so the variable must be FILE *fp.

2. Step 2: Check each option

   FILE *fp = fopen("file.txt", "r"); correctly declares FILE *fp and calls fopen with filename and mode "r".

3. Final Answer:

   FILE *fp = fopen("file.txt", "r"); -> Option A

4. Quick Check:

   Correct fopen syntax = FILE *fp = fopen("file.txt", "r"); [OK]

Hint: fopen returns FILE pointer, so use FILE * variable [OK]

Common Mistakes:

• Declaring FILE variable instead of pointer
• Using open() instead of fopen()
• Incorrect order of fopen parameters

3. What will be the output of this C code snippet?

FILE *fp = fopen("test.txt", "w");
fprintf(fp, "Hello\n");
fclose(fp);

fp = fopen("test.txt", "r");
char buffer[10];
fgets(buffer, 10, fp);
printf("%s", buffer);
fclose(fp);

medium

A. Hello\n

B. Compilation error

C. test.txt

D. Hello

Solution

1. Step 1: Write and close file

   The code opens test.txt for writing, writes "Hello\n", then closes the file.

2. Step 2: Read and print file content

   It reopens test.txt for reading, reads a line into buffer, then prints it. fgets reads until newline or buffer limit.

3. Final Answer:

   Hello (printed as Hello with newline) -> Option D

4. Quick Check:

   File write/read prints "Hello" [OK]

Hint: fgets reads line including newline, printf prints it [OK]

Common Mistakes:

• Expecting literal \n printed
• Confusing fprintf and printf output
• Forgetting to close file before reopening

4. Identify the error in this code snippet:

FILE *fp;
fp = fopen("data.txt", "r");
char c = fgetc(fp);
while (c != EOF) {
    putchar(c);
    c = fgetc(fp);
}
fclose(fp);

medium

A. fgetc returns int, not char, causing wrong EOF check

B. fopen mode "r" is incorrect for reading

C. Missing fclose call

D. putchar cannot print characters

Solution

1. Step 1: Check fgetc return type

   fgetc returns int to allow EOF (-1) detection, storing in char loses EOF info.

2. Step 2: Understand EOF check failure

   Comparing char c to EOF fails because c cannot hold EOF value properly, causing infinite loop or wrong output.

3. Final Answer:

   fgetc returns int, not char, causing wrong EOF check -> Option A

4. Quick Check:

   Use int for fgetc result to detect EOF [OK]

Hint: Store fgetc result in int to detect EOF correctly [OK]

Common Mistakes:

• Using char instead of int for fgetc result
• Ignoring EOF check causing infinite loops
• Assuming putchar can't print chars

5. You want to append text "World" to an existing file "hello.txt" without deleting its content. Which is the correct way to open the file?

hard

A. FILE *fp = fopen("hello.txt", "r");

B. FILE *fp = fopen("hello.txt", "w");

C. FILE *fp = fopen("hello.txt", "a");

D. FILE *fp = fopen("hello.txt", "r+");

Solution

1. Step 1: Understand file modes

   "a" mode opens file for appending, preserving existing content and writing at the end.

2. Step 2: Compare other modes

   "w" overwrites file, "r" is read-only, "r+" allows read/write but does not append automatically.

3. Final Answer:

   FILE *fp = fopen("hello.txt", "a"); -> Option C

4. Quick Check:

   Append mode = "a" [OK]

Hint: Use "a" mode to add without erasing file [OK]

Common Mistakes:

• Using "w" which erases file content
• Using "r" which is read-only
• Confusing "r+" with append mode