203 lines
6.2 KiB
Plaintext
203 lines
6.2 KiB
Plaintext
Copyright (C) 2000-2019 Free Software Foundation, Inc.
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This file is intended to contain a few notes about writing C code
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within GCC so that it compiles without error on the full range of
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compilers GCC needs to be able to compile on.
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The problem is that many ISO-standard constructs are not accepted by
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either old or buggy compilers, and we keep getting bitten by them.
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This knowledge until now has been sparsely spread around, so I
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thought I'd collect it in one useful place. Please add and correct
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any problems as you come across them.
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I'm going to start from a base of the ISO C90 standard, since that is
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probably what most people code to naturally. Obviously using
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constructs introduced after that is not a good idea.
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For the complete coding style conventions used in GCC, please read
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http://gcc.gnu.org/codingconventions.html
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String literals
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---------------
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Some compilers like MSVC++ have fairly low limits on the maximum
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length of a string literal; 509 is the lowest we've come across. You
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may need to break up a long printf statement into many smaller ones.
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Empty macro arguments
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---------------------
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ISO C (6.8.3 in the 1990 standard) specifies the following:
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If (before argument substitution) any argument consists of no
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preprocessing tokens, the behavior is undefined.
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This was relaxed by ISO C99, but some older compilers emit an error,
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so code like
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#define foo(x, y) x y
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foo (bar, )
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needs to be coded in some other way.
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Avoid unnecessary test before free
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----------------------------------
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Since SunOS 4 stopped being a reasonable portability target,
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(which happened around 2007) there has been no need to guard
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against "free (NULL)". Thus, any guard like the following
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constitutes a redundant test:
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if (P)
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free (P);
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It is better to avoid the test.[*]
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Instead, simply free P, regardless of whether it is NULL.
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[*] However, if your profiling exposes a test like this in a
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performance-critical loop, say where P is nearly always NULL, and
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the cost of calling free on a NULL pointer would be prohibitively
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high, consider using __builtin_expect, e.g., like this:
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if (__builtin_expect (ptr != NULL, 0))
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free (ptr);
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Trigraphs
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---------
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You weren't going to use them anyway, but some otherwise ISO C
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compliant compilers do not accept trigraphs.
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Suffixes on Integer Constants
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-----------------------------
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You should never use a 'l' suffix on integer constants ('L' is fine),
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since it can easily be confused with the number '1'.
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Common Coding Pitfalls
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======================
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errno
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-----
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errno might be declared as a macro.
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Implicit int
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------------
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In C, the 'int' keyword can often be omitted from type declarations.
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For instance, you can write
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unsigned variable;
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as shorthand for
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unsigned int variable;
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There are several places where this can cause trouble. First, suppose
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'variable' is a long; then you might think
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(unsigned) variable
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would convert it to unsigned long. It does not. It converts to
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unsigned int. This mostly causes problems on 64-bit platforms, where
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long and int are not the same size.
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Second, if you write a function definition with no return type at
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all:
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operate (int a, int b)
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{
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...
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}
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that function is expected to return int, *not* void. GCC will warn
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about this.
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Implicit function declarations always have return type int. So if you
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correct the above definition to
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void
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operate (int a, int b)
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...
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but operate() is called above its definition, you will get an error
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about a "type mismatch with previous implicit declaration". The cure
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is to prototype all functions at the top of the file, or in an
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appropriate header.
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Char vs unsigned char vs int
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----------------------------
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In C, unqualified 'char' may be either signed or unsigned; it is the
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implementation's choice. When you are processing 7-bit ASCII, it does
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not matter. But when your program must handle arbitrary binary data,
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or fully 8-bit character sets, you have a problem. The most obvious
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issue is if you have a look-up table indexed by characters.
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For instance, the character '\341' in ISO Latin 1 is SMALL LETTER A
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WITH ACUTE ACCENT. In the proper locale, isalpha('\341') will be
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true. But if you read '\341' from a file and store it in a plain
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char, isalpha(c) may look up character 225, or it may look up
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character -31. And the ctype table has no entry at offset -31, so
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your program will crash. (If you're lucky.)
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It is wise to use unsigned char everywhere you possibly can. This
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avoids all these problems. Unfortunately, the routines in <string.h>
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take plain char arguments, so you have to remember to cast them back
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and forth - or avoid the use of strxxx() functions, which is probably
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a good idea anyway.
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Another common mistake is to use either char or unsigned char to
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receive the result of getc() or related stdio functions. They may
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return EOF, which is outside the range of values representable by
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char. If you use char, some legal character value may be confused
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with EOF, such as '\377' (SMALL LETTER Y WITH UMLAUT, in Latin-1).
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The correct choice is int.
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A more subtle version of the same mistake might look like this:
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unsigned char pushback[NPUSHBACK];
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int pbidx;
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#define unget(c) (assert(pbidx < NPUSHBACK), pushback[pbidx++] = (c))
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#define get(c) (pbidx ? pushback[--pbidx] : getchar())
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...
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unget(EOF);
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which will mysteriously turn a pushed-back EOF into a SMALL LETTER Y
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WITH UMLAUT.
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Other common pitfalls
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---------------------
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o Expecting 'plain' char to be either sign or unsigned extending.
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o Shifting an item by a negative amount or by greater than or equal to
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the number of bits in a type (expecting shifts by 32 to be sensible
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has caused quite a number of bugs at least in the early days).
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o Expecting ints shifted right to be sign extended.
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o Modifying the same value twice within one sequence point.
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o Host vs. target floating point representation, including emitting NaNs
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and Infinities in a form that the assembler handles.
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o qsort being an unstable sort function (unstable in the sense that
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multiple items that sort the same may be sorted in different orders
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by different qsort functions).
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o Passing incorrect types to fprintf and friends.
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o Adding a function declaration for a module declared in another file to
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a .c file instead of to a .h file.
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