CODEWITHSUNDEEP
cscopelinkage
user3646717
user3646717

Reputation: 1095

What is the difference between scope and linkage?

I tried different websites but I don't get it. Could you explain it in simple english?

Upvotes: 9

Views: 2814

Answers (3)

Chris Bao
Chris Bao

Reputation: 2868

Share what I learned about this issue.

Scope is for the benefit of the compiler, while linkage is for the benefit of the linker.

The compiler uses the scope of an identifier to determine whether or not it's legal to refer to the identifier at a given point in a file. When the compiler translate a source file into object code, it notes which names have external linkage, eventually storing these names in a table inside the object file.

Thus, the linker has access to names with external linkage; names with internal linkage or no linkage are invisible to linker.

(The above text is from book "C programming: A modern approach", which contains an exactly same question).

Upvotes: 0

Paul Ogilvie
Paul Ogilvie

Reputation: 25286

"scope" is a namespace of the compiler; "linkage" is about compiled units.

I explain a bit more: A variable declared in a function has the scope of that function, i.e. it is visible only within that function. A variable declared as static in a source file, can be seen only by the code in that source file (and all included files!). Variables can also have global scope: they can be referred to in a source file, but not declared (allocated) in that source file but declared in another source file.

In stead of "source file" we should say "compilation unit" as it is the C source file being compiled, plus all included files. Scope refers to everything the compiler can "see" in a compilation unit. These are namespaces.

After compilation of a project there are a number of object files, one for each compile unit. Each may refer to variables used that are not declared in the compile unit. The linker must now resolve these references between object files: linkage.

This also holds for functions.

Upvotes: 5

DoxyLover
DoxyLover

Reputation: 3484

Keep reading on your page (http://msdn.microsoft.com/en-us/library/teta4z44.aspx). This is talking about visibility of objects between translation units (source files). It first talks about "internal linkage": objects defined as static, unique to the translation unit but available throughout.

Next it talks about "external linkage": a like-level object not declared static. These are shared between translation units.

Finally, "no linkage": an object such as a variable within a function, not declared extern, which is unique to that scope.

If you follow the links on the bottom of the page, it's all explained.

Upvotes: 1

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