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.