Copyright (C) 2000 Free Software Foundation, Inc. This file is intended to contain a few notes about writing C code within GCC so that it compiles without error on the full range of compilers GCC needs to be able to compile on. The problem is that many ISO-standard constructs are not accepted by either old or buggy compilers, and we keep getting bitten by them. This knowledge until know has been sparsely spread around, so I thought I'd collect it in one useful place. Please add and correct any problems as you come across them. I'm going to start from a base of the ISO C89 standard, since that is probably what most people code to naturally. Obviously using constructs introduced after that is not a good idea. The first section of this file deals strictly with portability issues, the second with common coding pitfalls. Portability Issues ================== Unary + ------- K+R C compilers and preprocessors have no notion of unary '+'. Thus the following code snippet contains 2 portability problems. int x = +2; /* int x = 2; */ #if +1 /* #if 1 */ #endif Pointers to void ---------------- K+R C compilers did not have a void pointer, and used char * as the pointer to anything. The macro PTR is defined as either void * or char * depending on whether you have a standards compliant compiler or a K+R one. Thus free ((void *) h->value.expansion); should be written free ((PTR) h->value.expansion); Further, an initial investigation indicates that pointers to functions returning void are okay. Thus the example given by "Calling functions through pointers to functions" below appears not to cause a problem. String literals --------------- Some SGI compilers choke on the parentheses in:- const char string[] = ("A string"); This is unfortunate since this is what the GNU gettext macro N_ produces. You need to find a different way to code it. K+R C did not allow concatenation of string literals like "This is a " "single string literal". Moreover, some compilers like MSVC++ have fairly low limits on the maximum length of a string literal; 509 is the lowest we've come across. You may need to break up a long printf statement into many smaller ones. Empty macro arguments --------------------- ISO C (6.8.3 in the 1990 standard) specifies the following: If (before argument substitution) any argument consists of no preprocessing tokens, the behavior is undefined. This was relaxed by ISO C99, but some older compilers emit an error, so code like #define foo(x, y) x y foo (bar, ) needs to be coded in some other way. signed keyword -------------- The signed keyword did not exist in K+R compilers; it was introduced in ISO C89, so you cannot use it. In both K+R and standard C, unqualified char and bitfields may be signed or unsigned. There is no way to portably declare signed chars or signed bitfields. All other arithmetic types are signed unless you use the 'unsigned' qualifier. For instance, it is safe to write short paramc; instead of signed short paramc; If you have an algorithm that depends on signed char or signed bitfields, you must find another way to write it before it can be integrated into GCC. Function prototypes ------------------- You need to provide a function prototype for every function before you use it, and functions must be defined K+R style. The function prototype should use the PARAMS macro, which takes a single argument. Therefore the parameter list must be enclosed in parentheses. For example, int myfunc PARAMS ((double, int *)); int myfunc (var1, var2) double var1; int *var2; { ... } This implies that if the function takes no arguments, it should be declared and defined as follows: int myfunc PARAMS ((void)); int myfunc () { ... } You also need to use PARAMS when referring to function protypes in other circumstances, for example see "Calling functions through pointers to functions" below. Variable-argument functions are best described by example:- void cpp_ice PARAMS ((cpp_reader *, const char *msgid, ...)); void cpp_ice VPARAMS ((cpp_reader *pfile, const char *msgid, ...)) { VA_OPEN (ap, msgid); VA_FIXEDARG (ap, cpp_reader *, pfile); VA_FIXEDARG (ap, const char *, msgid); ... VA_CLOSE (ap); } See ansidecl.h for the definitions of the above macros and more. One aspect of using K+R style function declarations, is you cannot have arguments whose types are char, short, or float, since without prototypes (ie, K+R rules), these types are promoted to int, int, and double respectively. Calling functions through pointers to functions ----------------------------------------------- K+R C compilers require parentheses around the dereferenced function pointer expression in the call, whereas ISO C relaxes the syntax. For example typedef void (* cl_directive_handler) PARAMS ((cpp_reader *, const char *)); *p->handler (pfile, p->arg); needs to become (*p->handler) (pfile, p->arg); Macros ------ The rules under K+R C and ISO C for achieving stringification and token pasting are quite different. Therefore some macros have been defined which will get it right depending upon the compiler. CONCAT2(a,b) CONCAT3(a,b,c) and CONCAT4(a,b,c,d) will paste the tokens passed as arguments. You must not leave any space around the commas. Also, STRINGX(x) will stringify an argument; to get the same result on K+R and ISO compilers x should not have spaces around it. Passing structures by value --------------------------- Avoid passing structures by value, either to or from functions. It seems some K+R compilers handle this differently or not at all. Enums ----- In K+R C, you have to cast enum types to use them as integers, and some compilers in particular give lots of warnings for using an enum as an array index. Bitfields --------- See also "signed keyword" above. In K+R C only unsigned int bitfields were defined (i.e. unsigned char, unsigned short, unsigned long. Using plain int/short/long was not allowed). free and realloc ---------------- Some implementations crash upon attempts to free or realloc the null pointer. Thus if mem might be null, you need to write if (mem) free (mem); Reserved Keywords ----------------- K+R C has "entry" as a reserved keyword, so you should not use it for your variable names. Type promotions --------------- K+R used unsigned-preserving rules for arithmetic expresssions, while ISO uses value-preserving. This means an unsigned char compared to an int is done as an unsigned comparison in K+R (since unsigned char promotes to unsigned) while it is signed in ISO (since all of the values in unsigned char fit in an int, it promotes to int). Trigraphs --------- You weren't going to use them anyway, but trigraphs were not defined in K+R C, and some otherwise ISO C compliant compilers do not accept them. Suffixes on Integer Constants ----------------------------- K+R C did not accept a 'u' suffix on integer constants. If you want to declare a constant to be be unsigned, you must use an explicit cast. You should never use a 'l' suffix on integer constants ('L' is fine), since it can easily be confused with the number '1'. Common Coding Pitfalls ====================== errno ----- errno might be declared as a macro. Implicit int ------------ In C, the 'int' keyword can often be omitted from type declarations. For instance, you can write unsigned variable; as shorthand for unsigned int variable; There are several places where this can cause trouble. First, suppose 'variable' is a long; then you might think (unsigned) variable would convert it to unsigned long. It does not. It converts to unsigned int. This mostly causes problems on 64-bit platforms, where long and int are not the same size. Second, if you write a function definition with no return type at all: operate (a, b) int a, b; { ... } that function is expected to return int, *not* void. GCC will warn about this. K+R C has no problem with 'void' as a return type, so you need not worry about that. Implicit function declarations always have return type int. So if you correct the above definition to void operate (a, b) int a, b; ... but operate() is called above its definition, you will get an error about a "type mismatch with previous implicit declaration". The cure is to prototype all functions at the top of the file, or in an appropriate header. Char vs unsigned char vs int ---------------------------- In C, unqualified 'char' may be either signed or unsigned; it is the implementation's choice. When you are processing 7-bit ASCII, it does not matter. But when your program must handle arbitrary binary data, or fully 8-bit character sets, you have a problem. The most obvious issue is if you have a look-up table indexed by characters. For instance, the character '\341' in ISO Latin 1 is SMALL LETTER A WITH ACUTE ACCENT. In the proper locale, isalpha('\341') will be true. But if you read '\341' from a file and store it in a plain char, isalpha(c) may look up character 225, or it may look up character -31. And the ctype table has no entry at offset -31, so your program will crash. (If you're lucky.) It is wise to use unsigned char everywhere you possibly can. This avoids all these problems. Unfortunately, the routines in take plain char arguments, so you have to remember to cast them back and forth - or avoid the use of strxxx() functions, which is probably a good idea anyway. Another common mistake is to use either char or unsigned char to receive the result of getc() or related stdio functions. They may return EOF, which is outside the range of values representable by char. If you use char, some legal character value may be confused with EOF, such as '\377' (SMALL LETTER Y WITH UMLAUT, in Latin-1). The correct choice is int. A more subtle version of the same mistake might look like this: unsigned char pushback[NPUSHBACK]; int pbidx; #define unget(c) (assert(pbidx < NPUSHBACK), pushback[pbidx++] = (c)) #define get(c) (pbidx ? pushback[--pbidx] : getchar()) ... unget(EOF); which will mysteriously turn a pushed-back EOF into a SMALL LETTER Y WITH UMLAUT. Other common pitfalls --------------------- o Expecting 'plain' char to be either sign or unsigned extending o Shifting an item by a negative amount or by greater than or equal to the number of bits in a type (expecting shifts by 32 to be sensible has caused quite a number of bugs at least in the early days). o Expecting ints shifted right to be sign extended. o Modifying the same value twice within one sequence point. o Host vs. target floating point representation, including emitting NaNs and Infinities in a form that the assembler handles. o qsort being an unstable sort function (unstable in the sense that multiple items that sort the same may be sorted in different orders by different qsort functions). o Passing incorrect types to fprintf and friends. o Adding a function declaration for a module declared in another file to a .c file instead of to a .h file.