errno

errno is a preprocessor macro used for error indication. It 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.

Example

#include <iostream>
#include <cmath>
#include <cerrno>
#include <cstring>
#include <clocale>
 
int main()
{
    double not_a_number = std::log(-1.0);
    if (errno == EDOM) {
        std::cout << "log(-1) failed: " << std::strerror(errno) << '\n';
        std::setlocale(LC_MESSAGES, "de_DE.utf8");
        std::cout << "Or, in German, " << std::strerror(errno) << '\n';
    }
}

log(-1) failed: Numerical argument out of domain
Or, in German, Das numerische Argument ist ausserhalb des Definitionsbereiches

See also