errno

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

Utilities library

Type support (basic types, RTTI, type traits)

Dynamic memory management

Error handling

Program utilities

Variadic functions

Date and time

Function objects

initializer_list (C++11)

bitset

hash (C++11)

Relational operators

rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=

Pairs and tuples

pair

tuple (C++11)

piecewise_construct_t (C++11)

piecewise_construct (C++11)

Swap, forward and move

swap

forward (C++11)

move (C++11)

move_if_noexcept (C++11)

declval (C++11)

 

Error handling

Exception handling

exception

uncaught_exception

exception_ptr (C++11)

make_exception_ptr (C++11)

current_exception (C++11)

rethrow_exception (C++11)

nested_exception (C++11)

throw_with_nested (C++11)

rethrow_if_nested (C++11)

Exception handling failures

terminate

terminate_handler

get_terminate (C++11)

set_terminate

unexpected (deprecated)

bad_exception

unexpected_handler (deprecated)

get_unexpected (C++11)(deprecated)

set_unexpected (deprecated)

Exception categories

logic_error

invalid_argument

domain_error

length_error

out_of_range

runtime_error

range_error

overflow_error

underflow_error

Error codes

Error codes

errno

Assertions

assert

system_error facility

error_category (C++11)

generic_category (C++11)

system_category (C++11)

error_condition (C++11)

errc (C++11)

error_code (C++11)

system_error (C++11)

 

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

Run this code

#include <iostream>
#include <cmath>
#include <cerrno>
#include <cstring>
 
int main()
{
    double not_a_number = std::log(-1.0);
    if (errno == EDOM) {
        std::cout << "log(-1) failed: " << std::strerror(errno) << '\n';
    }
}

Output:

log(-1) failed: Numerical argument out of domain

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)