How to Use errno in C: Simple Guide with Examples

In C, errno is a global variable set by system calls and library functions when an error occurs. To use it, include <errno.h>, check if a function signals an error, then read errno to find the error code and use perror() or strerror() to get a readable message.

Syntax

errno is a global integer variable defined in <errno.h>. It is set by system calls and some library functions when an error happens. You should check the function's return value first, then read errno if an error is indicated.

Common usage:

• Include <errno.h>
• Call a function that may fail
• If failure detected, read errno
• Use perror() or strerror(errno) to print error message

c

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

// Example usage pattern
int result = some_function();
if (result == -1) {  // check error condition
    int err = errno;  // save errno immediately
    perror("Error message");  // print error description
    // or use strerror(err) to get string
}

Example

This example tries to open a file that does not exist. When fopen() fails, it sets errno. We print the error message using perror().

c

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

int main() {
    FILE *file = fopen("nonexistent.txt", "r");
    if (file == NULL) {
        perror("Failed to open file");
        return 1;
    }
    fclose(file);
    return 0;
}

Output

Failed to open file: No such file or directory

Common Pitfalls

• Not checking function return values: errno is only meaningful if the function signals an error (like returning NULL or -1).
• Using errno without resetting: errno is not cleared automatically, so old errors may remain if you don't reset it before a call.
• Reading errno after another function call: Always save errno immediately after the error, because other calls may change it.

c

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

int main() {
    errno = 0; // reset errno before call
    FILE *file = fopen("missing.txt", "r");
    if (file == NULL) {
        int err = errno; // save errno immediately
        // Some other function call here could change errno
        perror("Error opening file");
        printf("Errno saved: %d\n", err);
    }
    return 0;
}

Output

Error opening file: No such file or directory
Errno saved: 2

Quick Reference

Remember these key points when using errno:

• Include <errno.h> to access errno.
• Check function return values before reading errno.
• Reset errno to 0 before calls if you want to detect new errors.
• Use perror() or strerror(errno) to get readable error messages.
• Save errno immediately after an error to avoid overwriting.

Key Takeaways

Always check a function's return value before using errno to detect errors.

Include and use perror() or strerror(errno) to print error messages.

Reset errno to 0 before a call if you want to detect new errors reliably.

Save errno immediately after an error to prevent it from being overwritten.

errno is set only when an error occurs; it is not cleared automatically.