I would like to have macro which will undefine constant passed to it when called.
Something like this:
#define CONSTANT1 123
#define macro(const) \
#ifdef const \
const \
#undef const \
#else \
#error "constant already used once" \
#endif
int main(){
int a = macro(CONSTANT1); // a = 123
int b = macro(CONSTANT1); // <- preprocessor error "constant already used once"
return 0;
}
It is possible to archive this functionality with preprocessor?
I don't think it is possible to get it with standard C preprocessor but it is possible to do it with GCC/CLANG pragmas like push_macro/pop_macro:
// define a macro that will generate error on expansion
#define CONSTANT1 _Pragma("GCC error \"CONSTANT1 expanded more than once\"")
// save this macro and undefine it
#pragma push_macro("CONSTANT1")
#undef CONSTANT1
// let CONSTANT1 expand to 123, but replace with
// previous error-generation macro
#define CONSTANT1 123 _Pragma("pop_macro(\"CONSTANT1\")")
int a = CONSTANT1;
int b = CONSTANT1;
Compiling with gcc/clang produced:
prog.c:8:11: error: CONSTANT1 expanded more than once
8 | int b = CONSTANT1;
Note that pragmas push_macro/pop_macro are quite portable and they are supported by GCC,CLANG,MSVS and Intel C Compiler.
A bit more portable version of failing CONSTANT1 could be:
#define CONSTANT1 sizeof(struct {_Static_assert(0, "CONSTANT1 expanded more than once"); int x; })
It requires C11 compatible compiler.
Related
Consider the following simple C/C++ example:
#define FOO
...
#ifdef FOO
bar++;
#endif
OK, now I would like to fit that (and any other similar) conditional into one line for the code readability sake (the code has tens of single-line statements that all need to be conditional each depending on different define). Something that, when used, would look like:
#define FOO
...
MY_IFDEF(FOO,bar++;) //Single-line conditional
The goal is to have a reusable macro that can take an arbitrary identifier and, compile the statement if such identifier has been #define-d previously, and do it all in a single line.
Any ideas?
UPDATE0: the code must compile for both C and C++
You can't use #ifdef while expanding macros, but you can totally precheck it and declare empty statement if condition is not met.
#ifdef FOO
#define MY_IFDEF(x,y) some-processing-you-need
#else
#define MY_IFDEF(x,y) ;
#endif
Also check out new feature of C++: constexpr this can also be usable.
UPDATE4: As pointed out by #Alex Bakanov you can't use #ifdef while expanding macros, so there is no single line solution for your question which works in all cases. Nevertheless, I hope the idea I wrote here may be useful.
if you use #define FOO 1 or #define FOO 0, combination of #define and if constexpr can be used. Note that it gives an error if FOO is not defined. This program gives 1 as result:
#include<iostream>
#define FOO 1
#define MY_IFDEF(x,y) if constexpr (x) y;
int main()
{
int bar =0;
MY_IFDEF(FOO,bar++)
std::cout << bar << "\n";
}
UPDATE: Based on #eerorika's comment to avoid the error if FOO is not defined, the following declaration has to be added:
constexpr bool FOO = false;
UPDATE2: This version works in any circumstances, the only question is that is it worth the effort?
#ifdef FOO
constexpr bool USED_FOO = true;
#else
constexpr bool USED_FOO = false;
#endif
#define MY_IFDEF(x,y) if constexpr (USED_##x) y ;
UPDATE3: C compatible version. Note that in this case in theory it is evaluated runtime not compile time, but the compiler will realize that it is always true/false and generates the code accordingly:
#ifdef FOO
const static bool USED_FOO = true;
#else
const static bool USED_FOO = false;
#endif
#define MY_IFDEF(x,y) if (USED_##x) y;
I know how to mark enums/functions as deprecated by using
__attribute__ ((deprecated)). But how can I mark constant macro
as deprecated?
#define MACRO1 4 //This is deprecated macro
GCC (and possibly others)
__attribute__((deprecated))
For your particular example with just a constant expression, you can use this:
Change
#define X (4)
to
#define X_old (4)
and then add
const int dep __attribute__((deprecated)) = 0;
#define X ((void)dep, X_old)
Addition also works:
#define X (X_old + dep)
For a procedure macro you can do this:
#define P_old do { ... } while(0)
#define P do { (void)dep; P_old; } while(0)
The only function of (void) is to suppress warnings. Thanks Kevin.
#pragma message
Another solution is to put all deprecated macros in a separate header file and use pragma. You could combine this with #ifdef and such:
#pragma message ("This header contains deprecated macros")
All compilers
Unreferenced label
Use an unreferenced label:
#define P_old do { ... } while(0)
#define P do { P_IS_DEPRECATED: P_old; } while(0)
This does not work for constant expression macros and requires you to compile with -Wall to get a warning. Will trigger error if used more than once.
Unused variable:
#define P_old do { ... } while(0)
#define P do { int P_IS_DEPRECATED; P_old; } while(0)
Does not work on constant expressions either. Also requires -Wall but can be used more than once.
Sidenote
Remember to encapsulate constant expression macros in parentheses. The macro #define X 2+3 would make an expression like 2*X to expand to 2*2+3 instead of 2*(2+3).
What I want to do is access code with macros. But the complier gives me this error
identifier "BUTTON___button" is undefined
#define BUTTON_1 HAL_GPIO_ReadPin(GPIOB, GPIO_PIN_8)
#define BUTTON_2 HAL_GPIO_ReadPin(GPIOB, GPIO_PIN_9)
#define BUTTON_3 HAL_GPIO_ReadPin(GPIOB, GPIO_PIN_4)
#define BUTTON_4 HAL_GPIO_ReadPin(GPIOB, GPIO_PIN_5)
#define BUTTON_5 HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_13)
#define BUTTON_6 HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_14)
#define BUTTON_7 HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_12)
#define BUTTON_8 HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_11)
#define BUTTON_9 HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_15)
#define BUTTON_10 HAL_GPIO_ReadPin(GPIOD, GPIO_PIN_0)
#define BUTTON_11 HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_10)
#define BUTTON_12 HAL_GPIO_ReadPin(GPIOA, GPIO_PIN_15)
#define BUTTON_13 HAL_GPIO_ReadPin(GPIOB, GPIO_PIN_1)
#define BUTTON_14 HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_2)
#define BUTTON_15 HAL_GPIO_ReadPin(GPIOB, GPIO_PIN_11)
#define BUTTON_16 HAL_GPIO_ReadPin(GPIOB, GPIO_PIN_2)
#define BUTTON_17 HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_0)
#define BUTTON_18 HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_1)
#define BUTTON_19 HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_5)
#define BUTTON_20 HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_4)
#define BUTTON(...) BUTTON_##__VA_ARGS__
for(uint8_t _button = 1; _button < 21; _button++)
BUTTON(__button) //=> This should give me BUTTON(1) , BUTTON(2) ... each iteration.But not working
By using variadic macros, can I get what I want?
You must remember that preprocessor things happen before the code is compiled, and is strictly text-only replacement.
Thus, depending on a variable that has different values because of a run-time loop makes no sense, and doesn't work.
The proper way to do this is to put the port addresses (GPIOA etc) in an array, together with the corresponding pin for each port:
static const struct {
const GPIO_TypeDef *port;
uint32_t pin;
} buttons[] = {
{ GPIOB, GPIO_PIN_8 },
{ GPIOB, GPIO_PIN_9 },
...
};
then iterate over the data in the array:
for (size_t i = 0; i < sizeof buttons / sizeof *buttons; ++i)
{
if (HAL_GPIO_ReadPin(buttons[i].port, buttons[i].pin))
{
...
}
}
The preprocessor works at compile-time, and is involved very early (read about translation phases). So you can't generate macros with a for loop. Read documentation of cpp and later your C standard (e.g. n1570).
You could generate your C code with a different program -maybe some other preprocessor like GPP or m4, or some script (or your own other program), and generating C files is a common practice since the previous century (for examples, look into yacc or rpcgen, but you'll find many others). You then want to configure your build automation tool (perhaps make or ninja) to invoke such a C code generator appropriately.
You can obtain the preprocessed form, e.g. with gcc -C -E if using GCC. So you can understand what the compiler (its translation phases after preprocessing) is getting.
By using variadic macros, can I get what I want?
No you can't. Read documentation of variadic macros.
Variadic macros won't solve anything here. You have two options, look-up tables or X macros. Look-up tables are preferable since they are most readable. But in case you need to minimize code repetition, X macros can be useful, although they are a bit hard to read.
Look-up table version:
typedef struct // I'm not sure about the exact types used here
{
volatile uint8_t* port;
uint8_t pin;
} button_t;
const button_t BUTTON[20] =
{
{&GPIOB, 8},
{&GPIOB, 9},
...
};
for(uint8_t i = 0; i<20; i++)
{
HAL_GPIO_ReadPin(BUTTON[i].port, BUTTON[i].pin);
}
X macro version:
#define BUTTON_LIST \
/*button port pin */ \
X(1, B, 8) \
X(2, B, 9) \
X(3, B, 4) \
X(4, B, 5) \
X(5, C, 13)
...
for(uint8_t button = 0; button<20; button++)
{
#define X(button, port, pin) HAL_GPIO_ReadPin(GPIO##port, GPIO_PIN_##pin);
BUTTON_LIST
#undef X
}
I know that this will not work, but hopefully you can see what I'm trying to do
#if ASSIGN_ALLOWED
#define MAYBE_SKIP_REST_OF_LINE
#else
#define MAYBE_SKIP_REST_OF_LINE ; //
#endif
char str[80] MAYBE_SKIP_REST_OF_LINE = "Hello\n";
long array[3] MAYBE_SKIP_REST_OF_LINE = { 7,8,9 };
int x MAYBE_SKIP_REST_OF_LINE = 3;
//...many many more similar lines...
Is there a way to do this such that it works?
Sure:
#ifdef ASSIGN_ALLOWED
#define OPTIONAL_INITIALISER(x) = x
#else
#define OPTIONAL_INITIALISER(x)
#endif
char str[80] OPTIONAL_INTIALISER("Hello\n");
#define ARRAY_INIT { 7,8,9 }
long array[3] OPTIONAL_INITIALISER(ARRAY_INIT);
#undef ARRAY_INIT
int x OPTIONAL_INITIALISER(3);
Any initialisers containing commas, like for array in the example, will need to be expanded from their own macro, like ARRAY_INIT in the above. If your compiler supports C99 varargs macros, then you can instead do it in a cleaner way:
#ifdef ASSIGN_ALLOWED
#define OPTIONAL_INITIALISER(...) = __VA_ARGS__
#else
#define OPTIONAL_INITIALISER(...)
#endif
char str[80] OPTIONAL_INTIALISER("Hello\n");
long array[3] OPTIONAL_INITIALISER({ 7,8,9 });
int x OPTIONAL_INITIALISER(3);
As comments are filtered out in the preprocessor run, i dont think so
It would depend on how the preprocessor worked with comments and macros. If it strips comments after macro expansion then your smooth sailing, but otherwise it might not work simply due to the preprocessor implementation.
You could try this? (it would be messy though).
#define MAYBE_SKIP(code) code
#define MAYBE_SKIP(code) /* code */
The preprocessor strips out commented sections. Try running
gcc -E source.c
This will run the preprocessor on your code but will not actually compile it, allowing you to see what happens after macro expansion. You should notice that all the comments are gone from any expanded macros.
I'm tidying up some older code that uses 'magic numbers' all over the place to set hardware registers, and I would like to use constants instead of these numbers to make the code somewhat more expressive (in fact they will map to the names/values used to document the registers).
However, I'm concerned that with the volume of changes I might break the magic numbers. Here is a simplified example (the register set is more complex):
const short mode0 = 0;
const short mode1 = 1;
const short mode2 = 2;
const short state0 = 0;
const short state1 = 4;
const short state2 = 8;
so instead of :
set_register(5);
we have:
set_register(state1|mode1);
What I'm looking for is a build time version of:
ASSERT(5==(state1|mode1));
Update
#Christian, thanks for the quick response, I'm interested on a C / non-boost environment answer too because this is driver/kernel code.
NEW ANSWER :
In my original answer (below), I had to have two different macros to support assertions in a function scope and at the global scope. I wondered if it was possible to come up with a single solution that would work in both scopes.
I was able to find a solution that worked for Visual Studio and Comeau compilers using extern character arrays. But I was able to find a more complex solution that works for GCC. But GCC's solution doesn't work for Visual Studio. :( But adding a '#ifdef __ GNUC __', it's easy to choose the right set of macros for a given compiler.
Solution:
#ifdef __GNUC__
#define STATIC_ASSERT_HELPER(expr, msg) \
(!!sizeof \ (struct { unsigned int STATIC_ASSERTION__##msg: (expr) ? 1 : -1; }))
#define STATIC_ASSERT(expr, msg) \
extern int (*assert_function__(void)) [STATIC_ASSERT_HELPER(expr, msg)]
#else
#define STATIC_ASSERT(expr, msg) \
extern char STATIC_ASSERTION__##msg[1]; \
extern char STATIC_ASSERTION__##msg[(expr)?1:2]
#endif /* #ifdef __GNUC__ */
Here are the error messages reported for STATIC_ASSERT(1==1, test_message); at line 22 of test.c:
GCC:
line 22: error: negative width in bit-field `STATIC_ASSERTION__test_message'
Visual Studio:
test.c(22) : error C2369: 'STATIC_ASSERTION__test_message' : redefinition; different subscripts
test.c(22) : see declaration of 'STATIC_ASSERTION__test_message'
Comeau:
line 22: error: declaration is incompatible with
"char STATIC_ASSERTION__test_message[1]" (declared at line 22)
ORIGINAL ANSWER :
I do something very similar to what Checkers does. But I include a message that'll show up in many compilers:
#define STATIC_ASSERT(expr, msg) \
{ \
char STATIC_ASSERTION__##msg[(expr)?1:-1]; \
(void)STATIC_ASSERTION__##msg[0]; \
}
And for doing something at the global scope (outside a function) use this:
#define GLOBAL_STATIC_ASSERT(expr, msg) \
extern char STATIC_ASSERTION__##msg[1]; \
extern char STATIC_ASSERTION__##msg[(expr)?1:2]
There is an article by
Ralf Holly that examines different options for static asserts in C.
He presents three different approaches:
switch case values must be unique
arrays must not have negative dimensions
division by zero for constant expressions
His conclusion for the best implementation is this:
#define assert_static(e) \
do { \
enum { assert_static__ = 1/(e) }; \
} while (0)
Checkout boost's static assert
You can roll your own static assert if you don't have access to a third-party library static assert function (like boost):
#define STATIC_ASSERT(x) \
do { \
const static char dummy[(x)?1:-1] = {0};\
} while(0)
The downside is, of course, that error message is not going to be very helpful, but at least, it will give you the line number.
#define static_assert(expr) \
int __static_assert(int static_assert_failed[(expr)?1:-1])
It can be used anywhere, any times.
I think it is the easiest solution.
Before usage, test it with your compiler carefully.
Any of the techniques listed here should work and when C++0x becomes available you will be able to use the built-in static_assert keyword.
If you have Boost then using BOOST_STATIC_ASSERT is the way to go. If you're using C or don't want to get Boost
here's my c_assert.h file that defines (and explains the workings of) a few macros to handle static assertions.
It's a bit more convoluted that it should be because in ANSI C code you need 2 different macros - one that can work in the area where you have declarations and one that can work in the area where normal statements go. There is a also a bit of work that goes into making the macro work at global scope or in block scope and a bunch of gunk to ensure that there are no name collisions.
STATIC_ASSERT() can be used in the variable declaration block or global scope.
STATIC_ASSERT_EX() can be among regular statements.
For C++ code (or C99 code that allow declarations mixed with statements) STATIC_ASSERT() will work anywhere.
/*
Define macros to allow compile-time assertions.
If the expression is false, an error something like
test.c(9) : error XXXXX: negative subscript
will be issued (the exact error and its format is dependent
on the compiler).
The techique used for C is to declare an extern (which can be used in
file or block scope) array with a size of 1 if the expr is TRUE and
a size of -1 if the expr is false (which will result in a compiler error).
A counter or line number is appended to the name to help make it unique.
Note that this is not a foolproof technique, but compilers are
supposed to accept multiple identical extern declarations anyway.
This technique doesn't work in all cases for C++ because extern declarations
are not permitted inside classes. To get a CPP_ASSERT(), there is an
implementation of something similar to Boost's BOOST_STATIC_ASSERT(). Boost's
approach uses template specialization; when expr evaluates to 1, a typedef
for the type
::interslice::StaticAssert_test< sizeof( ::interslice::StaticAssert_failed<true>) >
which boils down to
::interslice::StaticAssert_test< 1>
which boils down to
struct StaticAssert_test
is declared. If expr is 0, the compiler will be unable to find a specialization for
::interslice::StaticAssert_failed<false>.
STATIC_ASSERT() or C_ASSERT should work in either C or C++ code (and they do the same thing)
CPP_ASSERT is defined only for C++ code.
Since declarations can only occur at file scope or at the start of a block in
standard C, the C_ASSERT() or STATIC_ASSERT() macros will only work there. For situations
where you want to perform compile-time asserts elsewhere, use C_ASSERT_EX() or
STATIC_ASSERT_X() which wrap an enum declaration inside it's own block.
*/
#ifndef C_ASSERT_H_3803b949_b422_4377_8713_ce606f29d546
#define C_ASSERT_H_3803b949_b422_4377_8713_ce606f29d546
/* first some utility macros to paste a line number or counter to the end of an identifier
* this will let us have some chance of generating names that are unique
* there may be problems if a static assert ends up on the same line number in different headers
* to avoid that problem in C++ use namespaces
*/
#if !defined( PASTE)
#define PASTE2( x, y) x##y
#define PASTE( x, y) PASTE2( x, y)
#endif /* PASTE */
#if !defined( PASTE_LINE)
#define PASTE_LINE( x) PASTE( x, __LINE__)
#endif /* PASTE_LINE */
#if!defined( PASTE_COUNTER)
#if (_MSC_VER >= 1300) /* __COUNTER__ introduced in VS 7 (VS.NET 2002) */
#define PASTE_COUNTER( x) PASTE( x, __COUNTER__) /* __COUNTER__ is a an _MSC_VER >= 1300 non-Ansi extension */
#else
#define PASTE_COUNTER( x) PASTE( x, __LINE__) /* since there's no __COUNTER__ use __LINE__ as a more or less reasonable substitute */
#endif
#endif /* PASTE_COUNTER */
#if __cplusplus
extern "C++" { // required in case we're included inside an extern "C" block
namespace interslice {
template<bool b> struct StaticAssert_failed;
template<> struct StaticAssert_failed<true> { enum {val = 1 }; };
template<int x> struct StaticAssert_test { };
}
}
#define CPP_ASSERT( expr) typedef ::interslice::StaticAssert_test< sizeof( ::interslice::StaticAssert_failed< (bool) (expr) >) > PASTE_COUNTER( IntersliceStaticAssertType_)
#define STATIC_ASSERT( expr) CPP_ASSERT( expr)
#define STATIC_ASSERT_EX( expr) CPP_ASSERT( expr)
#else
#define C_ASSERT_STORAGE_CLASS extern /* change to typedef might be needed for some compilers? */
#define C_ASSERT_GUID 4964f7ac50fa4661a1377e4c17509495 /* used to make sure our extern name doesn't collide with something else */
#define STATIC_ASSERT( expr) C_ASSERT_STORAGE_CLASS char PASTE( PASTE( c_assert_, C_ASSERT_GUID), [(expr) ? 1 : -1])
#define STATIC_ASSERT_EX(expr) do { enum { c_assert__ = 1/((expr) ? 1 : 0) }; } while (0)
#endif /* __cplusplus */
#if !defined( C_ASSERT) /* C_ASSERT() might be defined by winnt.h */
#define C_ASSERT( expr) STATIC_ASSERT( expr)
#endif /* !defined( C_ASSERT) */
#define C_ASSERT_EX( expr) STATIC_ASSERT_EX( expr)
#ifdef TEST_IMPLEMENTATION
C_ASSERT( 1 < 2);
C_ASSERT( 1 < 2);
int main( )
{
C_ASSERT( 1 < 2);
C_ASSERT( 1 < 2);
int x;
x = 1 + 4;
C_ASSERT_EX( 1 < 2);
C_ASSERT_EX( 1 < 2);
return( 0);
}
#endif /* TEST_IMPLEMENTATION */
#endif /* C_ASSERT_H_3803b949_b422_4377_8713_ce606f29d546 */
Try:
#define STATIC_ASSERT(x, error) \
do { \
static const char error[(x)?1:-1];\
} while(0)
Then you can write:
STATIC_ASSERT(a == b, a_not_equal_to_b);
Which may give you a better error message (depending on your compiler).
The common, portable option is
#if 5 != (state1|mode1)
# error "aaugh!"
#endif
but it doesn't work in this case, because they're C constants and not #defines.
You can see the Linux kernel's BUILD_BUG_ON macro for something that handles your case:
#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
When condition is true, this becomes ((void)sizeof(char[-1])), which is illegal and should fail at compile time, and otherwise it becomes ((void)sizeof(char[1])), which is just fine.
Ensure you compile with a sufficiently recent compiler (e.g. gcc -std=c11).
Then your statement is simply:
_Static_assert(state1|mode1 == 5, "Unexpected change of bitflags");
#define MODE0 0
#define MODE1 1
#define MODE2 2
#define STATE0 0
#define STATE1 4
#define STATE2 8
set_register(STATE1|STATE1); //set_register(5);
#if (!(5==(STATE1|STATE1))) //MY_ASSERT(5==(state1|mode1)); note the !
#error "error blah blah"
#endif
This is not as elegant as a one line MY_ASSERT(expr) solution. You could use sed, awk, or m4 macro processor before compiling your C code to generate the DEBUG code expansion of MY_ASSERT(expr) to multiple lines or NODEBUG code which removes them for production.