perror and strerror functions - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of perror after a failed fopen?

Consider the following C code snippet. What will be printed to the standard error when fopen fails to open a non-existent file?

C

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

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

A Error opening file: File exists

B Error opening file: Permission denied

C Error opening file: Unknown error

D Error opening file: No such file or directory

💡 Hint

The file does not exist, so fopen sets errno to ENOENT.

When fopen fails because the file does not exist, errno is set to ENOENT. perror prints the string passed to it, followed by a colon, a space, and the error message corresponding to errno.

❓ Predict Output

intermediate

What does strerror return for errno 13?

What string does strerror return when errno is set to 13?

C

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

int main() {
    errno = 13;
    printf("%s\n", strerror(errno));
    return 0;
}

A Bad file descriptor

B No such file or directory

C Permission denied

D Input/output error

💡 Hint

Errno 13 corresponds to a permission problem.

Errno 13 is defined as EACCES, which means 'Permission denied'. strerror returns the string describing the error code.

🔧 Debug

advanced

Why does this perror call print an empty message?

Examine the code below. Why does perror print only a newline without any message?

C

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

int main() {
    errno = 0;
    perror("");
    return 0;
}

A Because the string passed to perror is empty, it causes a runtime error.

B Because errno is zero, perror prints only a newline with no message.

C Because perror requires a non-empty string, this code causes a segmentation fault.

D Because errno is zero, perror prints 'Success' message.

💡 Hint

Check what errno value means zero and how perror behaves with empty string.

When errno is zero, it means no error. perror prints the string passed to it, then ': ', then the error message. If the string is empty and errno is zero, it prints only a newline.

📝 Syntax

advanced

Which option correctly uses strerror to print an error message?

Select the code snippet that correctly prints the error message for the current errno using strerror.

A printf("Error: %s\n", strerror(errno));

B printf("Error: %s\n", strerror);

C printf("Error: %s\n", strerror());

D printf("Error: %s\n", strerror(errno()));

💡 Hint

strerror requires an integer error code as argument.

strerror takes an integer error code (like errno) and returns a string describing the error. Option A correctly passes errno to strerror.

🚀 Application

expert

How to safely print a custom error message with strerror in a multithreaded program?

In a multithreaded C program, which approach is safest to print a custom error message with the error string corresponding to errno?

A Use strerror_r(errno, buffer, size) to get the error string, then print it with the custom message.

B Manually map errno values to strings in a global array and print from there.

C Use strerror(errno) directly and print it with the custom message, ignoring thread safety.

D Use perror with a custom message directly, since it is thread-safe.

💡 Hint

strerror is not thread-safe, but strerror_r is designed for thread safety.

strerror is not thread-safe because it may return a pointer to a static buffer. strerror_r is a thread-safe alternative that writes the error message into a user-provided buffer.

Practice

1. What does the perror function do in C?

easy

A. Prints a custom message followed by the system error message

B. Returns a string describing an error number

C. Terminates the program immediately

D. Clears the error state of the program

Solution

1. Step 1: Understand perror purpose

   perror prints a message you provide, then adds the system error message related to the last error.

2. Step 2: Compare with other options

   strerror returns a string but does not print; program termination or clearing errors are unrelated.

3. Final Answer:

   Prints a custom message followed by the system error message -> Option A

4. Quick Check:

   perror prints message + system error [OK]

Hint: perror prints errors; strerror returns error strings [OK]

Common Mistakes:

• Confusing perror with strerror
• Thinking perror returns a string
• Assuming perror stops the program

2. Which of the following is the correct syntax to print an error message using perror with the text "File error"?

easy

A. perror(File_error);

B. perror(File error);

C. perror("File error");

D. perror('File error');

Solution

1. Step 1: Check correct string syntax in C

   Strings must be in double quotes, so "File error" is correct.

2. Step 2: Validate perror usage

   perror takes a const char* string, so perror("File error") is correct syntax.

3. Final Answer:

   perror("File error"); -> Option C

4. Quick Check:

   Use double quotes for strings in perror [OK]

Hint: Use double quotes for strings in perror [OK]

Common Mistakes:

• Using single quotes instead of double quotes
• Passing variable names without quotes
• Missing parentheses in function call

3. What will be the output of the following C code snippet if errno is set to ENOENT (No such file or directory)?

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

int main() {
    printf("Error: %s\n", strerror(errno));
    return 0;
}

medium

A. Error: strerror function not found

B. Error: Permission denied

C. Error: Unknown error

D. Error: No such file or directory

Solution

1. Step 1: Understand strerror(errno)

   strerror returns a string describing the error number stored in errno.

2. Step 2: Match errno value to message

   ENOENT means "No such file or directory", so strerror(errno) returns that message.

3. Final Answer:

   Error: No such file or directory -> Option D

4. Quick Check:

   strerror(ENOENT) = "No such file or directory" [OK]

Hint: strerror(errno) shows error message for current error [OK]

Common Mistakes:

• Confusing error messages for different errno values
• Assuming strerror prints instead of returns string
• Not including header

4. Identify the error in this code snippet that tries to print an error message:

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

int main() {
    int errnum = ENOENT;
    printf("Error: %s\n", perror(errnum));
    return 0;
}

medium

A. Using perror inside printf is incorrect; perror prints directly and returns void

B. Missing header

C. Incorrect error number assigned to errnum

D. printf format specifier %s should be %d

Solution

1. Step 1: Check perror return type

   perror prints directly to stderr and returns void; it does not return a string.

2. Step 2: Analyze usage inside printf

   Passing perror(errnum) inside printf is invalid because perror returns void, not a string.

3. Final Answer:

   Using perror inside printf is incorrect; perror prints directly and returns void -> Option A

4. Quick Check:

   perror prints itself, don't use inside printf [OK]

Hint: perror prints error; don't use inside printf [OK]

Common Mistakes:

• Using perror inside printf expecting a string
• Confusing perror and strerror return types
• Ignoring compiler warnings about void return

5. You want to write a function that opens a file and prints a clear error message if it fails. Which approach correctly uses perror and strerror together to achieve this?

FILE *open_file(const char *filename) {
    FILE *f = fopen(filename, "r");
    if (!f) {
        // What to do here?
    }
    return f;
}

hard

A. Call perror(filename) without message, then return NULL

B. Call perror("Failed to open file") to print error, then return NULL

C. Print printf("Error: %s\n", strerror(errno)) only, then return NULL

D. Return NULL without printing any error message

Solution

1. Step 1: Understand error reporting with perror

   perror prints your message plus the system error, making it clear why fopen failed.

2. Step 2: Compare options for clarity

   Call perror("Failed to open file") to print error, then return NULL uses perror("Failed to open file"), which is clear and standard. Print printf("Error: %s\n", strerror(errno)) only, then return NULL prints only strerror message without context. Call perror(filename) without message, then return NULL prints filename only, which is less clear. Return NULL without printing any error message prints nothing.

3. Final Answer:

   Call perror("Failed to open file") to print error, then return NULL -> Option B

4. Quick Check:

   Use perror with custom message for clear errors [OK]

Hint: Use perror with custom text for clear error messages [OK]

Common Mistakes:

• Printing strerror without context message
• Using perror with filename only, confusing output
• Not printing any error message on failure