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Declaration of global variables in one header file

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Read-Only Author Tobias Ilschner Posted 21-Jun-2005 12:34 GMT Toolset C166	Declaration of global variables in one header file Tobias Ilschner Hello everyone, I'm trying to split the C source code of one file into nearly 10 different. It does not work properly due to global variables declared in one header file but used in more than one C files. Compiling of such a project works fine but the linking errors L104 and L118 occur. My question: How can I prevent this errors without creating one header file for every C file? My idea is the following, but it is complicated and additionally unsure if it works: #ifdef CPVAR extern unsigned char global_var1; extern unsigned int global_var2; extern unsigned long global_var3; #elseif unsigned char global_var1; unsigned int global_var2; unsigned long global_var3; #define CPVAR #endif Maybe one of the experts knows a less complicated opportunity to prevent (these) linker errors.

Read-Only Author Hans-Bernhard Broeker Posted 21-Jun-2005 13:03 GMT Toolset C166	RE: Declaration of global variables in one header file Hans-Bernhard Broeker Your problem is that this global variables declared in one header file but used in more than one C files is not what you're actually doing. You define the symbols in the header file, instead of just declaring them. If this distinction doesn't mean anything to you, get a proper C textbook and look it up. The trick you're trying to apply won't work, because it only affects a single translation unit. It is quite similar to one that would work. You have to declare the variables in all translation units that use them (by doing it in the header), and define them in exactly one of the source files.

Read-Only Author Tobias Ilschner Posted 21-Jun-2005 13:26 GMT Toolset C166	RE: Declaration of global variables in one header file Tobias Ilschner Unfortunately a proper C book ist not available at the moment... Do you mean with extern it is called declaration and without definition? However, does the trick work if it is included in the one and only header file?

Read-Only Author erik malund Posted 21-Jun-2005 13:43 GMT Toolset C166	RE: Declaration of global variables in one header file erik malund #elseif elseif what try #else instead. Erik

Read-Only Author Jay Daniel Posted 21-Jun-2005 13:55 GMT Toolset C166	RE: Declaration of global variables in one header file Jay Daniel Tobias, In short, yes... when you use "extern" that's considered a declaration. While many will decry what you're trying to do with global variables to be heresy, there is a fairly easy way to do it. Make a file called something obvious like GLOBALS.H so that everyone who has to maintain this code will know of your treason. In it, do something like this: #ifdef DEFINEGLOBALSHERE #define GLOBAL #else #define GLOBAL extern #endif GLOBAL unsigned char global_var1; GLOBAL unsigned int global_var2; GLOBAL unsigned long global_var3; Then you need to pick a file in which the variables will actually be defined. Main.c is probably a good choice. To do this, on the first line of main.c (before you #include your global.h file), put: #define DEFINEGLOBALSHERE What this will do is automatically replace the tag GLOBAL before all these variables with "extern" for every file in which the DEFINEGLOBALSHERE tag isn't defined. For the one in which is is defined, it will just delete the GLOBAL tag. Note that this only works with plain vanilla definitions. Initializations won't work. Using the _at_ keyword won't work. So, you make some tradeoffs for this simplicity. Good luck and I hope that helps.

Read-Only Author A.W. Neil Posted 21-Jun-2005 18:25 GMT Toolset C166	RE: Declaration of global variables in one header file A.W. Neil "Note that this only works with plain vanilla definitions. Initializations won't work. Using the _at_ keyword won't work." It is left as an exercise for the student to devise a scheme that would support initialisers and the _at_ keyword...

Read-Only Author Jay Daniel Posted 21-Jun-2005 20:03 GMT Toolset C166	RE: Declaration of global variables in one header file Jay Daniel yes... we can only post so much in the way of preprocessor kung-fu in any one thread. :)

Read-Only Author C B Posted 21-Jun-2005 14:02 GMT Toolset C166	RE: Declaration of global variables in one header file C B Tobias, I try to desrcibe with in simple examples: this is how you define the variables in every C file: in file1.c unsigned char global_var1; in file2.c unsigned int global_var2; in file3.c unsigned long global_var3; To use these variables in other files you need to use external declarations. Like this: header.h extern unsigned char global_var1; extern unsigned int global_var2; extern unsigned long global_var3; Of course you need to include the file header.h in all C files where you need to know the global variables. Note, that you also can declare variables externally in other C files. CB

Read-Only Author erik malund Posted 21-Jun-2005 15:12 GMT Toolset C166	RE: Declaration of global variables in one header file erik malund While many will decry what you're trying to do with global variables to be heresy Of course it is heresy to use a technique that guarantees that the "internal" and external is the same. Making programs that automatically are correct may cost someone a job fixing the bugs your other method has introduced. If a method that makes programming safer> makes it "impure" so friggin what. Is good, maintainable = code not more important than "pure" code. Erik

Read-Only Author Jay Daniel Posted 21-Jun-2005 18:07 GMT Toolset C166	RE: Declaration of global variables in one header file Jay Daniel erik, Your fervor to rebuke me seems to have overpowered your ability to convey your point. Of course it is heresy to use a technique that guarantees that the "internal" and external is the same. Guarantees that the "internal" and external what are the same? Linkage? All I've done is define a kludged linkage specifier. As you well know, none of those variables can or will have both internal and external linkage in the same compilation unit. Making programs that automatically are correct may cost someone a job fixing the bugs your other method has introduced. How does my method affect the "automatic"-ness of anything? If someone misunderstands and tries to put a #define DECLARGLOBALSHERE in another compilation unit, link will fail. If someone doesn't understand and fails to put the definition in ANY unit, link will fail. As such, I'm not sure how this is any less automatic than someone knowing to extern a variable they want to use. If a method that makes programming safer> makes it "impure" so friggin what. Is good, maintainable = code not more important than "pure" code. I'm not sure where the discussion of the relative "purity" of anything came up? Can you elaborate on what you mean?

Read-Only Author erik malund Posted 21-Jun-2005 18:47 GMT Toolset C166	RE: Declaration of global variables in one header file erik malund sorry, about missing the preview, the highlights in my previous post are all wrong. I was fighting an intruder at the same time. Your fervor to rebuke me seems to have overpowered your ability to convey your point. rebuke???, I agreed with you. I supported your statement "While many will decry what you're trying to do with global variables to be heresy" which, as shown by your post you do not. I'm not sure where the discussion of the relative "purity" of anything came up? Can you elaborate on what you mean? Those that "decry what you're trying to do with global variables to be heresy" are the same that like yo use the expression "not real C" about anything that is legible. Erik

Read-Only Author Jay Daniel Posted 21-Jun-2005 19:44 GMT Toolset C166	RE: Declaration of global variables in one header file Jay Daniel erik, Wow... I guess I totally missed your point. My apologies. Yes, I think when you use that particular method, that things end up being fairly readable, and I personally like it better than having global variables smattered at the top of each C file. Nonetheless, I do find that things I write tend to devolve into ugliness when I'm doing C51 code with this method. For instance, I almost always need to have some variables located with the _at_ keyword, which leads to the following sort of construct: #ifdef DEFINEGLOBALSHERE #define GLOBAL unsigned char somespecialvar _at_ 0x0200; #else #define GLOBAL extern extern unsigned char somespecialvar; #endif GLOBAL unsigned char somenormalvar; and so forth. Then, if I'm unlucky enough to need to initialize a global where it's defined, I have to do it similarly. Anyway... I guess my point is just that it's useful, but will be extremely ugly if you're not careful with it.

Read-Only Author Jon Ward Posted 22-Jun-2005 09:06 GMT Toolset C166	RE: Declaration of global variables in one header file Jon Ward Take a look at the following knowledgebase article for several other ideas about how to do this: http://www.keil.com/support/docs/1868.htm Jon

Read-Only Author Tobias Ilschner Posted 23-Jun-2005 11:54 GMT Toolset C166	RE: Declaration of global variables in one header file Tobias Ilschner Problem is solved. Thank you guys!

Read-Only Author William McGinley Posted 15-Jul-2005 05:21 GMT Toolset C166	RE: Declaration of global variables in one header file William McGinley "Managing software engineers is like trying to herd cats" -Anonymous silicon valley manager

Read-Only Author erik malund Posted 15-Jul-2005 15:13 GMT Toolset C166	RE: Declaration of global variables in one header file erik malund "Managing software engineers is like trying to herd cats" But what if the result is the cat's miauw? Erik

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