How to use errno in C++?

In C++ errno is a preprocessor macro that is used for error indication and reporting errors that occur during a function call. It contains error codes so if a call to a function fails somehow then the errno is set to a value that corresponds to the error.

errno is defined in the header file <cerrno> in C++.

Working of `errno` in C++

The errno works as the explain below:

Initially at the beginning of the program, the errno is set to zero and it changes when any error occurs.
If a function call fails it sets errno, then we check its value. There is no change in the value errno if no errors occur.

Examples of errorno in C++

The following examples illustrate how we can use errno in different scenarios.

Example 1

The below example demonstrates the usage of errno for reading a file because when we try to open a file it may throw an error.

C++

// C++ program to demonstrate the use of errno for reading a
// file
#include <cerrno>
#include <cstring>
#include <fstream>
#include <iostream>
using namespace std;

int main()
{
    // Defining the filename to be opened.
    const char* filename = "myfile.txt";

    // Attempting to open the file using ifstream.
    ifstream file(filename);

    // Checking if the file was not opened successfully or
    // not.
    if (!file) {
        // Switch case based on the error number (errno).
        switch (errno) {
        case ENOENT: // Case for file not existing.
            cerr << "Error: File '" << filename
                 << "' doesn't exist." << endl;
            break;
        case EACCES: // Case for access denied.
            cerr << "Error: Permission denied for file '"
                 << filename << "'." << endl;
            break;
        default: // Default case for other errors.
            cerr << "Error opening file '" << filename
                 << "': " << strerror(errno) << endl;
            break;
        }
        // Return with error code 1 to indicate failure.
        return 1;
    }

    // Closing the file if it was opened successfully.
    file.close();

    // Return 0 to indicate successful completion.
    return 0;
}

Output

Error: File 'myfile.txt' doesn't exist.

Explanation: The above example shows how to handle error that we get when we try to open a file using ifstream. It checks for conditions that may give error like file not existing or access being denied by using the errno variable.

Note: Always clear errno after handling errors and use errno=0 to reset the value after handling an error.

Example 2

The below example demonstrates the use of errno to detect error in mathematical operations.

C++

// C++ program to demonstrate the use of errno to detect
// error in mathematical operations.
#include <cerrno>
#include <cmath>
#include <iostream>
using namespace std;

int main()
{
    // Variable initialization with a value that will cause
    // a domain error in sqrt
    double x = -1.0;

    // Attempt to calculate the square root of x
    double result = sqrt(x);

    // Check if errno is set to EDOM, indicating a domain
    // error in the sqrt function
    if (errno == EDOM) {
        // Print an error message to stderr and return with
        // error code 1
        cerr << "Error: Cannot take the square root "
                "of a negative number."
             << endl;
        return 1;
    }

    // If no error, print the result
    cout << "Square root of " << x << " is: " << result
         << endl;

    // Return 0 indicating success
    return 0;
}

Output

Error: Cannot take the square root of a negative number.

Explanation: The above example checks for EDOM after taking the mathematical values Error. When we try to calculate the square root of a negative number like (-2), that are actually not defined and leads to a domain error.

Error Codes Associated with errorno in C++

There are lots of codes associated with errorno in C++ but some of the commonly used error codes are listed in the below table:

errorno Code	Associated Macro	Description
1	EPERM	Operation not permitted
2	ENOENT	No such file or directory
3	ESRCH	No such process
5	EIO	I/O error
7	E2BIG	Argument list too long
9	EBADF	Bad file number
10	ECHILD	No child processes
11	EAGAIN	Try again
12	ENOMEM	Out of memory
13	EACCES	Permission denied