How to use errno in C++? - Coding

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

Reffered: https://www.geeksforgeeks.org

C++

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