Error handling in files in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this file error check?

Consider this C code snippet that tries to open a file and checks for errors. What will it print if the file does not exist?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("nonexistent.txt", "r");
    if (fp == NULL) {
        perror("Error opening file");
        return 1;
    }
    fclose(fp);
    return 0;
}

A Error opening file: No such file or directory

B File opened successfully

C Segmentation fault

D Compilation error

💡 Hint

Check what fopen returns when the file does not exist and how perror works.

When fopen fails to open a file, it returns NULL. perror prints the string you give it, followed by a colon, a space, and the error message corresponding to errno. Since the file does not exist, it prints 'No such file or directory'.

❓ Predict Output

intermediate

What error does this code raise?

What error will this C code produce when trying to read from a file pointer that is NULL?

C

#include <stdio.h>

int main() {
    FILE *fp = NULL;
    char buffer[10];
    fgets(buffer, 10, fp);
    return 0;
}

A No error, program runs fine

B Runtime error: file not found

C Compilation error

D Segmentation fault

💡 Hint

Think about what happens if you try to use a NULL pointer with fgets.

fgets expects a valid FILE pointer. Passing NULL causes undefined behavior, usually a segmentation fault because the program tries to access memory through a NULL pointer.

🧠 Conceptual

advanced

Which option correctly checks for file open errors?

You want to open a file for writing and handle errors properly. Which code snippet correctly checks if the file was opened successfully?

A

FILE *fp = fopen("file.txt", "w");
if (fp != NULL) {
    perror("Failed to open file");
    return 1;
}

B

FILE *fp = fopen("file.txt", "w");
if (fp == NULL) {
    perror("Failed to open file");
    return 1;
}

C

FILE *fp = fopen("file.txt", "w");
if (fp == 0) {
    perror("Failed to open file");
    return 1;
}

D

FILE *fp = fopen("file.txt", "w");
if (fp = NULL) {
    perror("Failed to open file");
    return 1;
}

💡 Hint

Remember how to compare pointers to NULL in C.

Option B correctly compares the FILE pointer to NULL using '=='. Option B incorrectly checks if fp is not NULL to detect error, which is wrong. Option B compares to 0, which is allowed but less clear; however, in C NULL is defined as 0, so it compiles but is less readable. Option B uses assignment '=' instead of comparison '==', causing a bug.

❓ Predict Output

advanced

What is the output of this code with error handling?

What will this program print if the file "data.txt" exists and contains the text "Hello"?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("data.txt", "r");
    if (!fp) {
        printf("Cannot open file\n");
        return 1;
    }
    int c = fgetc(fp);
    if (c == EOF) {
        printf("Empty file\n");
    } else {
        printf("First char: %c\n", c);
    }
    fclose(fp);
    return 0;
}

A First char: H

B Cannot open file

C Empty file

D First char: EOF

💡 Hint

Check what fgetc returns when reading the first character of a non-empty file.

Since the file exists and contains "Hello", fgetc reads the first character 'H'. It is not EOF, so the program prints 'First char: H'.

❓ Predict Output

expert

What is the value of 'count' after this file reading loop?

This program counts how many characters it reads from a file. What is the value of 'count' after running if the file "input.txt" contains exactly 3 characters?

C

#include <stdio.h>

int main() {
    FILE *fp = fopen("input.txt", "r");
    if (!fp) return 1;
    int count = 0;
    int ch;
    while ((ch = fgetc(fp)) != EOF) {
        count++;
    }
    fclose(fp);
    printf("%d\n", count);
    return 0;
}

A 0

B 4

C 3

D Undefined behavior

💡 Hint

Remember that fgetc returns EOF after reading all characters.

The loop increments count for each character read until EOF is reached. Since the file has 3 characters, count becomes 3.