Error numbers

< c‎ | error

Each of the macros defined in <errno.h> expands to an integer constant expression with type int and with a unique positive value. The following constants are defined by ISO C. The implementation may define more, as long as they begin with 'E' followed by digits or uppercase letters.

Defined in header <errno.h>
Mathematics argument out of domain of function
(macro constant)
Illegal byte sequence
(macro constant)
Result too large
(macro constant)


Many additional errno constants are defined by POSIX and by the C++ standard library, and individual implementations may define even more, e.g. errno(3) on Linux or intro(2) on BSD and OS X.


#include <stdio.h>
#include <math.h>
#include <errno.h>
#include <string.h>
int main(void)
    errno = 0;
    printf("log(-1.0) = %f\n", log(-1.0));
    errno = 0;
    printf("log(0.0)  = %f\n", log(0.0));

Possible output:

log(-1.0) = nan
Numerical argument out of domain
log(0.0)  = -inf
Numerical result out of range


  • C11 standard (ISO/IEC 9899:2011):
  • 7.5/2 Errors <errno.h> (p: 205)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.5/2 Errors <errno.h> (p: 186)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 4.1.3 Errors <errno.h>

See also

macro which expands to POSIX-compatible thread-local error number variable
(macro variable)
displays a character string corresponding of the current error to stderr
returns a text version of a given error code
C++ documentation for Error numbers