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.