Writing to files in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C program writing to a file?

Consider the following C code that writes to a file named output.txt. What will be the content of the file after running this program?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("output.txt", "w");
    if (fp == NULL) {
        return 1;
    }
    fprintf(fp, "Hello\n");
    fprintf(fp, "World\n");
    fclose(fp);
    return 0;
}

A

The file contains:
Hello
World

B

The file contains:
HelloWorld

C The program crashes and no file is created

D The file is empty

💡 Hint

Look at how fprintf is used with newline characters.

The program opens the file in write mode, writes "Hello" followed by a newline, then "World" followed by a newline, then closes the file. So the file contains two lines: "Hello" and "World".

❓ Predict Output

intermediate

What error does this code produce when writing to a file?

What error will this C code produce when executed?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("readonly.txt", "r");
    fprintf(fp, "Trying to write\n");
    fclose(fp);
    return 0;
}

A Segmentation fault at fprintf

B Runtime error: writing to a file opened in read mode

C No error, file is written successfully

D Compilation error: fprintf requires a writable file pointer

💡 Hint

Think about the mode used to open the file and what fprintf expects.

The file is opened in read mode, so writing to it with fprintf causes a runtime error because the file pointer is not writable.

🔧 Debug

advanced

Why does this file writing code produce no output in the file?

The following C code is supposed to write "Data" to data.txt, but the file remains empty after running. What is the cause?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("data.txt", "w");
    if (fp == NULL) {
        return 1;
    }
    fputs("Data", fp);
    // Missing fclose(fp);
    return 0;
}

A The file pointer is NULL, so nothing is written

B The file is opened in write mode but no data is written

C fputs does not write to files, only to stdout

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

💡 Hint

Think about how buffered output works and what happens if you don't close the file.

Output to files is buffered. Without calling fclose or fflush, the buffer may not be written to the file, leaving it empty.

📝 Syntax

advanced

Which option contains a syntax error in file writing code?

Identify the option that contains a syntax error preventing compilation.

A FILE *fp = fopen("file.txt", "w") if (fp == NULL) { return 1; } fprintf(fp, "Hello"); fclose(fp);

B FILE *fp = fopen("file.txt", "w"); if (fp == NULL) { return 1; } fprintf(fp, "Hello"); fclose(fp);

C FILE *fp = fopen("file.txt", "w"); if (fp != NULL) { fprintf(fp, "Hello"); fclose(fp); }

D ;)pf(esolcf ;)"olleH" ,pf(ftnirpf } ;1 nruter { )LLUN == pf( fi ;)"w" ,"txt.elif"(nepof = pf* ELIF

💡 Hint

Look carefully for missing punctuation in the code.

Option A is missing a semicolon after the fopen call, causing a syntax error.

🚀 Application

expert

How many bytes are written to the file by this program?

This C program writes a string to a file. How many bytes will the file contain after running?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("test.txt", "w");
    if (!fp) return 1;
    const char *str = "Line1\nLine2\n";
    int count = fprintf(fp, "%s", str);
    fclose(fp);
    return 0;
}

A 10 bytes

B 11 bytes

C 12 bytes

D 13 bytes

💡 Hint

Count all characters including newline characters in the string.

The string "Line1\nLine2\n" has 12 characters: 5 letters + 1 newline + 5 letters + 1 newline = 12 bytes.