Using errno in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code using errno after fopen fails?

Consider the following C code snippet that tries to open a non-existent file. What will it print?

C

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

int main() {
    FILE *fp = fopen("nonexistent.txt", "r");
    if (!fp) {
        printf("Error code: %d\n", errno);
        printf("Error message: %s\n", strerror(errno));
    }
    return 0;
}

A

Error code: 0
Error message: Success

B

Error code: 2
Error message: No such file or directory

C

Error code: 13
Error message: Permission denied

D

Error code: 22
Error message: Invalid argument

💡 Hint

errno is set by system calls and library functions when an error occurs. fopen returns NULL on failure and sets errno.

When fopen fails to open a file that does not exist, errno is set to 2, which means 'No such file or directory'.

🧠 Conceptual

intermediate

What does errno represent in C?

Choose the best description of what errno is used for in C programming.

A A global variable set by system calls and library functions to indicate the error code of the last error.

B A function that returns the last error message as a string.

C A constant integer representing the number of errors encountered since program start.

D A macro that clears all errors in the program.

💡 Hint

Think about how C functions communicate errors without exceptions.

errno is a global variable that is set by system calls and library functions when an error occurs to indicate the error code.

🔧 Debug

advanced

Why does this code print 'Error code: 0' even after fopen fails?

Examine the code below. It tries to open a file that does not exist but prints 'Error code: 0'. What is the cause?

C

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

int main() {
    errno = 0;
    FILE *fp = fopen("missing.txt", "r");
    if (!fp) {
        printf("Error code: %d\n", errno);
        printf("Error message: %s\n", strerror(errno));
    }
    return 0;
}

A errno must be checked immediately after fopen; some functions reset errno if not checked quickly.

B errno was set to 0 before fopen, but fopen did not set errno on failure in this environment.

C fopen succeeded, so errno remains 0.

D The program forgot to include <errno.h>, so errno is undefined.

💡 Hint

errno is only meaningful immediately after a failing function call.

errno is only set when an error occurs. If you do other calls before checking errno, it may be reset or unchanged. Always check errno immediately after the failing call.

📝 Syntax

advanced

Which option correctly declares and uses errno to check an error after a system call?

Select the code snippet that correctly uses errno to detect an error after calling a system function.

A

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

int main() {
    int errno = 0;
    int result = remove("file.txt");
    if (result == -1) {
        printf("Error: %s\n", strerror(errno));
    }
    return 0;
}

B

#include <stdio.h>

int main() {
    int result = remove("file.txt");
    if (result != 0) {
        printf("Error: %s\n", strerror(errno));
    }
    return 0;
}

C

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

int main() {
    int result = remove("file.txt");
    if (result != 0) {
        printf("Error: %s\n", strerror(errno));
    }
    return 0;
}

D

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

int main() {
    int result = remove("file.txt");
    if (result == 0) {
        printf("Error: %s\n", strerror(errno));
    }
    return 0;
}

💡 Hint

Do not redeclare errno as a local variable. Check the function's return value correctly.

Option C correctly includes errno.h, does not redeclare errno, and checks remove() return value properly (non-zero means error).

🚀 Application

expert

How many distinct error messages can this code print when opening files?

Given the code below, how many different error messages can it print if the file opening fails?

C

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

int main() {
    const char *files[] = {"file1.txt", "file2.txt", "file3.txt"};
    for (int i = 0; i < 3; i++) {
        FILE *fp = fopen(files[i], "r");
        if (!fp) {
            printf("File: %s, Error: %s\n", files[i], strerror(errno));
        } else {
            fclose(fp);
        }
    }
    return 0;
}

A 0

B 1

C 3

D Depends on the system and file permissions

💡 Hint

Error messages depend on why fopen fails, which can vary by file and system.

The number of distinct error messages depends on the system state and permissions for each file. It can vary, so the answer depends on the environment.