Using errno in C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to include the header needed to use errno.

C

#include <stdio.h>
#include [1]

int main() {
    return 0;
}

Drag options to blanks, or click blank then click option'

A<errno.h>

B<string.h>

C<stdlib.h>

D<unistd.h>

Attempts:

3 left

💡 Hint

Common Mistakes

Including instead of

Forgetting to include any header for errno

The <errno.h> header defines the errno variable and error codes.

2fill in blank

medium

Complete the code to reset errno before a function call.

C

#include <stdio.h>
#include <errno.h>

int main() {
    errno = [1];
    return 0;
}

Drag options to blanks, or click blank then click option'

ANULL

B1

C-1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting errno to 1 or -1 instead of 0

Using NULL which is for pointers

Setting errno to 0 clears previous errors before a new function call.

3fill in blank

hard

Fix the error in checking if fopen failed by using errno.

C

#include <stdio.h>
#include <errno.h>

int main() {
    FILE *file = fopen("missing.txt", "r");
    if (file == NULL) {
        if (errno == [1]) {
            perror("File open error");
        }
    }
    return 0;
}

Drag options to blanks, or click blank then click option'

AENOENT

BEACCES

CEINVAL

DEEXIST

Attempts:

3 left

💡 Hint

Common Mistakes

Using EACCES which means permission denied

Using EINVAL which means invalid argument

ENOENT means the file or directory does not exist, which is common when fopen fails.

4fill in blank

hard

Fill both blanks to print a custom error message with errno and its description.

C

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main() {
    errno = 0;
    FILE *f = fopen("nofile.txt", "r");
    if (f == NULL) {
        printf("Error %[1]: %s\n", errno, [2](errno));
    }
    return 0;
}

Drag options to blanks, or click blank then click option'

Ad

Bi

Cstrerror

Dperror

Attempts:

3 left

💡 Hint

Common Mistakes

Using %d with perror (which prints directly)

Using %s with errno (which is an int)

The %i format prints an integer (errno), and strerror(errno) returns the error message string.

5fill in blank

hard

Fill all three blanks to check errno after a failed open and print a message.

C

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main() {
    errno = 0;
    FILE *fp = fopen("data.txt", "r");
    if (fp == NULL) {
        if (errno == [1]) {
            printf("Permission denied: %[2] - %s\n", errno, [3](errno));
        }
    }
    return 0;
}

Drag options to blanks, or click blank then click option'

AEACCES

Bi

Cstrerror

DENOENT

Attempts:

3 left

💡 Hint

Common Mistakes

Using ENOENT instead of EACCES for permission errors

Using %s for errno integer

Using perror instead of strerror in printf

EACCES means permission denied. Use %i to print errno and strerror(errno) for the message.