Using macros in C to change behavior in other C files - c

Is it possible to define a macro in a C file, and then change behavior of another C file using this definition?
For instance, suppose we have a file named b.c:
#include <stdio.h>
#define ABC
void function() {
printf("b.c\n");
}
And another file named a.c:
#include <stdio.h>
int main() {
#ifdef ABC
printf("b.c defined this\n");
#endif
printf("a.c\n");
}
These files are compiled together. I want the definition of ABC in b.c to influence a.c.
I know I was supposed to be using a header file, but I don't want to do that. Is there another way to achieve this?
In my scenario, b.c is a sort of sub-application that will complement the behavior of a.c, and it should also be able to override behavior in there.
Is there an alternative to macros that can achieve this?

Use global variables or use callbacks. The "best" solution depends on precisely how much flexibility you need.

As others said, you can't do it.
Your options:
1. Don't use macros. Generally, it's good advice. There are some things that can't be done without macros, but normally you don't see them. You'll get better code without macros.
2. Define the macro in a header file, which is included by both C files.

It doesn't work the way you describe.
#define ABC only defines ABC within the scope of the b.c file, and does not impact other files (unless b.c is #included).
The only way to define ABC within the scope of a.c file without header files is to define ABC using the command line arguments of the compiler: gcc -D ABC ...

No this is not possible this way. But you may instruct compiler to define a symbol using command line options

Related

is it possible to have only header file in C without source file

I would like to write a C library with fast access by including just header files without using compiled library. For that I have included my code directly in my header file.
The header file contains:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#ifndef INC_TEST_H_
#define INC_TEST_H_
void test(){
printf("hello\n");
}
#endif
My program doesn't compile because I have multiple reference to function test(). If I had a correct source file with my header it works without error.
Is it possible to use only header file by including code inside in a C app?
Including code in a header is generally a really bad idea.
If you have file1.c and file2.c, and in each of them you include your coded.h, then at the link part of the compilation, there will be 2 test functions with global scope (one in file1.c and the other one in file2.c).
You can use the word "static" in order to say that the function will be restricted so it is only visible in the .c file which includes coded.h, but then again, it's a bad idea.
Last but not least: how do you intend to make a library without a .so/.a file? This is not a library; this is copy/paste code directly in your project.
And when a bug is found in your "library", you will be left with no solution apart correcting your code, redispatch it in every project, and recompile every project, missing the very point of a dynamic library: The ability to "just" correct the library without touching every program using it.
If I understand what you're asking correctly, you want to create a "library" which is strictly source code that gets #incuded as necessary, rather than compiled separately and linked.
As you have discovered, this is not easy when you're dealing with functions - the compiler complains of multiple definitions (you will have the same problem with object definitions).
You have a couple of options at this point.
You could declare the function static:
static void test( void )
{
...
}
The static keyword limits the function's visibility to the current translation unit, so you don't run into multiple definition errors at link time. It means that each translation unit is creating its own separate "instance" of the function, leading to a bit of code bloat and slightly longer build times. If you can live with that, this is the easiest solution.
You could use a macro in place of a function:
#define TEST() (printf( "hello\n" ))
except that macros are not functions and do not behave like functions. While macro-based "libraries" do exist, they are not trivial to implement correctly and require quite a bit of thought. Remember that macro arguments are not evaluated, they're just expanded in place, which can lead to problems if you pass expressions with side effects. The classic example is:
#define SQUARE(x) ((x)*(x))
...
y = SQUARE(z++);
SQUARE(z++) expands to ((z++)*(z++)), which leads to undefined behavior.
Separate compilation is a Good Thing, and you should not try to avoid it. Doing everything in one source file is not scalable, and leads to maintenance headaches.
My program do not compiled because I have multiple reference to test() function
That is because the .h file with the function is included and compiled in multiple C source files. As a result, the linker encounters the function with global scope multiple times.
You could have defined the function as static, which means it will have scope only for the curent compilation unit, so:
static void test()
{
printf("hello\n");
}

Calling function with no declaration in header

Say I have two header and implementation files, A and B.
B:
#include "B.h"
void funcFromB(); //prototype
...
void funcFromB()
{
...
}
A:
#include "B.h"
void funcFromB(); //prototype
...
funcFromB(); //will this work correctly?
Will calling funcFromB() from A work correctly if the function is not defined in header of B (B.h)?
Yes, the function will work correctly, provided B.o is linked to A. The .h files and .c files have nothing inherently to do with each other. For clarity purposes, they SHOULD, but nothing forces them to do so. There is no requirement for .h files at all, but coders find them useful to organize information and avoid repeating oneself.
The code fragment in your question both compiles ("works") and smells
The question is clearly a bit in the grey area, but here's my interpretation of it and the solution.
This is your B.h
void funcFromB();
i.e. you have the function signature in the header file.
I am not sure what exactly your A.h file has, but I assume it is not really important for our discussion, so we will leave it to that.
Now your B.c file will contains the implementation of the method funcFromB(). In that case, you don't have to do
void funcFromB(); //prototype
The above line is not required. When you #include your header file, the method signature will be available. So this is how your B.c file will look like.
#include "B.h"
void funcFromB()
{
...
}
Now, for you to be able to use this function in A.c, all you need to do is
#include "B.h"
funcFromB(); //This will work correctly
Here, the function will get the signature of the function from B.h, and the implementation is will get from B.c at compile time. All you need to do is, just call the function.
Now, coming to what have we done here. Well, what we wanted to do was to organise our code in a better way. For that, we wanted to use a function, defined in a different file and use it in another file. Now, B.c contains the definition of the function. B.h contains the method signature. If you want to use the function in A.c, you just need to tell this compilation unit, that how does the function look like. And when you link the files together later during compile time, the method implementation will be found out by the compiler on it's own.
Cheers.
It will work,given you define your funFromB() function, preferably in the B.c file; In that case, just #include the B.h file containing the funcFromB() prototype for the preprocessor in your A.c and B.c(this is the one where you will be defining the function) files , and you should be good to go. No need to re-define the function in A.c, nor re-declare the prototype in (any) .c file, as long as it is declared in the b.h file, and every .c file using the function includes the corresponding (B).h file.
PS: Using headers is a great way of improving readability, so I wouldn't skip that part, by declaring the prototype in the .c file. If you want to avoid Headers, you could still just use one single file, let's call it 'master.h', where you would declare ALL the prototypes used by ALL .c files, and include the master.h in all your.c files.
Headers tend to help keeping a better overview of how your files are linked to another, and establish the proper dependencies

What is the conventional way of defining macros in main.c that are needed in other header files?

For example I have a can.h header file that contains
#ifdef MC
extern int speed;
// many more variables and function prototypes
#endif
EDIT: I would want an option for the user to define MC that enables such variable and function prototypes. It is to be defined in C main alongside include directives (e.g. #define MC) however, other header files cannot link to main.c. Instead, i ended up defining such macros in can.h header itself. All i can think is writing a main.h too where can.h will include main.h. are there other ways to go around this problem?
It's not entirely clear to me what you are trying to do, but perhaps you want this:
/* in file can.h */
extern int speed;
and then
/* in file main.c */
#include "can.h"
int speed;
The header can.h just declares the existence of speed so that other modules can refer to it as an external symbol. The storage for this object is then allocated in main.c, when you write the the definition of speed.
You can either create a configuration header that has all the macros that you want defined and #include it in every header, or you can define configuration macros on the command line.
Use #include - Then you get those bits that you require
Inclusion is simply file content insertion by the preprocessor, anything declared or defined prior to inclusion will be defined in the include file. So:
#define MC
#include "can.h"
While that achieves what you have asked, I would not necessarily recommend it however, and might question the design.
You can have such macro added at your project level.
Such macro are also known as Preprocessor definitions. Visual studio provides such definition in project settings/configurations..
So you can have it added there and compile the solution for including mentioned declarations or remove it if you don't want to include it.
Adding the screen shot.

Best practice for using includes in C

I am learning C and I am unsure where to include files. Basically I can do this in .c or in .h files:
Option 1
test.h
int my_func(char **var);
test.c
#include <stdio.h>
#include "test.h"
int my_func(char **var) {printf("%s\n", "foo");}
int main() {...}
Option 2
test.h
#include <stdio.h>
int my_func(char **var);
test.c
#include "test.h"
int my_func(char **var) {printf("%s\n", "foo");}
int main() {...}
With option 2 I would only need to include test.h in whatever .c file I need the library. Most of the examples I see use option 1.
Are there some general rules when to do what or is this a question of personal preferences?
Don't use includes, you don't need.
I'd choose something like "Option 1". Why "something like" ? Because I'd create a separate file for the main and I'd keep all declaraions inside the .h and all definitions inside the corresponding .c.
Of course, both options are valid.
If you want to include only one header in your main, you can just create a header file, containing only includes - that's a common practice. This way, you can include only one header, instead of several.
I tend to prefer Option 1, as cyclic dependencies will come and bite you very quickly in option 2, and reducing the input size is the best way to guarantee faster compile times. Option 2 tends towards including everything everywhere, whether you really need it or not.
That said, it might be best to experiment a little with what works for structuring your projects. Hard and fast rules tend to not apply universally to these kinds of questions.
both options are correct. the C standard allows both solutions
All C standard headers must be made such that they can be included several times and in any order:
Standard headers may be included in any order; each may be included
more than once in a given scope, with no effect different from being
included only once
(From Preprocessor #ifndef)
I don't think that there is a universal rule (from a "grammatical" point of view, both your options are correct and will work). What is often done is to include in the .h file the headers you need for the library (as in your option 1), either because you'll need them when working with the library (thus avoiding always including the same set of header files in your .c files, which is more error prone), or because they are mentioned in the .h file itself (e.g., if you use a int32_t as type in the function prototypes of the .h files, you will of course need to include <stdint.h> in the .h file).
I prefer to use includes in c file.
If your program is getting bigger you might forgot to include something in one header file, but it is included in one other you use.
By including them in c-file you won't lose includes, while editing other files.
I prefer Option 1. I want to know what I used in my project, and in much time, Option 1 works more effective than Option 2 in time and efficiency.
There is no rule specifying you have following a particular fashion. Even if you include / not include in test.c file, it is not going to bother much, provided you include it in test.h file and include that test.h file in test.c. Hope you are clear with that.
This is because, you have preprocessor directives like #ifndef, #define, #endif. They are called Include guards. These are used in the inbuild header files. However, when you include a file written by you, either go with your option 2, or use include guards to be safe.
The include guards work as follows.
#ifndef ANYTHING
#define ANYTHING
..
..
..
#endif
So when you include for the first time, ANYTHING is not yet defined. So ifndef returns true, then ANYTHING gets defined, and so...on.. But the next time if you include the same file ( by mistake) ifndef would return a false since ANYTHING is now defined and so the file would not be included at all. This protection is necessary to avoid duplicate declarations of variable present in the header file. That would give you a compilation error.
Hope that helps
Cheers

possible flaws in 'including *.c files' style C programming

I came across some codes in the following way
//file.c
#include <stdlib.h>
void print(void){
printf("Hello world\n");
}
and
//file main.c
#include <stdio.h>
#include "file.c"
int main(int argc, char *argv[]){
print();
return EXIT_SUCCESS;
}
Is there any flaw in this kind of programming style? I am not able to make out the flaw although I feel so, because somewhere I read that separating the implementation into *.h and *.c file helps compiler check for consistency. I don't understand what is meant by consistency.
I would be deeply thankful for some suggestions.
--thanks
Nothing prevents you from including .c files. However, separating declaration (in .h files) and implementation (and .c files) and then including only .h files has several advantages :
Compile time. Your declaration usually changes less than your implementation. If you include only .h files, and makes a change in your implementation (in the .c file), then you only have to recompile one .c file, instead of all the files which include the modified file.
Readability and management of interfaces. All your declarations can be checked in a single glance at the (usually) small .h file whereas the .c file is filled with lines and lines of code. Moreover it helps you determine which file see which functions and variables. For example, to avoid having a global variable included where you don't want it.
It's a common expectation that the compiler should compile .c files. .h files are not directly given to the compiler. They are usually only included within .c files.
Thus, your code is expected to compile by something like:
gcc main.c file.c
rather than only gcc main.c. That command would fail in the linking stage as it sees duplicate symbols.
You're going to have problems if you include file.c in more than one source code file which combine to make a library/executable, since you'll have duplicate method implementations. The above strikes me as a poor means of sharing/reusing code, and is not to be recommended.
It is not uncommon to include data in another file if it is more convenient to separate it from the code. For example, XPM or raw BMP data in a char array could be included to embed an image in the program in this way. If the data is generated from another build step then it makes sense to include the file.
I would suggest using a different file extension to avoid confusion (e.g. *.inc, *.dat, etc.).
Yes, this is permitted.
Using this is an advanced topic.
It slows down development compile time (cheaper to compile only what is necessary).
It speeds up deployment compile time (all files are out of date).
It allows the compiler to inline functions across module boundaries.
It allows a trick to control exported symbols from a library while keeping it modular.
It might confuse the debugger.
There is nothing wrong from a C language perspective with including a .c file in a program. The C language cares not about the extension of the file, C++ in fact often omits an extension on certain header files to avoid conflicts.
But from the perspective of a programmer, yeah this is odd. Most programmers will operate on the assumption that a .c file will not be included and this can cause problems via incorrect assumptions. It's best to avoid this practice. If you find you must use it, it's a sign of poor design.
In the .h files you should place function prototypes. For example, in your code you should have:
//file.h
void print(void);
//file.c
void
print(void)
{
printf("Hello world\n");
}
//file main.c
#include <stdio.h>
#include "file.h"
int main(int argc, char *argv[]){
print();
return EXIT_SUCCESS;
}
There are only two reasons that I know of for including C files (and which make sense):
inline functions which are non trivial, but that's really a matter of style
share implementation of private (static) functions by including the same file in several other files. That's actually the only way to do it in a purely platform independent way (but toolchain specific tricks like hidden attribute for gcc, etc... are much better if they are available)
The flaws:
you compile several times the same code
if not used sparingly, it quickly leads to multiple defined symbols for public symbols, in a way which is difficult to debug (include files which include other files...)
It is bad style, but one other reason to do it is that it can be used a part of a trick using the ## token concatenation operator to do a kind of poor man's templating in C.
Note that this fairly evil, isn't recommended, and will produce code that's hard to debug and maintain, but you can do something like the following:
mytemplate.c:
MyTemplateFunction_ ## MYTYPE(MYTYPE x)
{
// function code that works with all used TYPEs
}
main.c:
#define MYTYPE float
#include "mytemplate.c"
#undef MYTYPE
#define MYTYPE int
#include "mytemplate.c"
#undef MYTYPE
int main(int, char*)
{
float f;
int i;
MyTemplateFunction_float(f);
MyTemplateFunction_int(i);
return 0;
}
The evils of macros can be exacerbated:
file1.c
#define bottom arse
file2.c
int f()
{
int arse = 4;
bottom = 3;
printf("%d", arse);
}
main.c
#include "file1.c"
#include "file2.c"
void main()
{
f();
}
Indeed a convoluted example. But usually you wouldn't notice it because a macro's scope is the file it is in.
I did actually get this bug, I was importing some lib code into a new project and couldn't be bothered to write the Makefile, so I just generated an all.cpp which included all the sources from the library. And it didn't work as expected due to macro pollution. Took me a while to figure it out.
It's fine for the programs of this length.

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