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Diffstat (limited to 'gcc-4.2.1/gcc/README.Portability')
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diff --git a/gcc-4.2.1/gcc/README.Portability b/gcc-4.2.1/gcc/README.Portability deleted file mode 100644 index 32a33e27b..000000000 --- a/gcc-4.2.1/gcc/README.Portability +++ /dev/null @@ -1,197 +0,0 @@ -Copyright (C) 2000, 2003 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 C90 standard, since that is -probably what most people code to naturally. Obviously using -constructs introduced after that is not a good idea. - -For the complete coding style conventions used in GCC, please read -http://gcc.gnu.org/codingconventions.html - - -String literals ---------------- - -Irix6 "cc -n32" and OSF4 "cc" have problems with constant string -initializers with parens around it, e.g. - -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. - -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. - - -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); - - -Trigraphs ---------- - -You weren't going to use them anyway, but some otherwise ISO C -compliant compilers do not accept trigraphs. - - -Suffixes on Integer Constants ------------------------------ - -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 (int a, int b) - { - ... - } - -that function is expected to return int, *not* void. GCC will warn -about this. - -Implicit function declarations always have return type int. So if you -correct the above definition to - - void - operate (int a, int 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 <string.h> -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. - |