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Defined in header <cerrno> | ||
#define errno /* implementation-defined */ | ||
errno is a preprocessor macro used for error indication. It expands to astatic(until C++11)thread-local(since C++11) modifiable lvalue of typeint.
Several standard library functions indicate errors by writing positive integers toerrno. Typically, the value oferrno is set to one of the error codes, listed in<cerrno> as macro constants that begin with the letterE, followed by uppercase letters or digits.
The value oferrno is0 at program startup, and although library functions are allowed to write positive integers toerrno whether or not an error occurred, library functions never store0 inerrno.
#include <cerrno>#include <clocale>#include <cmath>#include <cstring>#include <iostream> int main(){constdouble 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:
nanlog(-1) failed: Numerical argument out of domainOr, in German, Das numerische Argument ist ausserhalb des Definitionsbereiches
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 310 | C++98 | it was unclear whethererrno is a macro or an identifier with external linkage | errno must be a macro |
| macros for standard POSIX-compatible error conditions (macro constant)[edit] | |
| displays a character string corresponding of the current error tostderr (function)[edit] | |
| returns a text version of a given error code (function)[edit] | |
C documentation forerrno | |
