errno

 

C

Language

headers

Type support

Dynamic memory management

Error handling

Program utilities

Variadic function support

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Thread support (C11)

Atomic operations (C11)

Technical Specifications

 

Error handling

Error codes

Error codes

errno

Assertions

assert

static_assert (C11)

Bounds checking

set_constraint_handler_s (C11)

abort_handler_s (C11)

ignore_handler_s (C11)

 

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

errno is a preprocessor macro that expands to a thread-local (since C11) 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 <errno.h> as macro constants beginning 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.

Library functions perror and strerror can be used to obtain textual descriptions of the error conditions that correspond to the current errno value.