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.

Practice

1. What is the primary purpose of errno in C programming?

easy

A. To store error codes set by system or library functions when they fail

B. To count the number of errors in a program

C. To reset the program state after an error

D. To display error messages automatically

Solution

1. Step 1: Understand what errno stores

   errno is a global variable that stores error codes set by system or library functions when they fail.

2. Step 2: Differentiate from other options

   It does not count errors, reset program state, or display messages automatically.

3. Final Answer:

   To store error codes set by system or library functions when they fail -> Option A

4. Quick Check:

   errno stores error codes = C [OK]

Hint: Remember: errno holds error codes after failure [OK]

Common Mistakes:

• Thinking errno counts errors
• Assuming errno resets program state
• Believing errno prints messages automatically

2. Which of the following is the correct way to include errno in a C program?

easy

A. #include

B. #include

C. #include

D. #include

Solution

1. Step 1: Identify the standard header for errno

   The standard header file to use errno is <errno.h>.

2. Step 2: Verify other options

   <error.h>, <err.h>, and <errors.h> are not standard headers for errno.

3. Final Answer:

   #include <errno.h> -> Option B

4. Quick Check:

   errno header = <errno.h> [OK]

Hint: Use #include to access errno [OK]

Common Mistakes:

• Using wrong header files
• Forgetting to include errno.h
• Confusing with similar-sounding headers

3. What will the following C code print if the file nonexistent.txt does not exist?

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

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

medium

A. Error: No such file or directory

B. Error: Permission denied

C. Error: File exists

D. Error: Unknown error

Solution

1. Step 1: Understand fopen failure and errno

   When fopen fails to open a non-existing file for reading, it sets errno to indicate the error, usually ENOENT meaning "No such file or directory".

2. Step 2: Use strerror to get error message

   strerror(errno) converts the error code to a readable string, which will be "No such file or directory".

3. Final Answer:

   Error: No such file or directory -> Option A

4. Quick Check:

   fopen missing file error = "No such file or directory" [OK]

Hint: fopen missing file sets errno to ENOENT message [OK]

Common Mistakes:

• Assuming errno is zero without checking fopen result
• Confusing error messages for different errors
• Not including string.h for strerror

4. Identify the error in this code snippet that uses errno:

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

int main() {
    errno = 0;
    FILE *f = fopen("file.txt", "r");
    if (f == NULL) {
        perror("File open error");
    }
    if (errno != 0) {
        printf("Error code: %d\n", errno);
    }
    return 0;
}

medium

A. Using perror instead of strerror is incorrect

B. Setting errno to 0 before fopen is unnecessary

C. Checking errno after fopen without checking fopen result first

D. Not closing the file pointer after fopen

Solution

1. Step 1: Analyze errno usage after fopen

   errno should only be checked if the function indicates failure. Here, errno is checked regardless of fopen result, which can be misleading.

2. Step 2: Confirm correct error checking pattern

   The correct pattern is to check if fopen returned NULL before reading errno. This code checks errno even if fopen succeeded.

3. Final Answer:

   Checking errno after fopen without checking fopen result first -> Option C

4. Quick Check:

   Check function result before errno [OK]

Hint: Always check function result before reading errno [OK]

Common Mistakes:

• Reading errno without function failure
• Confusing perror and strerror usage
• Ignoring file close after fopen

5. You want to open a file for reading and print a custom error message if it fails. Which code snippet correctly uses errno and strerror to do this?

A) FILE *f = fopen("data.txt", "r");
   if (!f) {
       printf("Error: %s\n", strerror(errno));
   }

B) errno = 0;
   FILE *f = fopen("data.txt", "r");
   if (f == NULL) {
       printf("Error: %s\n", strerror(errno));
   }

C) FILE *f = fopen("data.txt", "r");
   if (f == NULL) {
       errno = 0;
       printf("Error: %s\n", strerror(errno));
   }

D) FILE *f = fopen("data.txt", "r");
   if (f == NULL) {
       perror("Error opening file");
   }

hard

A. Option A

B. Option B

C. Option C

D. Option D

Solution

1. Step 1: Check correct error handling with errno

   Option D uses perror, which is the standard and simplest way to print a custom error message along with the error string corresponding to errno. It automatically handles errno correctly.

2. Step 2: Identify issues in other options

   Option D does not reset errno before fopen, which is less safe. Option D resets errno before fopen and checks failure, but it's generally unnecessary to reset errno before calls that set it on failure. Option D resets errno after failure, clearing the error code before printing, which is incorrect.

3. Final Answer:

   Option D -> Option D

4. Quick Check:

   Use perror for simple and correct error messages [OK]

Hint: Use perror for automatic error message printing [OK]

Common Mistakes:

• Resetting errno unnecessarily
• Resetting errno after failure
• Using strerror without valid errno