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In my Kernel module, I have two very short C source files (the main file and a daq.c and daq.h file). The header file contains a number of variables and functions. When I compile the functions normally, by declaring the variable in the daq.h file:

volatile uint32_t *gpio;

I receive the following error:

error In function `.LANCHOR1': daq.c:(.bss+0x50): multiple definition of `gpio'

This error is only solved by declaring the variable as static, as follows:

static volatile uint32_t *gpio;

The kernel module works fine but in this case I receive the following warning as a static declaration creates a separated copy on every file in which it is included:

warning: ‘gpio’ defined but not used [-Wunused-variable]: static volatile uint32_t *gpio;

I've tried to declare it extern or just simply declaring the variable as uint32_t *gpio instead of declaring it static but then I get a:

WARNING: "gpio" undefined!

followed by an "out-of-tree definition" message in the dmesg log when the module is inserted (and the module is not loaded).

So, in this case, how is the best way to proceed in order to declare the variable? Is it correct to declare it as:

static volatile uint32_t *gpio;

and just omit the "defined but not used" warning given by the compiler? I don't think just "omitting" the warnings is a good practice, especially when it comes to kernel modules.

Thanks in advance.

drsambernardo
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    In the header file you need to *declare* a variable using `extern` keyword. [Multiple definition of ... linker error](https://stackoverflow.com/questions/17764661/multiple-definition-of-linker-error) Then you may **define** the variable in the `.c` file (without `static`). – Tsyvarev Nov 23 '19 at 16:57
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    In a header file variables should be `extern`. In a `.c` file they should not be. – S.S. Anne Nov 23 '19 at 17:19

1 Answers1

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Your question is not very clear because a full context is missing; but anyway I think some advice can be given.

First, your sources will be used together with other files; those files define things you have to be aware of, otherwise things like multiple definition of 'gpio' come out. In this case (gpio), you must decide if you want to interact with the "other" gpio variable - if not, you must use another name (it is not mandatory, but it is better).

Second, you must understand how the C compiler works, especially if you interface with the kernel, which adds some mechanisms. In general, you don't want to define variables in a header (.h) file; instead, you do declare them, together with data types, macros, and function prototypes. In the source files (.c) you #include headers in order to use the declarations found in them.

Variables are defined in .c files, and made available to other modules through header files, if this is wanted. Otherwise, make them static and do not mention them in the header.

The difference between declaration and definition is this: a declaration tells the compiler "you will encounter (perhaps) this name, somewhere, which has the following properties"; a definition instead means "I create this name, with these properties, and this thing is exactly here.".

When more than a single piece of software are used together, often happens that one piece does something needed in another one... declaration are the way to let these pieces work together.

Hope this helps a little.

linuxfan says Reinstate Monica
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  • Your answer was pretty useful because of its clarity! I took the static variables from the daq.h file and placed them (without the "static") into the daq.c file and it worked fine! Thank you very much! – drsambernardo Nov 26 '19 at 11:18