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Standard library header <errno.h>

From cppreference.net
C
Standard Library headers

This header is part of the error handling library.

Error numbers

macro which expands to POSIX-compatible thread-local error number variable
(macro variable)
macros for standard POSIX-compatible error conditions
(macro constant)

Synopsis 

#define EDOM   /* implementation-defined */
#define EILSEQ /* implementation-defined */
#define ERANGE /* implementation-defined */
#define errno  /* implementation-defined */
// Only if the implementation defines __STDC_LIB_EXT1__ and additionally the user code
// defines __STDC_WANT_LIB_EXT1__ before any inclusion of <errno.h>:
#ifdef __STDC_WANT_LIB_EXT1__
#define __STDC_LIB_EXT1__  /* implementation-defined */
#define errno_t            /* implementation-defined */
#endif