Opening and closing files - Mini Project: Build & Apply

Opening and closing files

📖 Scenario: You are creating a simple program to read data from a text file. Before you can read the data, you need to open the file safely and then close it when done.

🎯 Goal: Build a C program that opens a file called data.txt in read mode, checks if the file opened successfully, and then closes the file.

📋 What You'll Learn

Create a FILE* pointer variable named file.

Open the file data.txt in read mode using fopen.

Check if the file opened successfully by testing if file is not NULL.

Close the file using fclose.

Print "File opened and closed successfully." if the file opens, otherwise print "Failed to open file.".

💡 Why This Matters

🌍 Real World

Opening and closing files is a basic task in many programs that need to read or write data, such as reading configuration files or saving user data.

💼 Career

Understanding file handling is essential for software developers, especially those working with system programming, data processing, or embedded systems.

1

Create a FILE* pointer variable

Create a FILE* pointer variable called file and set it to NULL.

C

#include 

int main() {
    // Create a FILE* pointer called file and set it to NULL
    // Your code here

    return 0;
}

Hint

Use FILE *file = NULL; to declare the file pointer.

2

Open the file data.txt in read mode

Use fopen to open the file "data.txt" in read mode "r" and assign it to the variable file.

C

#include 

int main() {
    FILE *file = NULL;
    // Open "data.txt" in read mode and assign to file
    // Your code here

    return 0;
}

Hint

Use file = fopen("data.txt", "r"); to open the file.

3

Check if the file opened successfully

Write an if statement to check if file is not NULL. If it is not NULL, print "File opened and closed successfully.". Otherwise, print "Failed to open file.".

C

#include 

int main() {
    FILE *file = NULL;
    file = fopen("data.txt", "r");

    // Check if file is not NULL
    // Your code here

    return 0;
}

Hint

Use if (file != NULL) to check if the file opened.

4

Close the file

Inside the if block where file is not NULL, close the file using fclose(file);.

C

#include 

int main() {
    FILE *file = NULL;
    file = fopen("data.txt", "r");

    if (file != NULL) {
        // Close the file
        // Your code here
        printf("File opened and closed successfully.\n");
    } else {
        printf("Failed to open file.\n");
    }

    return 0;
}

Hint

Use fclose(file); to close the file inside the if block.

Practice

1. What does the fopen function do in C programming?

easy

A. It closes an open file stream.

B. It opens a file and returns a pointer to the file stream.

C. It reads data from a file.

D. It writes data to a file.

Solution

1. Step 1: Understand the purpose of fopen

   The fopen function is used to open a file and returns a pointer to the file stream if successful.

2. Step 2: Differentiate fopen from other file functions

   Functions like fclose close files, fread reads data, and fwrite writes data. Only fopen opens files.

3. Final Answer:

   It opens a file and returns a pointer to the file stream. -> Option B

4. Quick Check:

   fopen = open file pointer [OK]

Hint: fopen opens files, fclose closes them [OK]

Common Mistakes:

• Confusing fopen with fclose
• Thinking fopen reads or writes data
• Not knowing fopen returns NULL on failure

2. Which of the following is the correct syntax to open a file named "data.txt" for writing in C?

easy

A. FILE *fp = fopen("data.txt", "w");

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

C. FILE *fp = open("data.txt", "w");

D. FILE fp = fopen("data.txt", "w");

Solution

1. Step 1: Identify correct fopen syntax

   The correct syntax uses FILE * pointer and fopen with filename and mode as strings.

2. Step 2: Check mode for writing

   Mode "w" opens the file for writing. "r" is for reading, and open is not a standard C function for files.

3. Final Answer:

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

4. Quick Check:

   fopen + "w" = open for writing [OK]

Hint: Use FILE * and fopen with mode "w" to write [OK]

Common Mistakes:

• Using open instead of fopen
• Declaring FILE variable instead of pointer
• Using wrong mode like "r" for writing

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

FILE *fp = fopen("nonexistent.txt", "r");
if (fp == NULL) {
    printf("File not found\n");
} else {
    printf("File opened successfully\n");
    fclose(fp);
}

medium

A. File opened successfully

B. No output

C. File not found

D. Compilation error

Solution

1. Step 1: Understand fopen with "r" mode on nonexistent file

   Opening a file for reading with "r" mode returns NULL if the file does not exist.

2. Step 2: Check the if condition

   The code checks if fp == NULL, then prints "File not found". Since file does not exist, this condition is true.

3. Final Answer:

   File not found -> Option C

4. Quick Check:

   fopen returns NULL if file missing [OK]

Hint: fopen returns NULL if file missing in "r" mode [OK]

Common Mistakes:

• Assuming fopen creates file in "r" mode
• Not checking for NULL before using file pointer
• Expecting "File opened successfully" always

4. Identify the error in the following code snippet:

FILE *fp;
fp = fopen("file.txt", "w");
printf("File opened\n");
// Missing fclose here

medium

A. fopen mode "w" is incorrect

B. File pointer is not checked for NULL

C. printf statement is incorrect

D. fclose is missing to close the file

Solution

1. Step 1: Check fopen usage and mode

   The fopen call with mode "w" is correct to open file for writing.

2. Step 2: Identify missing fclose

   The code opens the file but never closes it with fclose, which can cause resource leaks.

3. Final Answer:

   fclose is missing to close the file -> Option D

4. Quick Check:

   Always fclose after fopen [OK]

Hint: Always close files with fclose after opening [OK]

Common Mistakes:

• Ignoring fclose causes resource leaks
• Not checking fopen return value
• Thinking printf affects file handling

5. You want to append text to an existing file "log.txt" safely. Which code snippet correctly opens the file, checks for errors, writes "Hello\n", and closes the file?

hard

A. FILE *fp = fopen("log.txt", "a");
if (fp == NULL) {
perror("Error opening file");
} else {
fprintf(fp, "Hello\n");
fclose(fp);
}

B. FILE *fp = fopen("log.txt", "w");
if (fp != NULL) {
fprintf(fp, "Hello\n");
fclose(fp);
}

C. FILE *fp = fopen("log.txt", "r");
fprintf(fp, "Hello\n");
fclose(fp);

D. FILE *fp = fopen("log.txt", "a");
fprintf(fp, "Hello\n");
// no fclose

Solution

1. Step 1: Choose correct mode for appending

   Mode "a" opens file for appending without erasing existing content.

2. Step 2: Check for NULL and handle errors

   Always check if fopen returns NULL before writing. Use perror to show error if NULL.

3. Step 3: Write and close file properly

   Use fprintf to write text and fclose to close file to avoid leaks.

4. Final Answer:

   FILE *fp = fopen("log.txt", "a"); if (fp == NULL) { perror("Error opening file"); } else { fprintf(fp, "Hello\n"); fclose(fp); } -> Option A

5. Quick Check:

   Append mode + check NULL + fclose = safe file write [OK]

Hint: Use "a" mode, check NULL, fclose after writing [OK]

Common Mistakes:

• Using "w" mode erases file instead of appending
• Not checking fopen return value before writing
• Forgetting to close file with fclose