❗ C Error Handling – Using errno, perror, and strerror Effectively

🧲 Introduction – How Is Error Handling Done in C?

C programming uses a minimal but flexible error-handling model based on global variables and diagnostic functions. Since C lacks exceptions (like in C++ or Java), errors from standard library functions must be manually checked, interpreted, and reported using tools like errno, perror(), and strerror().

🎯 In this guide, you’ll learn:

• How to detect and interpret runtime errors
• The role of errno, perror(), and strerror()
• Real-world use cases and practical examples
• Best practices for robust and clear error reporting

🔍 What Is errno in C?

errno is a global integer variable defined in <errno.h> that gets set by many standard library functions when an error occurs. It holds an error code that you can convert into a human-readable string using strerror() or display using perror().

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

🧠 Always check the function’s return value before accessing errno.

💡 Common Functions That Set errno

🖨️ perror() – Print Descriptive Error Message

perror("message") prints the custom message, followed by a colon and system error description based on errno.

✅ Example:

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

🖨️ Output:

Error opening file: No such file or directory

🧾 strerror() – Get Error Message String

strerror(errno) returns a string corresponding to the current error code.

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

This method is useful when you want custom formatting or logging.

💻 Complete Example

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

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

📚 Real-World Use Cases

💡 Best Practices & Tips

📘 Best Practice:
Always check a function’s return value before checking errno, as errno may hold a stale error code.

💡 Tip:
Use errno, perror(), and strerror() in combination for flexible and developer-friendly error output.

⚠️ Pitfall:
Never assume errno == 0 means success—many functions only set errno on failure, and not all failures update it.

📌 Summary – Recap & Next Steps

C’s error handling model is simple but powerful when used properly. By using errno in tandem with perror() and strerror(), you can detect, diagnose, and report errors gracefully.

🔍 Key Takeaways:

• Use errno to capture error codes from failed functions
• Use perror() to print a system-generated message with context
• Use strerror(errno) to retrieve error message strings for logging
• Always validate the return value before checking errno

⚙️ Real-World Relevance:

Crucial in file I/O, memory allocation, system calls, and robust C applications requiring detailed failure reporting.

❓ Frequently Asked Questions (FAQ)

❓ What is errno used for?

✅ It holds the error code when a library/system call fails, helping identify what went wrong.

❓ What does perror() do?

✅ It prints a message followed by the textual representation of errno.

❓ Should I reset errno before a function call?

✅ It’s not required but good practice in some cases to avoid checking a stale value.

❓ What is the difference between perror() and strerror()?

✅ perror() prints to stderr automatically.
strerror() returns a message string that you can format and log manually.

❓ Where is errno defined?

✅ In the <errno.h> header.