Error handling

Error numbers

Defined in header `<errno.h>`
errno	macro which expands to POSIX-compatible thread-local error number variable (macro variable)
E2BIG, EACCES, ..., EXDEV	macros for standard POSIX-compatible error conditions (macro constant)

Assertions

Defined in header `<assert.h>`
assert	aborts the program if the user-specified condition is not `true`. May be disabled for release builds (function macro)
static_assert (C11)(removed in C23)	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>`
errno_t (C11)	a typedef for the type int, used to self-document functions that return `errno` values (typedef)
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>`
rsize_t (C11)	a typedef for the same type as `size_t`, used to self-document functions that range-check their parameters at runtime (typedef)
Defined in header `<stdint.h>`
RSIZE_MAX (C11)	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_constraint_handler_s (C11)	set the error callback for bounds-checked functions (function)
abort_handler_s (C11)	abort callback for the bounds-checked functions (function)
ignore_handler_s (C11)	ignore callback for the bounds-checked functions (function)

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)

Notes

Since C23, static_assert is itself a keyword, which may also be a predefined macro, so <assert.h> no longer provides it.

References

Extended content
C23 standard (ISO/IEC 9899:2024): 7.2 Diagnostics <assert.h> (p: TBD) 7.5 Errors <errno.h> (p: TBD) 7.19 Common definitions <stddef.h> (p: TBD) 7.20 Integer types <stdint.h> (p: TBD) 7.21 Input/output <stdio.h> (p: TBD) 7.22 General utilities <stdlib.h> (p: TBD) K.3.1.3 Use of errno (p: TBD) K.3.2/2 errno_t (p: TBD) K.3.3/2 rsize_t (p: TBD) K.3.4/2 RSIZE_MAX (p: TBD) 7.31.3 Errors <errno.h> (p: TBD) 7.31.10 Integer types <stdint.h> (p: TBD) 7.31.11 Input/output <stdio.h> (p: TBD) 7.31.12 General utilities <stdlib.h> (p: TBD) C17 standard (ISO/IEC 9899:2018): 7.2 Diagnostics <assert.h> (p: TBD) 7.5 Errors <errno.h> (p: TBD) 7.19 Common definitions <stddef.h> (p: TBD) 7.20 Integer types <stdint.h> (p: TBD) 7.21 Input/output <stdio.h> (p: TBD) 7.22 General utilities <stdlib.h> (p: TBD) K.3.1.3 Use of errno (p: TBD) K.3.2/2 errno_t (p: TBD) K.3.3/2 rsize_t (p: TBD) K.3.4/2 RSIZE_MAX (p: TBD) 7.31.3 Errors <errno.h> (p: TBD) 7.31.10 Integer types <stdint.h> (p: TBD) 7.31.11 Input/output <stdio.h> (p: TBD) 7.31.12 General utilities <stdlib.h> (p: TBD) 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>

Extended content

C23 standard (ISO/IEC 9899:2024):

7.2 Diagnostics <assert.h> (p: TBD)
7.5 Errors <errno.h> (p: TBD)
7.19 Common definitions <stddef.h> (p: TBD)
7.20 Integer types <stdint.h> (p: TBD)
7.21 Input/output <stdio.h> (p: TBD)
7.22 General utilities <stdlib.h> (p: TBD)
K.3.1.3 Use of errno (p: TBD)
K.3.2/2 errno_t (p: TBD)
K.3.3/2 rsize_t (p: TBD)
K.3.4/2 RSIZE_MAX (p: TBD)
7.31.3 Errors <errno.h> (p: TBD)
7.31.10 Integer types <stdint.h> (p: TBD)
7.31.11 Input/output <stdio.h> (p: TBD)
7.31.12 General utilities <stdlib.h> (p: TBD)

C17 standard (ISO/IEC 9899:2018):

7.2 Diagnostics <assert.h> (p: TBD)
7.5 Errors <errno.h> (p: TBD)
7.19 Common definitions <stddef.h> (p: TBD)
7.20 Integer types <stdint.h> (p: TBD)
7.21 Input/output <stdio.h> (p: TBD)
7.22 General utilities <stdlib.h> (p: TBD)
K.3.1.3 Use of errno (p: TBD)
K.3.2/2 errno_t (p: TBD)
K.3.3/2 rsize_t (p: TBD)
K.3.4/2 RSIZE_MAX (p: TBD)
7.31.3 Errors <errno.h> (p: TBD)
7.31.10 Integer types <stdint.h> (p: TBD)
7.31.11 Input/output <stdio.h> (p: TBD)
7.31.12 General utilities <stdlib.h> (p: TBD)

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>