Embedded Systems Design - March 2008 - (Page 12)

0308esd.p09to14 2/13/08 7:42 PM Page 12 programming pointers can place this function declaration in a header, include that header in several source ﬁles, compile each ﬁle, and link the compiled modules together. Subsequently, all calls to that foo from any of those source ﬁles will call the same function. A function declared at ﬁle or namespace scope with the keyword static has internal linkage. That is, when you declare: static int foo(int n); Written either way, these declarations are valid in both C and C++, and foo has internal linkage. However, just because you can do this doesn’t mean you should. Writing function declarations that differ only in their storage class speciﬁers is rarely, if ever, necessary, and can be confusing. If a single identifier has both internal and external linkage in the same translation unit, the resulting program has undefined behavior. That is, it’s an error, but you might not get an error message from the compiler all calls to foo in that translation unit will be to a Written either way, these declarations are function foo deﬁned in that same translation unit. If valid in both C and C++ . . . however, just the translation unit contains calls to foo, yet function foo isn’t deﬁned in that translation unit, the because you can do this doesn’t mean program won’t compile or link, even if there’s a deﬁyou should. nition for function foo in another translation unit. Table 1 illustrates how C and C++ determine the linkage for a function from the storage class speciﬁer or linker. For example, reversing the order of the extern and static declarations from the previous examand the scope of the function’s declaration. For example, ple, as in: the row labeled extern shows that a function declared extern doesn’t always have external linkage. The row laextern int foo(int n); // external linkage beled none shows that a function declared without a stor age class speciﬁer at ﬁle, namespace, or block scope behaves as if it were declared with the keyword extern. static int foo(int n); // undeﬁned behavior For example, a single translation unit can contain the following declarations: yields undeﬁned behavior. All the more reason not to static int foo(int n); // internal linkage write things like this. extern int foo(int n); // still internal The extern declaration could be in an inner scope, as in: static int foo(int n); // internal linkage LINKAGE FOR OBJECTS Table 2 illustrates how C and C++ determine linkage and storage duration for an object from the storage class speciﬁer and the scope of the object’s declaration. It shows, for example, that a non-const object declared at ﬁle or namespace scope without a storage class speciﬁer, such as: int n; int bar(int m) { extern int foo(int n); // still internal } has external linkage and static storage duration. This declaration is also a deﬁnition in C++ but only a tentative deﬁnition in C. Linkage for functions in C and C++. storage class at file scope (C) or specifier namespace scope (C++) auto extern register static at block scope invalid normally external linkage, but possibly internal linkage* invalid invalid same as extern at class scope (only in C++ ) invalid invalid invalid internal linkage external linkage none Table 1 invalid normally external linkage, but possibly internal linkage* invalid internal linkage same as extern *The name has internal linkage if previously declared with internal linkage in a visible declaration. 12 MARCH 2008 | embedded systems design | www.embedded.com http://www.embedded.com

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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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