I am writing some code that I will want to use multiple times with slightly different function and variable names. I want to replace part of the function and variable names with a macro. gcc filename.c -E shows that the substitution is not being made. How do I rectify this?
Here is some code from the file, before substitution:
#define _CLASS Object
#define POOLLEVEL1 1024
#define POOLLEVEL2 1024
typedef struct {
int Self;
int Prev;
int Next;
int In_Use;
//----data----//
//----function pointers----//
} Object;
_CLASS* _CLASS_Pool[POOLLEVEL1] = { 0 };
//Note on POOLLEVEL1, POOLLEVEL2: _CLASS_Pool[] is an array of pointers to arrays of type _CLASS. The number of objects in these arrays is LEVEL2, the maximum number of arrays of type object is LEVEL1; The arrays of type object are allocated when needed.
int _CLASS_Available_Head = -1;
int _CLASS_Available_Tail = -1;
//Start and finish of list of available objects in pool.
// More follows
The preprocessor operates on tokens. And when it comes to identifiers _CLASS is one token, while _CLASS_Pool is another entirely, since they are different identifiers. The preprocessor is not going to stop in the middle of parsing an identifier to check if part of it is another identifier. No, it will gobble up all of _CLASS_Pool before recognizing what the identifier is.
If you ever heard the preprocessor does pure textual substitution, that was a gross over-simplification. It operates on tokens, something best to always keep in mind.
So what you need is a mechanism by which the preprocessor accepts _CLASS as a token, expands it, and then pastes it to another token. Fortunately for you, those mechanisms already exist. It can be written as follows:
#define CONCAT(a, b) CONCAT_(a, b)
#define CONCAT_(a, b) a ## b
To be used like this:
_CLASS* CONCAT(_CLASS, _Pool)[POOLLEVEL1] = { 0 };
int CONCAT(_CLASS, _Available_Head) = -1;
/* and so forth */
The first CONCAT accepts your arguments, and forwards them to another function like macro. Forwarding them allows for any intermediate expansion, like _CLASS -> Object. Tokens that aren't object-like macros remains unchanged. CONCAT_ then simply applies the in-built token pasting operator. You can examine the result and tweak it further.
As an aside, the C standard reserves all identifiers that begin by an underscore, followed by an uppercase letter (_[A-Z][0-9a-zA-Z]*), to the implementation, for any use. Using them yourself leaves you open for undefined behavior. In general, try to avoid leading underscore in identifiers, unless you know all the rules for reserved identifiers by heart.
If I had the following code:
#include <stdio.h>
#define VAR_NAME a_variable
int VAR_NAME = 42;
int main()
{
printf("%i\n", VAR_NAME);
printf("%i\n", a_variable);
a_variable = 123;
printf("%i\n", VAR_NAME);
printf("%i\n", a_variable);
return 0;
}
Is the declaration of a_variable well defined? On my system, an int named a_variable is declared, but do I have to worry about alternate compilers declaring an int named VAR_NAME?
BTW, the above code outputs the following:
42
42
123
123
EDIT:
Furthermore, if the previous is well-defined, could I do the following?
#define VAR_NAME a_variable
...
#define DECL_VAR int VAR_NAME
// Declare an int named a_variable
DECL_VAR;
do I have to worry about alternate compilers declaring an int named VAR_NAME?
No, you do not have to worry about that. Unless someone redefines VAR_NAME to something else, you will consistently get an int named a_variable.
#define directive is processed by preprocessor before the compiler gets to process the code. By the time the compiler "sees" your code, there is no VAR_NAME in there, only a_variable is left in all places where VAR_NAME has been used.
Furthermore, if the previous is well-defined, could I do the following?
#define DECL_VAR int VAR_NAME
Yes, as long as VAR_NAME is defined, this will work as well.
Note: The fact that it is well defined and standard-compliant does not mean that it is necessarily a good idea to use code like that, because cascaded declarations are difficult to read for human readers of your program.
Assuming the implementation complies with the standard, the preprocessor will perform text substitution, so every instance of VAR_NAME will be expanded to a_variable.
There are a couple of exceptions (e.g. macros inside string literals are not expanded). But, if you use the macro in places where a variable name would normally be expected, the expansion will work as you expect.
It is also necessary to avoid reserved identifiers (identifiers that the standard reserves for use by the implementation - i.e. the compiler and standard headers). For example, if you pick a name for your macro or for your variable that is reserved (e.g. that starts with an underscore followed by an uppercase letter, such as _VAR_NAME rather than VAR_NAME) then the behaviour is undefined.
EDIT following edit of the original question: Yes, the expansion of DECL_VAR works as you expect.
I would caution you against such things though. Such tricks do not give many benefits, but can make your code harder to understand (after all, few people will know what DECL_VAR; will mean in your code unless they know you are playing with macros). Code that is harder to understand is harder to get right.
Which one is better to use among the below statements in C?
static const int var = 5;
or
#define var 5
or
enum { var = 5 };
It depends on what you need the value for. You (and everyone else so far) omitted the third alternative:
static const int var = 5;
#define var 5
enum { var = 5 };
Ignoring issues about the choice of name, then:
If you need to pass a pointer around, you must use (1).
Since (2) is apparently an option, you don't need to pass pointers around.
Both (1) and (3) have a symbol in the debugger's symbol table - that makes debugging easier. It is more likely that (2) will not have a symbol, leaving you wondering what it is.
(1) cannot be used as a dimension for arrays at global scope; both (2) and (3) can.
(1) cannot be used as a dimension for static arrays at function scope; both (2) and (3) can.
Under C99, all of these can be used for local arrays. Technically, using (1) would imply the use of a VLA (variable-length array), though the dimension referenced by 'var' would of course be fixed at size 5.
(1) cannot be used in places like switch statements; both (2) and (3) can.
(1) cannot be used to initialize static variables; both (2) and (3) can.
(2) can change code that you didn't want changed because it is used by the preprocessor; both (1) and (3) will not have unexpected side-effects like that.
You can detect whether (2) has been set in the preprocessor; neither (1) nor (3) allows that.
So, in most contexts, prefer the 'enum' over the alternatives. Otherwise, the first and last bullet points are likely to be the controlling factors — and you have to think harder if you need to satisfy both at once.
If you were asking about C++, then you'd use option (1) — the static const — every time.
Generally speaking:
static const
Because it respects scope and is type-safe.
The only caveat I could see: if you want the variable to be possibly defined on the command line. There is still an alternative:
#ifdef VAR // Very bad name, not long enough, too general, etc..
static int const var = VAR;
#else
static int const var = 5; // default value
#endif
Whenever possible, instead of macros / ellipsis, use a type-safe alternative.
If you really NEED to go with a macro (for example, you want __FILE__ or __LINE__), then you'd better name your macro VERY carefully: in its naming convention Boost recommends all upper-case, beginning by the name of the project (here BOOST_), while perusing the library you will notice this is (generally) followed by the name of the particular area (library) then with a meaningful name.
It generally makes for lengthy names :)
In C, specifically? In C the correct answer is: use #define (or, if appropriate, enum)
While it is beneficial to have the scoping and typing properties of a const object, in reality const objects in C (as opposed to C++) are not true constants and therefore are usually useless in most practical cases.
So, in C the choice should be determined by how you plan to use your constant. For example, you can't use a const int object as a case label (while a macro will work). You can't use a const int object as a bit-field width (while a macro will work). In C89/90 you can't use a const object to specify an array size (while a macro will work). Even in C99 you can't use a const object to specify an array size when you need a non-VLA array.
If this is important for you then it will determine your choice. Most of the time, you'll have no choice but to use #define in C. And don't forget another alternative, that produces true constants in C - enum.
In C++ const objects are true constants, so in C++ it is almost always better to prefer the const variant (no need for explicit static in C++ though).
The difference between static const and #define is that the former uses the memory and the later does not use the memory for storage. Secondly, you cannot pass the address of an #define whereas you can pass the address of a static const. Actually it is depending on what circumstance we are under, we need to select one among these two. Both are at their best under different circumstances. Please don't assume that one is better than the other... :-)
If that would have been the case, Dennis Ritchie would have kept the best one alone... hahaha... :-)
In C #define is much more popular. You can use those values for declaring array sizes for example:
#define MAXLEN 5
void foo(void) {
int bar[MAXLEN];
}
ANSI C doesn't allow you to use static consts in this context as far as I know. In C++ you should avoid macros in these cases. You can write
const int maxlen = 5;
void foo() {
int bar[maxlen];
}
and even leave out static because internal linkage is implied by const already [in C++ only].
Another drawback of const in C is that you can't use the value in initializing another const.
static int const NUMBER_OF_FINGERS_PER_HAND = 5;
static int const NUMBER_OF_HANDS = 2;
// initializer element is not constant, this does not work.
static int const NUMBER_OF_FINGERS = NUMBER_OF_FINGERS_PER_HAND
* NUMBER_OF_HANDS;
Even this does not work with a const since the compiler does not see it as a constant:
static uint8_t const ARRAY_SIZE = 16;
static int8_t const lookup_table[ARRAY_SIZE] = {
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}; // ARRAY_SIZE not a constant!
I'd be happy to use typed const in these cases, otherwise...
If you can get away with it, static const has a lot of advantages. It obeys the normal scope principles, is visible in a debugger, and generally obeys the rules that variables obey.
However, at least in the original C standard, it isn't actually a constant. If you use #define var 5, you can write int foo[var]; as a declaration, but you can't do that (except as a compiler extension" with static const int var = 5;. This is not the case in C++, where the static const version can be used anywhere the #define version can, and I believe this is also the case with C99.
However, never name a #define constant with a lowercase name. It will override any possible use of that name until the end of the translation unit. Macro constants should be in what is effectively their own namespace, which is traditionally all capital letters, perhaps with a prefix.
#define var 5 will cause you trouble if you have things like mystruct.var.
For example,
struct mystruct {
int var;
};
#define var 5
int main() {
struct mystruct foo;
foo.var = 1;
return 0;
}
The preprocessor will replace it and the code won't compile. For this reason, traditional coding style suggest all constant #defines uses capital letters to avoid conflict.
It is ALWAYS preferable to use const, instead of #define. That's because const is treated by the compiler and #define by the preprocessor. It is like #define itself is not part of the code (roughly speaking).
Example:
#define PI 3.1416
The symbolic name PI may never be seen by compilers; it may be removed by the preprocessor before the source code even gets to a compiler. As a result, the name PI may not get entered into the symbol table. This can be confusing if you get an error during compilation involving the use of the constant, because the error message may refer to 3.1416, not PI. If PI were defined in a header file you didn’t write, you’d have no idea where that 3.1416 came from.
This problem can also crop up in a symbolic debugger, because, again, the name you’re programming with may not be in the symbol table.
Solution:
const double PI = 3.1416; //or static const...
I wrote quick test program to demonstrate one difference:
#include <stdio.h>
enum {ENUM_DEFINED=16};
enum {ENUM_DEFINED=32};
#define DEFINED_DEFINED 16
#define DEFINED_DEFINED 32
int main(int argc, char *argv[]) {
printf("%d, %d\n", DEFINED_DEFINED, ENUM_DEFINED);
return(0);
}
This compiles with these errors and warnings:
main.c:6:7: error: redefinition of enumerator 'ENUM_DEFINED'
enum {ENUM_DEFINED=32};
^
main.c:5:7: note: previous definition is here
enum {ENUM_DEFINED=16};
^
main.c:9:9: warning: 'DEFINED_DEFINED' macro redefined [-Wmacro-redefined]
#define DEFINED_DEFINED 32
^
main.c:8:9: note: previous definition is here
#define DEFINED_DEFINED 16
^
Note that enum gives an error when define gives a warning.
The definition
const int const_value = 5;
does not always define a constant value. Some compilers (for example tcc 0.9.26) just allocate memory identified with the name "const_value". Using the identifier "const_value" you can not modify this memory. But you still could modify the memory using another identifier:
const int const_value = 5;
int *mutable_value = (int*) &const_value;
*mutable_value = 3;
printf("%i", const_value); // The output may be 5 or 3, depending on the compiler.
This means the definition
#define CONST_VALUE 5
is the only way to define a constant value which can not be modified by any means.
Although the question was about integers, it's worth noting that #define and enums are useless if you need a constant structure or string. These are both usually passed to functions as pointers. (With strings it's required; with structures it's much more efficient.)
As for integers, if you're in an embedded environment with very limited memory, you might need to worry about where the constant is stored and how accesses to it are compiled. The compiler might add two consts at run time, but add two #defines at compile time. A #define constant may be converted into one or more MOV [immediate] instructions, which means the constant is effectively stored in program memory. A const constant will be stored in the .const section in data memory. In systems with a Harvard architecture, there could be differences in performance and memory usage, although they'd likely be small. They might matter for hard-core optimization of inner loops.
Don't think there's an answer for "which is always best" but, as Matthieu said
static const
is type safe. My biggest pet peeve with #define, though, is when debugging in Visual Studio you cannot watch the variable. It gives an error that the symbol cannot be found.
Incidentally, an alternative to #define, which provides proper scoping but behaves like a "real" constant, is "enum". For example:
enum {number_ten = 10;}
In many cases, it's useful to define enumerated types and create variables of those types; if that is done, debuggers may be able to display variables according to their enumeration name.
One important caveat with doing that, however: in C++, enumerated types have limited compatibility with integers. For example, by default, one cannot perform arithmetic upon them. I find that to be a curious default behavior for enums; while it would have been nice to have a "strict enum" type, given the desire to have C++ generally compatible with C, I would think the default behavior of an "enum" type should be interchangeable with integers.
A simple difference:
At pre-processing time, the constant is replaced with its value.
So you could not apply the dereference operator to a define, but you can apply the dereference operator to a variable.
As you would suppose, define is faster that static const.
For example, having:
#define mymax 100
you can not do printf("address of constant is %p",&mymax);.
But having
const int mymax_var=100
you can do printf("address of constant is %p",&mymax_var);.
To be more clear, the define is replaced by its value at the pre-processing stage, so we do not have any variable stored in the program. We have just the code from the text segment of the program where the define was used.
However, for static const we have a variable that is allocated somewhere. For gcc, static const are allocated in the text segment of the program.
Above, I wanted to tell about the reference operator so replace dereference with reference.
We looked at the produced assembler code on the MBF16X... Both variants result in the same code for arithmetic operations (ADD Immediate, for example).
So const int is preferred for the type check while #define is old style. Maybe it is compiler-specific. So check your produced assembler code.
I am not sure if I am right but in my opinion calling #defined value is much faster than calling any other normally declared variable (or const value).
It's because when program is running and it needs to use some normally declared variable it needs to jump to exact place in memory to get that variable.
In opposite when it use #defined value, the program don't need to jump to any allocated memory, it just takes the value. If #define myValue 7 and the program calling myValue, it behaves exactly the same as when it just calls 7.
After reading through some of K&R's The C Programming Language I came across the #define symbolic constants. I decided to define...
#define INTEGER_EXAMPLE 2
#define CHAR_EXAMPLE 2
...so my question is how does C know if I'm defining an int or a char type?
#define-d names have no types. They just define textual replacements.
What the compiler is seeing is the preprocessed form. If using GCC, try gcc -C -E somesource.c and have a look at the (preprocessed) output.
In the 1980s the preprocessor was a separate program.
Read about the cpp preprocessor, and preprocessor and C preprocessor wikipages.
You could even define ill-defined names like
#define BAD #*?$ some crap $?
And even more scary you can define things which are syntactically incomplete like
#define BADTASTE 2 +
and later code BADTASTE 3
Actually, you want to use parenthesis when defining macros. If you have
#define BADPROD(x,y) x*y
then BADPROD(2+3,4+5) is expanded to 2+3*4+5 which the compiler understands like 2+ (3*4) +5; you really want
#define BETTERPROD(x,y) ((x)*(y))
So that BETTERPROD(2+3,4+5) is expanded to ((2+3)*(4+5))
Avoid side-effects in macro arguments, e.g. BETTERPROD(j++,j--)
In general, use macros with care and have them stay simple.
Regarding these defines, it doesn't, the expanded macros doesn't have a type. The pre-processor which processes the #define is just replacing text within the source code
When you use these defines somewhere, e.g.
int i = INTEGER_EXAMPLE;
This will expand to
int i = 2;
Here the literal 2 (which in this context is an int) is assigned to an int.
You could also do:
char c = INTEGER_EXAMPLE;
Here too, the literal 2 is an int, and it is assigned to a char. 2 is within the limits of a char though, so all is ok.
You could even do:
int INTEGER_EXAMPLE = 2;
This would expand to
int 2 = 2;
Which isn't valid C.
#define STRING VALUE
is just an instruction for the pre-processor to replace the STRING with VALUE
afterwards the compiler will take control and will check the types
It doesn't, this is the preprocessor. The type of the constant is dependent on the context in which it is used. For instance:
#define INT_EXAMPLE 257
char foo = INT_EXAMPLE;
will attempt to assign 257 in a char context which should generate a warning unless char has more than 8 bits on your computer.
#Defines are nothing but literal replacements of values. You might want to use
static const
As it respects scope and is type-safe. Try this:
#define main no_main
int main() // gets replaced as no_main by preprocessor
{
return 0;
}
Should give you linking errors. Or you could try and fool your teacher by this
#define I_Have_No_Main_Function main //--> Put this in header file 1.h
#include"1.h"
int I_Have_No_Main_Function()
{
return 0;
}
It doesn't. The #define statements are processed before the compiler starts its work. Basically the pre-processor does a search and replace for what you wrote and replaces it, for instance, all instances of INTEGER_EXAMPLE are replaced with the string 2.
It is up to the compiler to decide the type of that 2 based on where it's used:
int x = INTEGER_EXAMPLE; // 2 is an integer
char y = INTEGER_EXAMPLE; // 2 is a char
Preprocessor cannot know the type of the macro definition. Preprocessor will just replace all occurrence of 'CHAR_EXAMPLE' with '2'. I would use cast:
#define CHAR_EXAMPLE ((char)2)
I'm trying to figure out there best way to define a global array with a constant size and I've come to the following options, all with their own flaws.
// 1:
#define ASIZE 10
int array[ASIZE];
// 2:
enum {ASIZE = 10};
int array[ASIZE];
// 3:
#define ASIZE_DEF 10
static const int ASIZE = ASIZE_DEF;
int array[ASIZE_DEF];
The problem with the first two is that I can't get the value of ASIZE from GDB. I guess the third option is best because I can still dump the value of the const, but it also leaks in another macro. I can undef the macro after defining the array and const but if the #define and the const are in a separate file from the array declaration, then it gets a bit hairy.
Is there a better way?
Doing something for the sake of the debugger is wrong. Incidentally, gdb knows about this if you compile your code right.
Some languages, such as C and C++, provide a way to define and invoke
“preprocessor macros” which expand into strings of tokens. gdb can
evaluate expressions containing macro invocations, show the result of
macro expansion, and show a macro's definition, including where it was
defined.
Version 3.1 and later of gcc, the gnu C compiler, provides macro
information if you specify the options -gdwarf-2 and -g3; the former
option requests debugging information in the Dwarf 2 format, and the
latter requests “extra information”.
You are dealing with a GDB issue, not a C issue. You can also do #4, which is arguably better than #3.
enum {ASIZE = 10};
static const int ASIZE_FOR_GDB = ASIZE;
int array[ASIZE];
My understanding is that you are defining a constant, using it later to size one or more arrays, and also want that constant to be a symbol, preferably without a messy namespace. (If it were a matter of exporting the size of a single array, I would instead suggest sizeof(array) / sizeof(*array) as missingno did.)
static const int ASIZE = 10;
#define ASIZE 10
int array[ASIZE];
Here, there is a variable with the desired value which will be in the object file, but the preprocessor macro shadows it with the value itself, so the array definition also succeeds.
However, you may find the need to duplicate the value expression ugly. Wouldn't it be nice if we could define the variable in terms of the macro?
static const int ASIZE =
#define ASIZE 10
ASIZE;
int array[ASIZE];
I'm not sure whether this is actually a better idea maintainability-wise than the above, but it works (and I couldn't get gcc to be offended by it) and it contains no duplication other than of the identifier. And it's amusing.
Since you know array is a global array (and not just a pointer) you can find its length using
sizeof(array) / sizeof(*array)
without needing to set an extra variable for that.