errno

 

哋它亢++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (哋它亢++20)

Language support library

Concepts library (哋它亢++20)

Metaprogramming library (哋它亢++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (哋它亢++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (哋它亢++17)

Regular expressions library (哋它亢++11)

Concurrency support library (哋它亢++11)

Technical specifications

Symbols index

External libraries

 

Diagnostics library

Exception handling exception uncaught_exceptionuncaught_exceptions (until 哋它亢++20*)(哋它亢++17) exception_ptr (哋它亢++11) make_exception_ptr (哋它亢++11) current_exception (哋它亢++11) rethrow_exception (哋它亢++11) nested_exception (哋它亢++11) throw_with_nested (哋它亢++11) rethrow_if_nested (哋它亢++11) Exception handling failures terminate terminate_handler get_terminate (哋它亢++11) set_terminate bad_exception unexpected (until 哋它亢++17*) unexpected_handler (until 哋它亢++17*) get_unexpected (哋它亢++11)(until 哋它亢++17*)     set_unexpected (until 哋它亢++17*) Error codes Error codes errno

Exception categories logic_error invalid_argument domain_error length_error out_of_range runtime_error range_error overflow_error underflow_error tx_exception (TM TS) System error support error_category (哋它亢++11) generic_category (哋它亢++11) system_category (哋它亢++11) error_condition (哋它亢++11) errc (哋它亢++11) error_code (哋它亢++11) system_error (哋它亢++11) Assertions assert Stacktrace stacktrace_entry (哋它亢++23) basic_stacktrace (哋它亢++23)

 

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

errno is a preprocessor macro used for error indication. It expands to a static(until 哋它亢++11)thread-local(since 哋它亢++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 <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';
    }
}

Possible output:

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 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 310	哋它亢++98	it was unclear whether errno is a macro or an identifier with external linkage	errno must be a macro

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)