Hi all,

I just experienced an odd issue that I didn't think could happen. In my program I have several .c files. There are 0 external global variables defined. However I just created two modules that shared a variable name for internal use.

This caused some serious problems though with my logic. I was under the assumption that the variables are isolated from each other.

Is this just not the case?

Both the modules were using a pointer called "head". They had different structure types.

Thank you.

use modifier static:

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NevemTeve gave you the solution. That may be all you need, especially for just this time.

I will try to give you the answer, so you won't be confused over this next time.

That is apparently incorrect. I expect you mean you didn't use the keyword extern. But a global variable is defined every time you define a variable at module scope without the keyword static.

Your intent may have been internal use, but you defined each of those variables for global use.


Variables of the same name defined at function scope are isolated by scope. The lack of "static" at function scope does not make a variable global. (It makes a variable "automatic" meaning it acts as if it were created on entry to its scope and destroyed on exit, so its value is likely lost when exiting the scope and reentering it later.)

Variables defined at module scope without "static" are global. Module "scope" effectively acts like a scope only when the keyword static is used.

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Thank you for that explanation. Your correct in that I did not explicitly define them as external. My understanding of external vs static vs auto is limited. I was under the assumption that to access a variable from another module it had to be defined as external. I also did not know that for the variable to be persistent that it required being defined with the static keyword.

I hope that doesn't mean I confused you, so let me try again to summarize:

In a function scope, you have either auto (not persistent) or static (persistent).

In a module scope, you have either global (persistent) or static (also persistent).

Also, in case this detail was misleading: While you are in any kind of inner scope an auto variable may not be accessible, but its value persists. The value is only non persistent when you go outside its scope, not when you nest into some other scope that just makes it temporarily inaccessible.