errno

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

errno is a preprocessor macro used for error indication.

It expands to a static modifiable lvalue of type `int`.	(until C++11)
It expands to a thread-local modifiable lvalue of type `int`.	(since C++11)

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';
    }
}

Possible output:

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

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)

errno

Example

See also