Copyright (C) 2000, 2003 Free Software Foundation, Inc.
|
Copyright (C) 2000, 2003 Free Software Foundation, Inc.
|
|
|
This file is intended to contain a few notes about writing C code
|
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
|
within GCC so that it compiles without error on the full range of
|
compilers GCC needs to be able to compile on.
|
compilers GCC needs to be able to compile on.
|
|
|
The problem is that many ISO-standard constructs are not accepted by
|
The problem is that many ISO-standard constructs are not accepted by
|
either old or buggy compilers, and we keep getting bitten by them.
|
either old or buggy compilers, and we keep getting bitten by them.
|
This knowledge until know has been sparsely spread around, so I
|
This knowledge until know has been sparsely spread around, so I
|
thought I'd collect it in one useful place. Please add and correct
|
thought I'd collect it in one useful place. Please add and correct
|
any problems as you come across them.
|
any problems as you come across them.
|
|
|
I'm going to start from a base of the ISO C90 standard, since that is
|
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
|
probably what most people code to naturally. Obviously using
|
constructs introduced after that is not a good idea.
|
constructs introduced after that is not a good idea.
|
|
|
For the complete coding style conventions used in GCC, please read
|
For the complete coding style conventions used in GCC, please read
|
http://gcc.gnu.org/codingconventions.html
|
http://gcc.gnu.org/codingconventions.html
|
|
|
|
|
String literals
|
String literals
|
---------------
|
---------------
|
|
|
Irix6 "cc -n32" and OSF4 "cc" have problems with constant string
|
Irix6 "cc -n32" and OSF4 "cc" have problems with constant string
|
initializers with parens around it, e.g.
|
initializers with parens around it, e.g.
|
|
|
const char string[] = ("A string");
|
const char string[] = ("A string");
|
|
|
This is unfortunate since this is what the GNU gettext macro N_
|
This is unfortunate since this is what the GNU gettext macro N_
|
produces. You need to find a different way to code it.
|
produces. You need to find a different way to code it.
|
|
|
Some compilers like MSVC++ have fairly low limits on the maximum
|
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
|
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.
|
may need to break up a long printf statement into many smaller ones.
|
|
|
|
|
Empty macro arguments
|
Empty macro arguments
|
---------------------
|
---------------------
|
|
|
ISO C (6.8.3 in the 1990 standard) specifies the following:
|
ISO C (6.8.3 in the 1990 standard) specifies the following:
|
|
|
If (before argument substitution) any argument consists of no
|
If (before argument substitution) any argument consists of no
|
preprocessing tokens, the behavior is undefined.
|
preprocessing tokens, the behavior is undefined.
|
|
|
This was relaxed by ISO C99, but some older compilers emit an error,
|
This was relaxed by ISO C99, but some older compilers emit an error,
|
so code like
|
so code like
|
|
|
#define foo(x, y) x y
|
#define foo(x, y) x y
|
foo (bar, )
|
foo (bar, )
|
|
|
needs to be coded in some other way.
|
needs to be coded in some other way.
|
|
|
|
|
free and realloc
|
free and realloc
|
----------------
|
----------------
|
|
|
Some implementations crash upon attempts to free or realloc the null
|
Some implementations crash upon attempts to free or realloc the null
|
pointer. Thus if mem might be null, you need to write
|
pointer. Thus if mem might be null, you need to write
|
|
|
if (mem)
|
if (mem)
|
free (mem);
|
free (mem);
|
|
|
|
|
Trigraphs
|
Trigraphs
|
---------
|
---------
|
|
|
You weren't going to use them anyway, but some otherwise ISO C
|
You weren't going to use them anyway, but some otherwise ISO C
|
compliant compilers do not accept trigraphs.
|
compliant compilers do not accept trigraphs.
|
|
|
|
|
Suffixes on Integer Constants
|
Suffixes on Integer Constants
|
-----------------------------
|
-----------------------------
|
|
|
You should never use a 'l' suffix on integer constants ('L' is fine),
|
You should never use a 'l' suffix on integer constants ('L' is fine),
|
since it can easily be confused with the number '1'.
|
since it can easily be confused with the number '1'.
|
|
|
|
|
Common Coding Pitfalls
|
Common Coding Pitfalls
|
======================
|
======================
|
|
|
errno
|
errno
|
-----
|
-----
|
|
|
errno might be declared as a macro.
|
errno might be declared as a macro.
|
|
|
|
|
Implicit int
|
Implicit int
|
------------
|
------------
|
|
|
In C, the 'int' keyword can often be omitted from type declarations.
|
In C, the 'int' keyword can often be omitted from type declarations.
|
For instance, you can write
|
For instance, you can write
|
|
|
unsigned variable;
|
unsigned variable;
|
|
|
as shorthand for
|
as shorthand for
|
|
|
unsigned int variable;
|
unsigned int variable;
|
|
|
There are several places where this can cause trouble. First, suppose
|
There are several places where this can cause trouble. First, suppose
|
'variable' is a long; then you might think
|
'variable' is a long; then you might think
|
|
|
(unsigned) variable
|
(unsigned) variable
|
|
|
would convert it to unsigned long. It does not. It converts to
|
would convert it to unsigned long. It does not. It converts to
|
unsigned int. This mostly causes problems on 64-bit platforms, where
|
unsigned int. This mostly causes problems on 64-bit platforms, where
|
long and int are not the same size.
|
long and int are not the same size.
|
|
|
Second, if you write a function definition with no return type at
|
Second, if you write a function definition with no return type at
|
all:
|
all:
|
|
|
operate (int a, int b)
|
operate (int a, int b)
|
{
|
{
|
...
|
...
|
}
|
}
|
|
|
that function is expected to return int, *not* void. GCC will warn
|
that function is expected to return int, *not* void. GCC will warn
|
about this.
|
about this.
|
|
|
Implicit function declarations always have return type int. So if you
|
Implicit function declarations always have return type int. So if you
|
correct the above definition to
|
correct the above definition to
|
|
|
void
|
void
|
operate (int a, int b)
|
operate (int a, int b)
|
...
|
...
|
|
|
but operate() is called above its definition, you will get an error
|
but operate() is called above its definition, you will get an error
|
about a "type mismatch with previous implicit declaration". The cure
|
about a "type mismatch with previous implicit declaration". The cure
|
is to prototype all functions at the top of the file, or in an
|
is to prototype all functions at the top of the file, or in an
|
appropriate header.
|
appropriate header.
|
|
|
Char vs unsigned char vs int
|
Char vs unsigned char vs int
|
----------------------------
|
----------------------------
|
|
|
In C, unqualified 'char' may be either signed or unsigned; it is the
|
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
|
implementation's choice. When you are processing 7-bit ASCII, it does
|
not matter. But when your program must handle arbitrary binary data,
|
not matter. But when your program must handle arbitrary binary data,
|
or fully 8-bit character sets, you have a problem. The most obvious
|
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.
|
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
|
For instance, the character '\341' in ISO Latin 1 is SMALL LETTER A
|
WITH ACUTE ACCENT. In the proper locale, isalpha('\341') will be
|
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
|
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
|
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
|
character -31. And the ctype table has no entry at offset -31, so
|
your program will crash. (If you're lucky.)
|
your program will crash. (If you're lucky.)
|
|
|
It is wise to use unsigned char everywhere you possibly can. This
|
It is wise to use unsigned char everywhere you possibly can. This
|
avoids all these problems. Unfortunately, the routines in
|
avoids all these problems. Unfortunately, the routines in
|
take plain char arguments, so you have to remember to cast them back
|
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
|
and forth - or avoid the use of strxxx() functions, which is probably
|
a good idea anyway.
|
a good idea anyway.
|
|
|
Another common mistake is to use either char or unsigned char to
|
Another common mistake is to use either char or unsigned char to
|
receive the result of getc() or related stdio functions. They may
|
receive the result of getc() or related stdio functions. They may
|
return EOF, which is outside the range of values representable by
|
return EOF, which is outside the range of values representable by
|
char. If you use char, some legal character value may be confused
|
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).
|
with EOF, such as '\377' (SMALL LETTER Y WITH UMLAUT, in Latin-1).
|
The correct choice is int.
|
The correct choice is int.
|
|
|
A more subtle version of the same mistake might look like this:
|
A more subtle version of the same mistake might look like this:
|
|
|
unsigned char pushback[NPUSHBACK];
|
unsigned char pushback[NPUSHBACK];
|
int pbidx;
|
int pbidx;
|
#define unget(c) (assert(pbidx < NPUSHBACK), pushback[pbidx++] = (c))
|
#define unget(c) (assert(pbidx < NPUSHBACK), pushback[pbidx++] = (c))
|
#define get(c) (pbidx ? pushback[--pbidx] : getchar())
|
#define get(c) (pbidx ? pushback[--pbidx] : getchar())
|
...
|
...
|
unget(EOF);
|
unget(EOF);
|
|
|
which will mysteriously turn a pushed-back EOF into a SMALL LETTER Y
|
which will mysteriously turn a pushed-back EOF into a SMALL LETTER Y
|
WITH UMLAUT.
|
WITH UMLAUT.
|
|
|
|
|
Other common pitfalls
|
Other common pitfalls
|
---------------------
|
---------------------
|
|
|
o Expecting 'plain' char to be either sign or unsigned extending.
|
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
|
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
|
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).
|
has caused quite a number of bugs at least in the early days).
|
|
|
o Expecting ints shifted right to be sign extended.
|
o Expecting ints shifted right to be sign extended.
|
|
|
o Modifying the same value twice within one sequence point.
|
o Modifying the same value twice within one sequence point.
|
|
|
o Host vs. target floating point representation, including emitting NaNs
|
o Host vs. target floating point representation, including emitting NaNs
|
and Infinities in a form that the assembler handles.
|
and Infinities in a form that the assembler handles.
|
|
|
o qsort being an unstable sort function (unstable in the sense that
|
o qsort being an unstable sort function (unstable in the sense that
|
multiple items that sort the same may be sorted in different orders
|
multiple items that sort the same may be sorted in different orders
|
by different qsort functions).
|
by different qsort functions).
|
|
|
o Passing incorrect types to fprintf and friends.
|
o Passing incorrect types to fprintf and friends.
|
|
|
o Adding a function declaration for a module declared in another file to
|
o Adding a function declaration for a module declared in another file to
|
a .c file instead of to a .h file.
|
a .c file instead of to a .h file.
|
|
|
|
|
