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 <cerrno>
#include <clocale>
#include <cmath>
#include <cstring>
#include <iostream>
 
int main()
{
    const double not_a_number = std::log(-1.0);
    std::cout << not_a_number << '\n';
 
    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';
    }
}

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

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also