不要在C程序里用的标识符 |
| If you want to write a portable C program, you have to be careful not
to give your own definitions to any of the identifiers that are reserved
by the C standard. The standard tells you which identifiers are reserved,
but scatters the information through a rather thick (and expensive) book.
The basic principles of reserved identifiers are in ISO subclause 7.1.3 (ANSI section 4.1.2.1), "Reserved Identifiers". There you are warned: if a program declares or defines an identifier with the same name as an identifier reserved in that context .., the behavior is undefined.That means that if your program contains a statement like "extern int log;", the compiler is fully justified in turning your terminal into a large wart hog. Yes, the standard allows (3.16/1.6) such behavior, though market forces (and the laws of physics!) might not support it. More realistically, your program may or may not work right, and you may or may not get a diagnostic message. This page is intended mostly for my fellow C compiler users. Compiler writers have a related but different set of concerns. |
| These are two different languages. But C++ has its historical roots
in C, and many C programs (and C programmers!) get converted to C++. The
namespace
feature of C++ makes it very much easier for programmers to avoid identifier
collisions -- except for the C++ keywords, which are a superset of the
standard C keywords.
The C++ standard is written so that a C++ compiler can compile almost any correct C program and produce equivalent code. To keep your program from being one of the unlucky exceptions, you might want to avoid using the C++ keywords as identifiers. I've listed them in their alphabetical places in the table. Reference: Bjarne Stroustrup, The C++ Programming Language, Third Edition, section B.2.2. |
| This page reflects the first ISO/ANSI standard, finalized in 1989 ("C89"),
with subsequent corrections and addenda. It does not yet reflect the new
1999 standard ("C99").Realistically, I doubt I'll
be updating it for C99 any time soon. C99 programmers should consult the
standard or another good reference for the reserved identifiers added by
the new standard.
When this page refers to "the standard" or "standard C", it takes several documents into account, as listed below. None of them is available on the Web; they're sold by national standards bodies but at prices I can no longer afford. (For more information on how to acquire a copy of the standard, see Steve Summit's comp.lang.c FAQ.)
|
| The ISO and ANSI standards are substantively the same, but sections
are numbered differently. Specifically, ANSI sections 2 through 4 correspond
pretty closely (but not exactly) to ISO clauses 5 through 7.
The table of reserved identifiers below shows the ISO subclause and ANSI section numbers. In text, I show both numbers with the ISO number first, like this: 7.1.3/4.1.2.1. |
There are several kinds of safety to be considered:
My own preference is for maximum safety: I try never to use any identifier for my own purposes if it's in the C standard. This appeals to my simple mind because it's simple! <grin> But those context notations can be useful even so: if a program is misbehaving and you suspect a standard identifier has been used in a non-standard way, you can make sure by checking the context in the table. You need to be aware of some surprising consequences of the standard: two identifiers can be spelled the same yet be distinct, or they can be spelled differently yet be the same, as I'll discuss next. |
| (If you're going for maximum safety, just skip
to the next section.)
Yes, it's possible to have two different identifiers that are spelled exactly the same. For instance, struct ceil doesn't conflict with the ceil( ) function, even if you've included the math.h header that declares it. And extern int grouping doesn't conflict with grouping, the member of struct lconv defined in locale.h. In fact, you can legally define a different structure of your own with a member called grouping. (It may be confusing to humans, but it's legal.) There's no conflict because the identifiers in those examples are in different name spaces. The standard lists (6.1.2.3/3.1.2.3) these separate name spaces:
Macro names have their own name space, according to ISO 6.8.3/ANSI 3.8.3: "There is one name space for macro names." But of course you must not create a conflict between a macro and an ordinary name. |
Respect that first entry in the table below: never
make up any identifier that starts with an underscore.
Actually, you can legally use an identifier that starts with an underscore if the second character is a lower-case letter or a digit, and the identifier is used inside a function or a function prototype or as a structure member or label. Easier just not to use leading underscores!There are various other table entries with "[any]". For instance, no matter which headers you use, you must never create a non-static function or global variable that starts with the letters is, mem, str, to, or wcs. Additional classes of identifiers are reserved if you include particular headers, as indicated in the table. |
Here's an explanation of the columns of the table:
_ | A | B | C | D | E | F | G-H | I-J | L | M | N-O | P-Q | R | S | T | U | V | W-X |
identifier context header ISO ANSI _[any] (see Groups) 7.1.3 4.1.2.1 _[_A-Z][any] all 7.1.3 4.1.2.1 __DATE__ all (predef) 6.8.8 3.8.8 __FILE__ all (predef) 6.8.8 3.8.8 __LINE__ all (predef) 6.8.8 3.8.8 __STDC__ all (predef) 6.8.8 3.8.8 __STDC_VERSION__ all (predef) NA1 __TIME__ all (predef) 6.8.8 3.8.8 _IOFBF H stdio.h 7.9.1 4.9.1 _IOLBF H stdio.h 7.9.1 4.9.1 _IONBF H stdio.h 7.9.1 4.9.1 |
identifier context header ISO ANSI abort ext stdlib.h 7.10.4.1 4.10.4.1 abs ext stdlib.h 7.10.6.1 4.10.6.1 acos ext math.h 7.5.2.1 4.5.2.1 acosf, acosl ext math.h 7.13.4 4.13.4 and H/C++ iso646.h NA1 and_eq H/C++ iso646.h NA1 asctime ext time.h 7.12.3.1 4.12.3.1 asin ext math.h 7.5.2.2 4.5.2.2 asinf, asinl ext math.h 7.13.4 4.13.4 asm C++ only assert H assert.h 7.2 4.2 atan ext math.h 7.5.2.3 4.5.2.3 atan2 ext math.h 7.5.2.4 4.5.2.4 atan2f, atan2l ext math.h 7.13.4 4.13.4 atanf, atanl ext math.h 7.13.4 4.13.4 atexit ext stdlib.h 7.10.4.2 4.10.4.2 atof ext stdlib.h 7.10.1.1 4.10.1.1 atoi ext stdlib.h 7.10.1.2 4.10.1.2 atol ext stdlib.h 7.10.1.3 4.10.1.3 auto all (keyword) 6.1.1 3.1.1 |
identifier context header ISO ANSI bitand H/C++ iso646.h NA1 bitor H/C++ iso646.h NA1 bool C++ only break all (keyword) 6.1.1 3.1.1 bsearch ext stdlib.h 7.10.5.1 4.10.5.1 btowc extW wchar.h NA1 BUFSIZ H stdio.h 7.9.1 4.9.1 |
identifier context header ISO ANSI calloc ext stdlib.h 7.10.3.1 4.10.3.1 case all (keyword) 6.1.1 3.1.1 catch C++ only ceil ext math.h 7.5.6.1 4.5.6.1 ceilf, ceill ext math.h 7.13.4 4.13.4 char all (keyword) 6.1.1 3.1.1 CHAR_BIT H limits.h 5.2.4.2.1 2.2.4.2.1 CHAR_MAX H limits.h 5.2.4.2.1 2.2.4.2.1 CHAR_MIN H limits.h 5.2.4.2.1 2.2.4.2.1 class C++ only clearerr ext stdio.h 7.9.10.1 4.9.10.1 clock ext time.h 7.12.2.1 4.12.2.1 clock_t (type) H time.h 7.12.1 4.12.1 CLOCKS_PER_SEC H time.h 7.12.1 4.12.1 compl H/C++ iso646.h NA1 const all (keyword) 6.1.1 3.1.1 const_cast C++ only continue all (keyword) 6.1.1 3.1.1 cos ext math.h 7.5.2.5 4.5.2.5 cosf ext math.h 7.13.4 4.13.4 cosh ext math.h 7.5.3.1 4.5.3.1 coshf, coshl ext math.h 7.13.4 4.13.4 cosl ext math.h 7.13.4 4.13.4 ctime ext time.h 7.12.3.2 4.12.3.2 currency_symbol Hmem locale.h 7.4 4.4 |
identifier context header ISO ANSI DBL_DIG H float.h 5.2.4.2.2 2.2.4.2.2 DBL_EPSILON H float.h 5.2.4.2.2 2.2.4.2.2 DBL_MANT_DIG H float.h 5.2.4.2.2 2.2.4.2.2 DBL_MAX H float.h 5.2.4.2.2 2.2.4.2.2 DBL_MAX_10_EXP H float.h 5.2.4.2.2 2.2.4.2.2 DBL_MAX_EXP H float.h 5.2.4.2.2 2.2.4.2.2 DBL_MIN H float.h 5.2.4.2.2 2.2.4.2.2 DBL_MIN_10_EXP H float.h 5.2.4.2.2 2.2.4.2.2 DBL_MIN_EXP H float.h 5.2.4.2.2 2.2.4.2.2 decimal_point Hmem locale.h 7.4 4.4 default all (keyword) 6.1.1 3.1.1 defined all (predef) 6.8.8 3.8.8 delete C++ only difftime ext time.h 7.12.2.2 4.12.2.2 div ext stdlib.h 7.10.6.2 4.10.6.2 div_t (type) H stdlib.h 7.10 4.10 do all (keyword) 6.1.1 3.1.1 double all (keyword) 6.1.1 3.1.1 dynamic_cast C++ only |
identifier context header ISO ANSI E[0-9A-Z][any] H errno.h 7.13.1 4.13.1 EDOM H errno.h 7.1.4 4.1.3 EILSEQ H errno.h NA1 else all (keyword) 6.1.1 3.1.1 enum all (keyword) 6.1.1 3.1.1 EOF H stdio.h 7.9.1 4.9.1 ERANGE H errno.h 7.1.4 4.1.3 errno H errno.h 7.1.4 4.1.3 exit ext stdlib.h 7.10.4.3 4.10.4.3 EXIT_FAILURE H stdlib.h 7.10 4.10 EXIT_SUCCESS H stdlib.h 7.10 4.10 exp ext math.h 7.5.4.1 4.5.4.1 expf, expl ext math.h 7.13.4 4.13.4 explicit C++ only export C++ only extern all (keyword) 6.1.1 3.1.1 |
identifier context header ISO ANSI fabs ext math.h 7.5.6.2 4.5.6.2 fabsf, fabsl ext math.h 7.13.4 4.13.4 false C++ only fclose ext stdio.h 7.9.5.1 4.9.5.1 feof ext stdio.h 7.9.10.2 4.9.10.2 ferror ext stdio.h 7.9.10.3 4.9.10.3 fflush ext stdio.h 7.9.5.2 4.9.5.2 fgetc ext stdio.h 7.9.7.1 4.9.7.1 fgetpos ext stdio.h 7.9.9.1 4.9.9.1 fgets ext stdio.h 7.9.7.2 4.9.7.2 fgetwc extW wchar.h NA1 fgetws extW wchar.h NA1 FILE (type) H stdio.h 7.9.1 4.9.1 FILENAME_MAX H stdio.h 7.9.1 4.9.1 float all (keyword) 6.1.1 3.1.1 floor ext math.h 7.5.6.3 4.5.6.3 floorf, floorl ext math.h 7.13.4 4.13.4 FLT_DIG H float.h 5.2.4.2.2 2.2.4.2.2 FLT_EPSILON H float.h 5.2.4.2.2 2.2.4.2.2 FLT_MANT_DIG H float.h 5.2.4.2.2 2.2.4.2.2 FLT_MAX H float.h 5.2.4.2.2 2.2.4.2.2 FLT_MAX_10_EXP H float.h 5.2.4.2.2 2.2.4.2.2 FLT_MAX_EXP H float.h 5.2.4.2.2 2.2.4.2.2 FLT_MIN H float.h 5.2.4.2.2 2.2.4.2.2 FLT_MIN_10_EXP H float.h 5.2.4.2.2 2.2.4.2.2 FLT_MIN_EXP H float.h 5.2.4.2.2 2.2.4.2.2 FLT_RADIX H float.h 5.2.4.2.2 2.2.4.2.2 FLT_ROUNDS H float.h 5.2.4.2.2 2.2.4.2.2 fmod ext math.h 7.5.6.4 4.5.6.4 fmodf, fmodl ext math.h 7.13.4 4.13.4 fopen ext stdio.h 7.9.5.3 4.9.5.3 FOPEN_MAX H stdio.h 7.9.1 4.9.1 for all (keyword) 6.1.1 3.1.1 fpos_t (type) H stdio.h 7.9.1 4.9.1 fprintf ext stdio.h 7.9.6.1 4.9.6.1 fputc ext stdio.h 7.9.7.3 4.9.7.3 fputs ext stdio.h 7.9.7.4 4.9.7.4 fputwc extW wchar.h NA1 fputws extW wchar.h NA1 frac_digits Hmem locale.h 7.4 4.4 fread ext stdio.h 7.9.8.1 4.9.8.1 free ext stdlib.h 7.10.3.2 4.10.3.2 freopen ext stdio.h 7.9.5.4 4.9.5.4 frexp ext math.h 7.5.4.2 4.5.4.2 frexpf, frexpl ext math.h 7.13.4 4.13.4 friend C++ only fscanf ext stdio.h 7.9.6.2 4.9.6.2 fseek ext stdio.h 7.9.9.2 4.9.9.2 fsetpos ext stdio.h 7.9.9.3 4.9.9.3 ftell ext stdio.h 7.9.9.4 4.9.9.4 fwide extW wchar.h NA1 fwprintf extW wchar.h NA1 fwrite ext stdio.h 7.9.8.2 4.9.8.2 fwscanf extW wchar.h NA1 |
identifier context header ISO ANSI getc ext stdio.h 7.9.7.5 4.9.7.5 getchar ext stdio.h 7.9.7.6 4.9.7.6 getenv ext stdlib.h 7.10.4.4 4.10.4.4 gets ext stdio.h 7.9.7.7 4.9.7.7 getwc extW wchar.h NA1 getwchar extW wchar.h NA1 gmtime ext time.h 7.12.3.3 4.12.3.3 goto all (keyword) 6.1.1 3.1.1 grouping Hmem locale.h 7.4 4.4 HUGE_VAL H math.h 7.5 4.5 |
identifier context header ISO ANSI if all (keyword) 6.1.1 3.1.1 inline C++ only int all (keyword) 6.1.1 3.1.1 int_curr_symbol Hmem locale.h 7.4 4.4 int_frac_digits Hmem locale.h 7.4 4.4 INT_MAX H limits.h 5.2.4.2.1 2.2.4.2.1 INT_MIN H limits.h 5.2.4.2.1 2.2.4.2.1 is[a-z][any] ext ctype.h 7.13.2 4.13.2 isalnum ext ctype.h 7.3.1.1 4.3.1.1 isalpha ext ctype.h 7.3.1.2 4.3.1.2 iscntrl ext ctype.h 7.3.1.3 4.3.1.3 isdigit ext ctype.h 7.3.1.4 4.3.1.4 isgraph ext ctype.h 7.3.1.5 4.3.1.5 islower ext ctype.h 7.3.1.6 4.3.1.6 isprint ext ctype.h 7.3.1.7 4.3.1.7 ispunct ext ctype.h 7.3.1.8 4.3.1.8 isspace ext ctype.h 7.3.1.9 4.3.1.9 isupper ext ctype.h 7.3.1.10 4.3.1.10 iswalnum extW wctype.h NA1 iswalpha extW wctype.h NA1 iswcntrl extW wctype.h NA1 iswctype extW wctype.h NA1 iswdigit extW wctype.h NA1 iswgraph extW wctype.h NA1 iswlower extW wctype.h NA1 iswprint extW wctype.h NA1 iswpunct extW wctype.h NA1 iswspace extW wctype.h NA1 iswupper extW wctype.h NA1 iswxdigit extW wctype.h NA1 isxdigit ext ctype.h 7.3.1.11 4.3.1.11 jmp_buf (type) H setjmp.h 7.6 4.6 |
identifier context header ISO ANSI L_tmpnam H stdio.h 7.9.1 4.9.1 labs ext stdlib.h 7.10.6.3 4.10.6.3 LC_[A-Z][any] H locale.h 7.13.3 4.13.3 LC_ALL H locale.h 7.4 4.4 LC_COLLATE H locale.h 7.4 4.4 LC_CTYPE H locale.h 7.4 4.4 LC_MONETARY H locale.h 7.4 4.4 LC_NUMERIC H locale.h 7.4 4.4 LC_TIME H locale.h 7.4 4.4 lconv (struct) Htag locale.h 7.4 4.4 LDBL_DIG H float.h 5.2.4.2.2 2.2.4.2.2 LDBL_EPSILON H float.h 5.2.4.2.2 2.2.4.2.2 LDBL_MANT_DIG H float.h 5.2.4.2.2 2.2.4.2.2 LDBL_MAX H float.h 5.2.4.2.2 2.2.4.2.2 LDBL_MAX_10_EXP H float.h 5.2.4.2.2 2.2.4.2.2 LDBL_MAX_EXP H float.h 5.2.4.2.2 2.2.4.2.2 LDBL_MIN H float.h 5.2.4.2.2 2.2.4.2.2 LDBL_MIN_10_EXP H float.h 5.2.4.2.2 2.2.4.2.2 LDBL_MIN_EXP H float.h 5.2.4.2.2 2.2.4.2.2 ldexp ext math.h 7.5.4.3 4.5.4.3 ldexpf, ldexpl ext math.h 7.13.4 4.13.4 ldiv ext stdlib.h 7.10.6.4 4.10.6.4 ldiv_t (type) H stdlib.h 7.10 4.10 localeconv ext locale.h 7.4.2.1 4.4.2.1 localtime ext time.h 7.12.3.4 4.12.3.4 log ext math.h 7.5.4.4 4.5.4.4 log10 ext math.h 7.5.4.5 4.5.4.5 log10f, log10l ext math.h 7.13.4 4.13.4 logf, logl ext math.h 7.13.4 4.13.4 long all (keyword) 6.1.1 3.1.1 LONG_MAX H limits.h 5.2.4.2.1 2.2.4.2.1 LONG_MIN H limits.h 5.2.4.2.1 2.2.4.2.1 longjmp ext setjmp.h 7.6.2.1 4.6.2.1 |
identifier context header ISO ANSI malloc ext stdlib.h 7.10.3.3 4.10.3.3 MB_CUR_MAX H stdlib.h 7.10 4.10 MB_LEN_MAX H limits.h 5.2.4.2.1 2.2.4.2.1 mblen ext stdlib.h 7.10.7.1 4.10.7.1 mbrlen extW wchar.h NA1 mbrtowc extW wchar.h NA1 mbsinit extW wchar.h NA1 mbsrtowcs extW wchar.h NA1 mbstate_t (type) H wchar.h NA1 mbstowcs ext stdlib.h 7.10.8.1 4.10.8.1 mbtowc ext stdlib.h 7.10.7.2 4.10.7.2 mem[a-z][any] ext string.h 7.13.8 4.13.8 memchr ext string.h 7.11.5.1 4.11.5.1 memcmp ext string.h 7.11.4.1 4.11.4.1 memcpy ext string.h 7.11.2.1 4.11.2.1 memmove ext string.h 7.11.2.2 4.11.2.2 memset ext string.h 7.11.6.1 4.11.6.1 mktime ext time.h 7.12.2.3 4.12.2.3 modf ext math.h 7.5.4.6 4.5.4.6 modff, modfl ext math.h 7.13.4 4.13.4 mon_decimal_point Hmem locale.h 7.4 4.4 mon_grouping Hmem locale.h 7.4 4.4 mon_thousands_sep Hmem locale.h 7.4 4.4 mutable C++ only |
identifier context header ISO ANSI n_cs_precedes Hmem locale.h 7.4 4.4 n_sep_by_space Hmem locale.h 7.4 4.4 n_sign_posn Hmem locale.h 7.4 4.4 namespace C++ only NDEBUG H assert.h 7.2 4.2 negative_sign Hmem locale.h 7.4 4.4 new C++ only not H/C++ iso646.h NA1 not_eq H/C++ iso646.h NA1 NULL H many many offsetof H stddef.h 7.1.6 4.1.5 operator C++ only or H/C++ iso646.h NA1 or_eq H/C++ iso646.h NA1 |
identifier context header ISO ANSI p_cs_precedes Hmem locale.h 7.4 4.4 p_sep_by_space Hmem locale.h 7.4 4.4 p_sign_posn Hmem locale.h 7.4 4.4 perror ext stdio.h 7.9.10.4 4.9.10.4 positive_sign Hmem locale.h 7.4 4.4 pow ext math.h 7.5.5.1 4.5.5.1 powf, powl ext math.h 7.13.4 4.13.4 printf ext stdio.h 7.9.6.3 4.9.6.3 private C++ only protected C++ only ptrdiff_t (type) H stddef.h 7.1.6 4.1.5 public C++ only putc ext stdio.h 7.9.7.8 4.9.7.8 putchar ext stdio.h 7.9.7.9 4.9.7.9 puts ext stdio.h 7.9.7.10 4.9.7.10 putwc extW wchar.h NA1 putwchar extW wchar.h NA1 qsort ext stdlib.h 7.10.5.2 4.10.5.2 |
identifier context header ISO ANSI raise ext signal.h 7.7.2.1 4.7.2.1 rand ext stdlib.h 7.10.2.1 4.10.2.1 RAND_MAX H stdlib.h 7.10 4.10 realloc ext stdlib.h 7.10.3.4 4.10.3.4 register all (keyword) 6.1.1 3.1.1 reinterpret_cast C++ only remove ext stdio.h 7.9.4.1 4.9.4.1 rename ext stdio.h 7.9.4.2 4.9.4.2 return all (keyword) 6.1.1 3.1.1 rewind ext stdio.h 7.9.9.5 4.9.9.5 |
identifier context header ISO ANSI scanf ext stdio.h 7.9.6.4 4.9.6.4 SCHAR_MAX H limits.h 5.2.4.2.1 2.2.4.2.1 SCHAR_MIN H limits.h 5.2.4.2.1 2.2.4.2.1 SEEK_CUR H stdio.h 7.9.1 4.9.1 SEEK_END H stdio.h 7.9.1 4.9.1 SEEK_SET H stdio.h 7.9.1 4.9.1 setbuf ext stdio.h 7.9.5.5 4.9.5.5 setjmp H setjmp.h 7.6.1.1 4.6.1.1 setlocale ext locale.h 7.4.1.1 4.4.1.1 setvbuf ext stdio.h 7.9.5.6 4.9.5.6 short all (keyword) 6.1.1 3.1.1 SHRT_MAX H limits.h 5.2.4.2.1 2.2.4.2.1 SHRT_MIN H limits.h 5.2.4.2.1 2.2.4.2.1 SIG_[A-Z][any] H signal.h 7.7 4.7 sig_atomic_t(type) H signal.h 7.7 4.7 SIG_DFL H signal.h 7.7 4.7 SIG_ERR H signal.h 7.7 4.7 SIG_IGN H signal.h 7.7 4.7 SIG[A-Z][any] H signal.h 7.7 4.7 SIGABRT H signal.h 7.7 4.7 SIGFPE H signal.h 7.7 4.7 SIGILL H signal.h 7.7 4.7 SIGINT H signal.h 7.7 4.7 signal ext signal.h 7.7.1.1 4.7.1.1 signed all (keyword) 6.1.1 3.1.1 SIGSEGV H signal.h 7.7 4.7 SIGTERM H signal.h 7.7 4.7 sin ext math.h 7.5.2.6 4.5.2.6 sinf ext math.h 7.13.4 4.13.4 sinh ext math.h 7.5.3.2 4.5.3.2 sinhf, sinhl ext math.h 7.13.4 4.13.4 sinl ext math.h 7.13.4 4.13.4 size_t (type) ext many many sizeof all (keyword) 6.1.1 3.1.1 sprintf ext stdio.h 7.9.6.5 4.9.6.5 sqrt ext math.h 7.5.5.2 4.5.5.2 sqrtf, sqrtl ext math.h 7.13.4 4.13.4 srand ext stdlib.h 7.10.2.1 4.10.2.1 sscanf ext stdio.h 7.9.6.6 4.9.6.6 static all (keyword) 6.1.1 3.1.1 static_cast C++ only stderr H stdio.h 7.9.1 4.9.1 stdin H stdio.h 7.9.1 4.9.1 stdout H stdio.h 7.9.1 4.9.1 str[a-z][any] ext stdlib.h 7.13.7 4.13.7 str[a-z][any] ext string.h 7.13.8 4.13.8 strcat ext string.h 7.11.3.1 4.11.3.1 strchr ext string.h 7.11.5.2 4.11.5.2 strcmp ext string.h 7.11.4.2 4.11.4.2 strcoll ext string.h 7.11.4.3 4.11.4.3 strcpy ext string.h 7.11.2.3 4.11.2.3 strcspn ext string.h 7.11.5.3 4.11.5.3 strerror ext string.h 7.11.6.2 4.11.6.2 strftime ext time.h 7.12.3.5 4.12.3.5 strlen ext string.h 7.11.6.3 4.11.6.3 strncat ext string.h 7.11.3.2 4.11.3.2 strncmp ext string.h 7.11.4.4 4.11.4.4 strncpy ext string.h 7.11.2.4 4.11.2.4 strpbrk ext string.h 7.11.5.4 4.11.5.4 strrchr ext string.h 7.11.5.5 4.11.5.5 strspn ext string.h 7.11.5.6 4.11.5.6 strstr ext string.h 7.11.5.7 4.11.5.7 strtod ext stdlib.h 7.10.1.4 4.10.1.4 strtok ext string.h 7.11.5.8 4.11.5.8 strtol ext stdlib.h 7.10.1.5 4.10.1.5 strtoul ext stdlib.h 7.10.1.6 4.10.1.6 struct all (keyword) 6.1.1 3.1.1 strxfrm ext string.h 7.11.4.5 4.11.4.5 switch all (keyword) 6.1.1 3.1.1 swprintf extW wchar.h NA1 swscanf extW wchar.h NA1 system ext stdlib.h 7.10.4.5 4.10.4.5 |
identifier context header ISO ANSI tan ext math.h 7.5.2.7 4.5.2.7 tanf ext math.h 7.13.4 4.13.4 tanh ext math.h 7.5.3.3 4.5.3.3 tanhf, tanhl ext math.h 7.13.4 4.13.4 tanl ext math.h 7.13.4 4.13.4 template C++ only this C++ only thousands_sep Hmem locale.h 7.4 4.4 throw C++ only time ext time.h 7.12.2.4 4.12.2.4 time_t (type) H time.h 7.12.1 4.12.1 tm (struct) Htag time.h 7.12.1 4.12.1 tm (struct) Htag wchar.h NA1 tm_hour Hmem time.h 7.12.1 4.12.1 tm_isdst Hmem time.h 7.12.1 4.12.1 tm_mday Hmem time.h 7.12.1 4.12.1 tm_min Hmem time.h 7.12.1 4.12.1 tm_mon Hmem time.h 7.12.1 4.12.1 tm_sec Hmem time.h 7.12.1 4.12.1 tm_wday Hmem time.h 7.12.1 4.12.1 tm_yday Hmem time.h 7.12.1 4.12.1 tm_year Hmem time.h 7.12.1 4.12.1 TMP_MAX H stdio.h 7.9.1 4.9.1 tmpfile ext stdio.h 7.9.4.3 4.9.4.3 tmpnam ext stdio.h 7.9.4.4 4.9.4.4 to[a-z][any] ext ctype.h 7.13.2 4.13.2 tolower ext ctype.h 7.3.2.1 4.3.2.1 toupper ext ctype.h 7.3.2.2 4.3.2.2 towctrans extW wctype.h NA1 towlower extW wctype.h NA1 towupper extW wctype.h NA1 true C++ only try C++ only typedef all (keyword) 6.1.1 3.1.1 typeid C++ only typename C++ only |
identifier context header ISO ANSI UCHAR_MAX H limits.h 5.2.4.2.1 2.2.4.2.1 UINT_MAX H limits.h 5.2.4.2.1 2.2.4.2.1 ULONG_MAX H limits.h 5.2.4.2.1 2.2.4.2.1 ungetc ext stdio.h 7.9.7.11 4.9.7.11 ungetwc extW wchar.h NA1 union all (keyword) 6.1.1 3.1.1 unsigned all (keyword) 6.1.1 3.1.1 USHRT_MAX H limits.h 5.2.4.2.1 2.2.4.2.1 using C++ only |
identifier context header ISO ANSI va_arg H stdarg.h 7.8.1 4.8.1 va_end H stdarg.h 7.8.1 4.8.1 va_list (type) H stdarg.h 7.8 4.8 va_start H stdarg.h 7.8.1 4.8.1 vfprintf ext stdio.h 7.9.6.7 4.9.6.7 vfwprintf extW wchar.h NA1 virtual C++ only void all (keyword) 6.1.1 3.1.1 volatile all (keyword) 6.1.1 3.1.1 vprintf ext stdio.h 7.9.6.8 4.9.6.8 vsprintf ext stdio.h 7.9.6.9 4.9.6.9 vswprintf extW wchar.h NA1 vwprintf extW wchar.h NA1 |
identifier context header ISO ANSI WCHAR_MAX H wchar.h NA1 WCHAR_MIN H wchar.h NA1 wchar_t (type) H/C++ stddef.h 7.1.6 4.1.5 wchar_t (type) H/C++ stdlib.h 7.10 4.10 wchar_t (type) H/C++ wchar.h NA1 wcrtomb extW wchar.h NA1 wcs[a-z][any] ext string.h 7.13.8 4.13.8 wcs[a-z][any] extW wchar.h NA1 wcscat extW wchar.h NA1 wcschr extW wchar.h NA1 wcscmp extW wchar.h NA1 wcscoll extW wchar.h NA1 wcscpy extW wchar.h NA1 wcscspn extW wchar.h NA1 wcsftime extW wchar.h NA1 wcslen extW wchar.h NA1 wcsncat extW wchar.h NA1 wcsncmp extW wchar.h NA1 wcsncpy extW wchar.h NA1 wcspbrk extW wchar.h NA1 wcsrchr extW wchar.h NA1 wcsrtombs extW wchar.h NA1 wcsspn extW wchar.h NA1 wcsstr extW wchar.h NA1 wcstod extW wchar.h NA1 wcstok extW wchar.h NA1 wcstol extW wchar.h NA1 wcstombs ext stdlib.h 7.10.8.2 4.10.8.2 wcstoul extW wchar.h NA1 wcsxfrm extW wchar.h NA1 wctob extW wchar.h NA1 wctomb ext stdlib.h 7.10.7.3 4.10.7.3 wctrans extW wctype.h NA1 wctrans_t (type) H wctype.h NA1 wctype extW wctype.h NA1 wctype_t (type) H wctype.h NA1 WEOF H wchar.h NA1 WEOF H wctype.h NA1 while all (keyword) 6.1.1 3.1.1 wint_t (type) H wchar.h NA1 wint_t (type) H wctype.h NA1 wmemchr extW wchar.h NA1 wmemcmp extW wchar.h NA1 wmemcpy extW wchar.h NA1 wmemmove extW wchar.h NA1 wmemset extW wchar.h NA1 wprintf extW wchar.h NA1 wscanf extW wchar.h NA1 xor H/C++ iso646.h NA1 xor_eq H/C++ iso646.h NA1 |
| The standard lists (7.1.3/4.1.2.1) the categories of reserved identifiers,
where "reserved" means, essentially, prohibited for use by the individual
C programmer. Recall that "if a program declares or defines an identifier
with the same name as an identifier reserved in that context ..,
the behavior is undefined" (emphasis added). The "context" combines the
concepts of scope (6.1.2.1/3.1.2.1), linkage (6.1.2.1/3.1.2.2), and
name
space (6.1.2.3/3.1.2.3).
I've marked those contexts in the above table. The details follow, but remember that you don't have to be concerned with them: you'll be safe if you simply never use any of the above identifiers (or anything that starts with the same six characters in capitals or lower case) for your own purposes.
|
| Mark Brader (msb@sq.com) was kind enough to send me a copy
of his May 1992 Usenet article on the conversion between ANSI section and
ISO (sub)clause numbers. I am also indebted to him for pointing out that
user-selected external names must not match reserved external names even
in the first six characters, and for many other helpful
hints.
Clive Feather's writeup on the Normative Addendum was invaluable; see Standards, above, for the URL. My thanks to Steve Summit and Bruce Evans for suggesting improvements in the Usenet article through a couple of revision cycles in 1992. My thanks also to Jutta Degener for pointing out in 1997 the need to take account of name spaces, so that e.g. extern int grouping; is in fact quite legal. Among our wonderful Net resources are the Swedish sitelysator.liu.se, which preserved this article at <http://gopher.lysator.liu.se:70/0/information/C/c-predefined>, and the Alta Vista search engine at http://www.altavista.digital.com/ for finding it. I had mislaid my own copy, and thus was happy to recover it in December 1996. (If you haven't searched Alta Vista for your own name, you might find the results enlightening.) Thanks are also due to David Yuan, http://www.cs.latrobe.edu.au/~yuand/, who thought it worth while offering to his students, and thus galvanized me to make my own HTMLized version available. Finally, if imitation is the sincerest form of flattery, I hope Bradd Szonye feels flattered. I liked the look of his Web pages enough to adapt some of his ideas for these pages. |
|
| original page: http://oakroadsystems.com/tech/c-predef.htm |