Error handling

< c

Error numbers

Defined in header <errno.h>
macro which expands to POSIX-compatible thread-local error number variable
(macro variable)
macros for standard POSIX-compatible error conditions
(macro constant)


Defined in header <assert.h>
aborts the program if the user-specified condition is not true. May be disabled for release builds
(function macro)
issues a compile-time diagnostic if the value of a constant expression is false
(keyword macro)

Bounds checking

The standard library provides bounds-checked versions of some existing functions (gets_s, fopen_s, printf_s, strcpy_s, wcscpy_s, mbstowcs_s, qsort_s, getenv_s, etc). This functionality is optional and is only available if __STDC_LIB_EXT1__ is defined. The following macros and functions support this functionality.

Defined in header <errno.h>
Defined in header <stdio.h>
a typedef for the type int, used to self-document functions that return errno values
Defined in header <stddef.h>
Defined in header <stdio.h>
Defined in header <stdlib.h>
Defined in header <string.h>
Defined in header <time.h>
Defined in header <wchar.h>
a typedef for the same type as size_t, used to self-document functions that range-check their parameters at runtime
Defined in header <stdint.h>
largest acceptable size for bounds-checked functions, expands to either constant or variable which may change at runtime (e.g. as the currently allocated memory size changes)
(macro variable)
Defined in header <stdlib.h>
set the error callback for bounds-checked functions
abort callback for the bounds-checked functions
ignore callback for the bounds-checked functions

Note: implementations of bounds-checked functions are available as open-source libraries Safe C and Slibc, and as part of Watcom C. There is also an incompatible set of bounds-checked functions available in Visual Studio.

(since C11)


  • C11 standard (ISO/IEC 9899:2011):
  • 7.2 Diagnostics <assert.h> (p: 186-187)
  • 7.5 Errors <errno.h> (p: 205)
  • 7.19 Common definitions <stddef.h> (p: 288)
  • 7.20 Integer types <stdint.h> (p: 289-295)
  • 7.21 Input/output <stdio.h> (p: 296-339)
  • 7.22 General utilities <stdlib.h> (p: 340-360)
  • K.3.1.3 Use of errno (p: 584)
  • K.3.2/2 errno_t (p: 585)
  • K.3.3/2 rsize_t (p: 585)
  • K.3.4/2 RSIZE_MAX (p: 585)
  • 7.31.3 Errors <errno.h> (p: 455)
  • 7.31.10 Integer types <stdint.h> (p: 456)
  • 7.31.11 Input/output <stdio.h> (p: 456)
  • 7.31.12 General utilities <stdlib.h> (p: 456)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.2 Diagnostics <assert.h> (p: 169)
  • 7.5 Errors <errno.h> (p: 186)
  • 7.26.3 Errors <errno.h> (p: 401)
  • 7.26.8 Integer types <stdint.h> (p: 401)
  • 7.26.9 Input/output <stdio.h> (p: 402)
  • 7.26.10 General utilities <stdlib.h> (p: 402)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 4.2 DIAGNOSTICS <assert.h>
  • 4.1.3 Errors <errno.h>
  • 4.13.1 Errors <errno.h>
  • 4.13.6 Input/output <stdio.h>
  • 4.13.7 General utilities <stdlib.h>

See also

defines the error handling mechanism used by the common mathematical functions
(macro constant)