Opening and closing files - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C code when opening a file?

Consider the following C code snippet that attempts to open a file named data.txt in read mode. What will be printed if the file does not exist?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("data.txt", "r");
    if (fp == NULL) {
        printf("File not found\n");
    } else {
        printf("File opened successfully\n");
        fclose(fp);
    }
    return 0;
}

A File not found

B Runtime error

C Compilation error

D File opened successfully

💡 Hint

Think about what happens when you try to open a file for reading that does not exist.

If the file data.txt does not exist, fopen returns NULL. The program prints "File not found" in this case.

❓ Predict Output

intermediate

What happens if you try to close a file pointer that is NULL?

Look at this C code snippet. What will happen when it runs?

C

#include <stdio.h>

int main() {
    FILE *fp = NULL;
    fclose(fp);
    printf("Closed file pointer\n");
    return 0;
}

A Prints "Closed file pointer" without error

B Causes a segmentation fault or runtime error

C Compilation error due to fclose argument

D Prints nothing and exits

💡 Hint

Consider what happens if you try to close a file pointer that was never opened.

Calling fclose on a NULL pointer causes undefined behavior, often a segmentation fault or runtime error.

🔧 Debug

advanced

Why does this code fail to write to the file?

The following code tries to open a file for writing and write a string. However, the file remains empty after running. What is the problem?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("output.txt", "w");
    if (fp == NULL) {
        printf("Failed to open file\n");
        return 1;
    }
    fputs("Hello, world!", fp);
    // Missing fclose here
    return 0;
}

A fputs requires a newline character to write

B The file is opened in write mode but should be append mode

C The string "Hello, world!" is not null-terminated

D The file is not closed, so output is not flushed to disk

💡 Hint

Think about what happens to buffered output if the file is not closed.

Without calling fclose, the output buffer may not be flushed to the file, so the file remains empty.

📝 Syntax

advanced

Which option correctly opens a file for appending text?

Choose the correct C code line that opens a file named log.txt for appending text data.

A FILE *fp = fopen("log.txt", "a");

B FILE *fp = fopen("log.txt", "x");

C FILE *fp = fopen("log.txt", "w+");

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

💡 Hint

Appending means adding data to the end without deleting existing content.

Mode "a" opens the file for appending. "r+" opens for reading and writing but does not create the file if missing. "w+" truncates the file. "x" creates a new file but fails if it exists.

🚀 Application

expert

How many times is the file opened and closed in this program?

Analyze the following C program. How many times does it open and close the file test.txt?

C

#include <stdio.h>

void write_once() {
    FILE *fp = fopen("test.txt", "w");
    if (fp) {
        fputs("First write\n", fp);
        fclose(fp);
    }
}

void write_twice() {
    FILE *fp = fopen("test.txt", "a");
    if (fp) {
        fputs("Second write\n", fp);
        fclose(fp);
    }
}

int main() {
    write_once();
    write_twice();
    return 0;
}

A The file is opened twice but closed once

B The file is opened once and closed once

C The file is opened and closed twice

D The file is opened once but closed twice

💡 Hint

Count each fopen and fclose call that executes.

The function write_once opens and closes the file once. The function write_twice also opens and closes the file once. So total two opens and two closes.