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I was reading TLDP and noticed that all global variables and functions are declared static. After further reading, I understand that the variables are declared static to reduce namespace pollution.

According to this SO post,

static functions are functions that are only visible to other functions in the same file (more precisely the same translation unit).

Hence declaring functions as static will reduce namespace pollution. But, in the case of variables, according to TLDP:

When a Static variable is modified by a module, all other modules will see the new value.

it would increase namespace pollution. Aren't global static variables also visible to the same translation unit? If so how the above quoted statement true? I seem to be missing something.

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daltonfury42
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1 Answers1

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When a Static variable is modified by a module

I believe you are misinterpreting that sentence. It can be a bit confusing. It is not referring to the C static keyword. It is referring to the C standard's usage of "static storage duration". Refer to section 6.2.4 of the C standard:

1 An object has a storage duration that determines its lifetime. There are three storage durations: static, automatic, and allocated.

....

3 An object whose identifier is declared with external or internal linkage, or with the storage-class specifier static has static storage duration. Its lifetime is the entire execution of the program

That is, "static storage duration" variables include both global variables and variables declared with the C static keyword. In the TLDP article it is referring to the former.

kaylum
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  • In the case of a kernel module, what is a "program"? Is it the entire kernel or is it just the module? – daltonfury42 Oct 05 '16 at 05:33
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    The kernel link stage is more strict than user level programs in terms of global variables. The kernel build prevents accessing any global variables from modules unless they are explicitly marked by calling the `EXPORT_SYMBOL` macro. – kaylum Oct 05 '16 at 05:47
  • Can you tell me why all the global variables need to be declared static? – daltonfury42 Oct 05 '16 at 05:49