I have some testing code in Fortran which basically looks like
#define test1 0
#define test2 0
#define test3 0
...
#if test1
call test1()
#endif
#if test2
call test2()
#endif
#if test3
call test3()
#endif
...
At compilation, i want to change those values using -Dtest1=1.
I've seen some answers (like here) where they say you need to put
#ifndef test1
#define test1 0
#endif
But i have a lot of those and i would prefer not to add 40 lines of definition code.
Will compilation command -D flag take priority over my hard definition without the if clause?
EDIT : So i just tested it (which i should have done anyway before asking...) and compiler -D flag does not take priority over my own definition and just pulls out a warning . So any way for it to take priority and always use the compiler flags without passing through ifndef clauses?
-D effectively adds a #define at the beginning, before reading the input. So it would be just like adding an additional #define in your source code, which would naturally produce an error (or warning) about redefining the macro.
If you want to avoid that error, use the #ifndef idiom, just like everyone else does.
Related
Not including the #ifndef guard to prevent multiple header file inclusions, does not throw compile error. Why is that?
// main.c
#include <stdio.h>
#include "declare.h"
#include "declare.h" //Shouldn't I get compiler error here
int main(){
printf("Hello World\n");
}
// declare.h
#define a 1 //just this define in declare.h file
Command used to compile: gcc main.c -std=c89
Including a header multiple times is not an error, either with or without guards.
Guards prevent you from redefining objects and functions, when you do so.
But you haven't done that. You've just redefined a macro with the same value it had before. Which is fine.
You can test this by just compiling the following file:
#define a 1
#define a 1
It's fine.
[C89: 6.8.3]: [..] An identifier currently defined as a macro without use of lparen (an object-like macro) may be redefined by another #define preprocessing directive provided that the second definition is an object-like macro definition and the two replacement lists are identical. [..]
Start putting more complex stuff in that header and you'll see a problem.
The C preprocessor macro for concatenation (##) does not seem to work on a Mac using gfortran. Using other Fortran compilers on other systems works so I am looking for a workaround for gfortran. I have to use the ## to create many variables so I can't do without them.
Example code:
#define CONCAT(x,y) x##y
program main
integer, parameter:: CONCAT(ID,2) = 3
print*,"Hello", ID_2
end program main
Compilation error with gfortran on MAC
gfortran m.F90 -o m
m.F90:5.23:
integer, parameter:: ID##2 = 3
1
Error: PARAMETER at (1) is missing an initializer
## doesn't work in gfortran (any OS, not just Mac) because it runs CPP in the traditional mode.
According to this thread the gfortran mailing list the correct operator in the traditional mode is x/**/y, so you must distinguish between different compilers:
#ifdef __GFORTRAN__
#define CONCAT(x,y) x/**/y
#else
#define CONCAT(x,y) x ## y
#endif
Others (http://c-faq.com/cpp/oldpaste.html) use this form, which behaves better when a macro passed to the CONCAT (via Concatenating an expanded macro and a word using the Fortran preprocessor):
#ifdef __GFORTRAN__
#define PASTE(a) a
#define CONCAT(a,b) PASTE(a)b
#else
#define PASTE(a) a ## b
#define CONCAT(a,b) PASTE(a,b)
#endif
The indirect formulation helps to expand the passed macro before the strings are concatenated (it is too late after).
I'd like to have some sort of verbose debug printing in arduino that can be enabled/disabled by a flag. For example I'd like the ability to do something like
#define VERBOSE
#define VERBOSE_PRINT(text) #ifdef VERBOSE Serial.println(text); #endif
Later in code:
VERBOSE_PRINT("Doing something");
if VERBOSE is defined then I should get stuff out over the serial port and if it's not defined then that code won't be compiled. Unfortunately this isn't working. I get the error: "error: '#' is not followed by a macro parameter". So I'm wondering what's the best way to get an optionally compiled print (or anything for that matter). Of course I could manually write out the #ifdefs, but I'd like it streamlined so that it doesn't take up a ton space and so I don't have to write it out each time I'd like to use it. Is a function w/ the #ifdef inside the function the best way to do this?
#define VERBOSE
#ifdef VERBOSE
#define VERBOSE_PRINT(str) Serial.println(str)
#else
#define VERBOSE_PRINT(str)
#endif
VERBOSE_PRINT("Doing something");
I just saw this thread, describing how to add conditional macros:
Conditional value for a #define
but in my case I am defining a function within the condition.
#if TARGET_IPHONE_SIMULATOR
#define doSomething(){\
\\ does something
}\
#else
#define doSomething(){\
\\ does something else
}\
#endif
This does work, except I is causing gcc compiler to throw this warning:
"doSomething" redefined
This is the location of the previous arguments
Is there any workaround to help getting rid of the warnings?
UPDATE:
So I tried including the condition inside my definition:
#define doSomething(){\
#if TARGET_IPHONE_SIMULATOR
\\ do something
#else
\\ do something else
#endif
}\
but that throws an error:
error: '#' is not followed by a macro parameter.
I found the answer to my question here.
Conclusion:
you cannot include #ifdef etc... inside #define, because there should only be one pre-processing directive per line.
So although we can break the line with a backslash '\' this helps writing readable multiline macros,
but the preprocessor will see it as one line:
#define doSomething(){ #if TARGET_IPHONE_SIMULATOR ... #endif }
Which throws this error:
error: '#' is not followed by a macro parameter.
That makes sense, so I will have to rethink my implementation.
There is a quirk in your thinking which is by analogy/ extension. doSomething() has to be viewed as a function-like macro. As such its definition is ambivalent. Zoom out and see below:
doSomething() {
#if TARGET_IPHONE_SIMULATOR
// conditionally compiled code
#else
// platform-specific code
#endif
}
Also, this might address the error you received:# and ## have special purposes inside macro definitions. # is used to surround a macro parameter with double quotes. ## is used to concatenate two macro parameters. Example:#define ABC(X) #XABC(hello) results in "hello".#define XYZ(X,Y) X##YXYZ(O,K) results in OK.
Note that this feature is (possibly) unique to ANSI C.
Also, why would you be using a macro like this? Might a function work better for you?
One option to consider is creating a condition in a macro that will resolve at compile time. Consider the following:
If I would like to call a different function based on the value of 'c' as a pre-processor action, I can define a macro that checks the value of 'c' statically.
#define AorB(c) ((c>0) ? (Do_A(c)) : (Do_B(c)))
Then if you configure a level of optimization that removes branches that are never reachable, it should strip out which ever case wasn't performed. This may not exactly be what you were looking for.
How do I at compile time undefine a compiler macro using gcc. I tried some compile args to gcc like -D but I can't get to see the "not defined" message.
Thanks
#include <iostream>
#define MYDEF
int main(){
#ifdef MYDEF
std::cout<<"defined\n";
#else
std::cout<<"not defined\n";
#endif
}
You can use the -U option with gcc, but it won't undefine a macro defined in your source code. As far as I know, there's no way to do that.
You should wrap the MYDEF definition in a preprocessor macro, the presence of which (defined on the command line) would then prevent MYDEF from being defined. A bit convoluted to be sure but you can then control the build in the way you want from the command line (or Makefile). Example:
#ifndef DONT_DEFINE_MYDEF
#define MYDEF
#endif
Then from the command line when you don't want MYDEF:
gcc -DDONT_DEFINE_MYDEF ...
http://gcc.gnu.org/onlinedocs/gcc-4.4.2/gcc/Preprocessor-Options.html#Preprocessor-Options
The -U options seemed like what you could have needed... but then again you can't override a definition contained in your source code without resorting to more preprocessor directives.
You can resort to filtering source code and give this back to gcc for compilation, like this pseudo code:
grep -v "define MYDEF" yourFile.c | gcc -o yourFile.o -xc -
Hope it helps.
The code use case is not right. As I see, you have hard coded #define in the file. If compiler initially assumes MYDEF undefined, it will define it once it start processing the file.
You should remove the line #define MYDEF. And I hope your test case will work, if you pass MYDEF to -D and -U.
Here is one possibility that doesn't completely cover your use case but which I found to be helpful in my case.
If your MYDEF were #defined in a separate header file #included from the .c file you could force the definition of the #include guard macro with the -D option (thus preventing the MYDEF #definition) then either actively #define (still with the -D option) MYDEF to something else or just leave it undefined.
It is clear that anything else defined in the header file would also be missing but this was for me a solution to forcedly undefine a macro without changing the third-party code.