# errno

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< cpp‎ | error

Defined in header `<cerrno>` |  |
#define errno /*implementation-defined*/ |  |

`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

Run this code

    #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';
        }
    }

Possible output:

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

### 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)

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