Embedded Systems Design - March 2008 - (Page 9)

0308esd.p09to14 2/13/08 7:42 PM Page 9 BY Dan Saks programming pointers Linkage in C and C++ name declared in C or C++ In my earlier article on the scope may have attributes such as rules,1 I explained the distinction between declarations and deﬁnitype, scope, storage durations, but omitted some details. tion, and linkage. Not every name Those details are pertinent to the has all of these attributes. For examconcept of linkage, so let’s take a ple, a function name has a type, a look at them ﬁrst. scope, and a linkage, but no storage duration. A statement label name DECLARATIONS AND has only a scope. DEFINITIONS An object’s type determines the A declaration associates attributes object’s size and memory address with names. A declaration either inalignment, the values the object can troduces a name into the current have, and the operations that can be translation unit or redeclares a name performed on that object. A funcintroduced by a declaration that aption’s type speciﬁes the function’s peared earlier in the same translaparameter list and return type. tion unit. A declaration for a name A name’s scope is that portion of might also be a deﬁnition. Informala translation unit in which the ly, a deﬁnition is a declaration that name is visible.1 (A translation unit is the source code produced by the not only says “here’s a name,” but Scope determines what preprocessor from a source ﬁle and also “here’s all the information the you can see within a trans- compiler needs to create the code all its included headers.) C supports four different kinds of scope: for that name”. lation unit. Linkage deterﬁle scope, block scope, function C++ lets you declare all sorts of mines what you can see prototype scope, and function things that aren’t valid in C, such as across translation units. scope. C++ generalizes ﬁle scope classes, namespaces, and linkage into namespace scope, and adds speciﬁcations. Not surprisingly, these class scope. features make the distinction between deﬁnitions and other An object’s storage duration determines the lifetime of declarations, as well as the linkage rules, more complicated the storage for that object.2 That is, it determines how and in C++ than in C. However, we don’t really need to considwhen during program execution the storage for that ober all those features to explain linkage—using just functions ject comes and goes. Each object in C and C++ has one of and objects should provide adequate illustration. the following three storage durations: static, automatic, For function declarations that are valid in both C and and dynamic. Only objects have storage duration. EnuC++, the rule for deciding when a function declaration is meration constants, functions, labels, and types don’t. also a deﬁnition is the same for both languages, namely, Which brings us to linkage. A name’s linkage affects that a function declaration is also a function deﬁnition if whether two or more declarations for that name are valid, it includes a brace-enclosed body, as in: and if so, whether they refer to the same entity (such as a int abs(int v) function or an object). Understanding linkage clears up a { number of misconceptions about the behavior of C and return v < 0 ? -v : v; C++ programs. }; A Dan Saks is president of Saks & Associates, a C/C++ training and consulting company. For more information about Dan Saks, visit his website at www.dansaks.com. Dan also welcomes your feedback: e-mail him at dsaks@wittenberg.edu. A function heading without a body, as in: int abs(int v); is just a declaration. www.embedded.com | embedded systems design | MARCH 2008 9 http://www.dansaks.com http://www.embedded.com

Table of Contents Feed for the Digital Edition of Embedded Systems Design - March 2008

Embedded Systems Design - March 2008 Contents #Include Programming Pointers Designing DSP-based Motor Control Using Fuzzy Logic Hardware/Software Verification Enters the Atomic Age Efficient CRC Calculation with Minimal Memory Footprint Programming Your Own Microcontroller Advertising Index Break Points Marketplace

Embedded Systems Design - March 2008

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