I pass a macto during compilation:
% gcc -DIDENT="abcd" app.c
What is the right way to check during compilation the macro? For example the following works, but throws warning:
#ifdef IDENT == "abcd"
printf("abcd\n");
#endif
warning: extra tokens at end of #ifdef directive.
#ifdef tests whether a symbol is defined, not what the value is, so is valid only as
#ifdef SYMBOL
#if expression will test whether an expression is true, however is limited to integers only.
For more information, see the gcc preprocessor manual:
https://gcc.gnu.org/onlinedocs/cpp/If.html
https://gcc.gnu.org/onlinedocs/cpp/Ifdef.html#Ifdef
#ifdef in typical use is just about _whether the symbol is defined or not. It's not about its value, it's whether it's defined/exists.
#define IDENT
#ifdef IDENT {yields true}
Vs.
//#define IDENT {not defined}
#ifdef IDENT {yields false because line above is commented out
If there is a code applicable on more than 1 project / variant than we use pre-processor switches (compiler switches) to enable or disable the code. For example if MACRO1, MACRO2 and MACRO3 are preprocessor directives or compiler switches in a project than
#if (defined MACRO1 || defined MACRO2)
/* Do something */
#elif (defined MACRO2 && defined MACRO3)
/* Do something */
#else
/* Do something */
#endif
This conditional #if statement yields MISRA warning 12.6 as
"boolean expression required for operator: '!'; the -strong(B,...) option can help provide Boolean-by-enforcement"
Can someone please tell me if there is an alternate way to write this conditional statement or how to justify it?
I have the same problem as described by Jonathon Reinhart here:
Temporarily disable gcc warning on redefinition
That is because I have to use thrid party libraries (C only) which throws tons of warnings like this
Warning "__always_inline" redefined [enabled by default]
What I want is something like this:
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-W???"
#include "someheader.h"
#include "otherheader.h"
#pragma GCC diagnostic pop
Is there a way to disable warnings by gcc which are enabled by default with a
#pragma GCC diagnostic ignored
EDIT:
here is the block causing the warning (file: compiler.h):
#if defined(__CC_ARM)
# define __always_inline __forceinline
#elif (defined __GNUC__)
# define __always_inline inline __attribute__((__always_inline__))
#elif (defined __ICCARM__)
# define __always_inline _Pragma("inline=forced")
#endif
I fixed it by undefining all lines where __always_inline was defined. :-(
Thanks Jasen for helping!
I have some macros that are defined based on compiler flags. I'm trying to decide whether I would rather have the macro defined as (void)0 or have it undefined and cause a compile time error.
i.e.
#ifdef DEBUG
#define PRINTF(...) printf(__VA_ARGS__)
#else
#define PRINTF(...) (void)0
#endif
int main(void) {
...
PRINTF("something");
...
}
vs.
#ifdef DEBUG
#define PRINTF(...) printf(__VA_ARGS__)
#endif
int main(void) {
...
#ifdef DEBUG
PRINTF("something");
#endif
...
}
I'm not sure which technique I prefer. On one hand wrapping every PRINTF statement with #ifdef's would be ugly. On the other hand it would be nice to know at compile time if I've called a function that doesn't really work in the context.
I think the deciding factor will be whether or not having the (void)0 macros is going to affect the size of the executable.
When the code is compiled, what happens to the (void)0's? If PRINTF is defined as (void)0, does that mean the executable is going to contain some sort of (void)0 instruction or will it be completely ignored?
(void) 0;
is an expression statement with no side-effect. Any sane implementation will optimize this statement out (what else an implementation could do with such a statement?).
Having (void) 0 as a macro definition is endorsed by the C Standard as it appears in (C11) 7.2p1 for assert macro definition if NDEBUG is defined:
#define assert(ignore) ((void)0)
Note that defining:
#define PRINTF(...) (void)0
instead of
#define PRINTF(...)
has an advantage. In the first case, you have an expression (like a function that returns no value) and so it is usable for example in a comma expression or in a conditional expression.
For example:
// Comma expression
printf("test"), PRINTF("Hi Dennis");
// Conditional expression
test-expr ? perror("Hello") : PRINTF("world");
This two expression statements are only valid with the former PRINTF definition (with (void) 0).
It'll be completely ignored, you can confirm this by looking at the assembly output (gcc -S will generate file.s, the asm output), compare with and without the (void)0 line and see that it is completely the same.
A half way decent compiler will optimise away dead (unreachable) code, so you can:
#ifdef DEBUG
#define PRINTF(...) if (1) { printf(__VA_ARGS__) ; }
#else
#define PRINTF(...) if (0) { printf(__VA_ARGS__) ; }
#endif
which has the big advantage of allowing the compiler to check the debug code, no matter whether you are working with/without your DEBUG turned on -- which reduces the risk of ending up with painful teeth marks in your backside.
I regularly use object-like preprocessor macros as boolean flags in C code to turn on and off sections of code.
For example
#define DEBUG_PRINT 1
And then use it like
#if(DEBUG_PRINT == 1)
printf("%s", "Testing");
#endif
However, it comes a problem if the header file that contains the #define is forgotten to be included in the source code. Since the macro is not declared, the preprocessor treats it as if it equals 0, and the #if statement never runs.
When the header file is forgotten to be included, non-expected, unruly behaviour can occur.
Ideally, I would like to be able to both check that a macro is defined, and check that it equals a certain value, in one line. If it is not defined, the preprocessor throws an error (or warning).
I'm looking for something along the lines of:
#if-def-and-true-else-throw-error(DEBUG_PRINT)
...
#endif
It's like a combination of #ifdef and #if, and if it doesn't exist, uses #error.
I have explored a few avenues, however, preprocessor directives can't be used inside a #define block, and as far as I can tell, there is no preprocessor option to throw errors/warnings if a macro is not defined when used inside a #if statement.
This may not work for the general case (I don't think there's a general solution to what you're asking for), but for your specific example you might consider changing this sequence of code:
#if(DEBUG_PRINT == 1)
printf("%s", "Testing");
#endif
to:
if (DEBUG_PRINT == 1) {
printf("%s", "Testing");
}
It's no more verbose and will fail to compile if DEBUG_PRINT is not defined or if it's defined to be something that cannot be compared with 1.
as far as I can tell, there is no preprocessor option to throw errors/warnings if a macro is not defined when used inside a #if statement.
It can't be an error because the C standard specifies that behavior is legal. From section 6.10.1/3 of ISO C99 standard:
After all replacements due to macro expansion and the defined unary
operator have been performed, all remaining identifiers are replaced with the pp-number
0....
As Jim Balter notes in the comment below, though, some compilers (such as gcc) can issue warnings about it. However, since the behavior of substituting 0 for unrecognized preprocessor tokens is legal (and in many cases desirable), I'd expect that enabling such warnings in practice would generate a significant amount of noise.
There's no way to do exactly what you want. If you want to generate a compilation failure if the macro is not defined, you'll have to do it explicitly
#if !defined DEBUG_PRINT
#error DEBUG_PRINT is not defined.
#endif
for each source file that cares. Alternatively, you could convert your macro to a function-like macro and avoid using #if. For example, you could define a DEBUG_PRINT macro that expands to a printf call for debug builds but expands to nothing for non-debug builds. Any file that neglects to include the header defining the macro then would fail to compile.
Edit:
Regarding desirability, I have seen numerous times where code uses:
#if ENABLE_SOME_CODE
...
#endif
instead of:
#ifdef ENABLE_SOME_CODE
...
#endif
so that #define ENABLE_SOME_CODE 0 disables the code rather than enables it.
Rather than using DEBUG_PRINT directly in your source files, put this in the header file:
#if !defined(DEBUG_PRINT)
#error DEBUG_PRINT is not defined
#endif
#if DEBUG_PRINT
#define PrintDebug([args]) [definition]
#else
#define PrintDebug
#endif
Any source file that uses PrintDebug but doesn't include the header file will fail to compile.
If you need other code than calls to PrintDebug to be compiled based on DEBUG_PRINT, consider using Michael Burr's suggestion of using plain if rather than #if (yes, the optimizer will not generate code within a false constant test).
Edit:
And you can generalize PrintDebug above to include or exclude arbitrary code as long as you don't have commas that look like macro arguments:
#if !defined(IF_DEBUG)
#error IF_DEBUG is not defined
#endif
#if IF_DEBUG
#define IfDebug(code) code
#else
#define IfDebug(code)
#endif
Then you can write stuff like
IfDebug(int count1;) // IfDebug(int count1, count2;) won't work
IfDebug(int count2;)
...
IfDebug(count1++; count2++;)
Yes you can check both:
#if defined DEBUG && DEBUG == 1
# define D(...) printf(__VA_ARGS__)
#else
# define D(...)
#endif
In this example even when #define DEBUG 0 but it is not equal to 1 thus nothing will be printed.
You can do even this:
#if defined DEBUG && DEBUG
# define D(...) printf(__VA_ARGS__)
#else
# define D(...)
#endif
Here if you #define DEBUG 0 and then D(1,2,3) also nothing will be printed
DOC
Simply create a macro DEBUG_PRINT that does the actual printing:
#define DEBUG_PRINT(n, str) \
\
if(n == 1) \
{ \
printf("%s", str); \
} \
else if(n == 2) \
{ \
do_something_else(); \
} \
\
#endif
#include <stdio.h>
int main()
{
DEBUG_PRINT(1, "testing");
}
If the macro isn't defined, then you will get a compiler error because the symbol is not recognized.
#if 0 // 0/1
#define DEBUG_PRINT printf("%s", "Testing")
#else
#define DEBUG_PRINT printf("%s")
#endif
So when "if 0" it'll do nothing and when "if 1" it'll execute the defined macro.