errno

From cppreference.com

Defined in header `<cerrno>`

#define errno /implementation-defined/

errno is a preprocessor macro that expands to a static(until C++11) / thread-local(since C++11) modifiable lvalue of type int. Several standard library functions indicate errors by writing positive integers to errno. Typically, the value of errno is set to one of the error codes, listed in <cerrno> as macro constants that begin with the letter E, followed by uppercase letters or digits.

The value of errno is 0 at program startup, and although library functions are allowed to write positive integers to errno whether or not an error occurred, library functions never store 0 in errno.

[edit] Example

run this code

#include <iostream>
#include <cmath>
#include <cerrno>
#include <cstring>
 
int main()
{
    double not_a_number = std::log(-1.0);
    if (errno == EDOM) {
        std::cout << "log(-1) failed: " << std::strerror(errno) << '\n';
    }
}

Output:

log(-1) failed: Numerical argument out of domain

[edit] See also


E2BIG, EACCES, ..., EXDEV	macros for standard POSIX-compatible error conditions (macro constant)

perror	displays a character string corresponding of the current error to stderr (function)

strerror	returns a text version of a given error code (function)

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)