Cirrus Logic CS485 Bedienungsanleitung PDF herunterladen (Seite 35)

CS485G Spring 2015 35

(a) Programs deﬁne symbols and reference them:

1 void swap() {...} // exported global identifier

2 extern int myGlobal; // imported global identifier

3 int myGlobal; // also local, so exported, too

4 swap(&myGlobal, &myLocal); // reference identifiers

(b) The compiler uses an internal data structure called the symbol

table to keep track of all identiﬁers.

tion such as type, location, and global/local ﬂag.

(d) The compiler includes the global symbols as part of the object

ﬁle it outputs.

(e) The object ﬁle marks any reference to an imported global as

“dangling”.

(f) The linker resolves dangling references by connecting them to

the proper identiﬁer in another object ﬁle.

(g) The compiler has already resolved references to local symbols.

(h) It’s a link-time error if the linker discovers multiple possible res-

olutions.

(i) If desired, the linker then consults libraries to resolve any still-

dangling references by adding more object ﬁles.

(j) If the linker can resolve all dangling references, the result is an

executable ﬁle that the operating system can load and run.

(k) Otherwise, the result is an object ﬁle that can be used for further

linking steps.

(l) The early Unix convention was to call the executable ﬁle “a.out”;

now it usually has a name without an extension.

6. Shared object ﬁles

(a) If desired, the linker can store its result as a shared object ﬁle

(conventional extension “.so”), which the program can load

into memory dynamically (typically when the program starts)

in such a way that it is shared among all processes that need it.

(b) Libraries are usually shared object ﬁles.

object ﬁle, it leaves it dangling (but resolved); full resolution

happens when the shared object ﬁle is loaded into memory.

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Cirrus Logic CS485 Bedienungsanleitung Seite 35