Embedded Systems Design - March 2008 - (Page 14)

0308esd.p09to14 2/13/08 7:42 PM Page 14 programming pointers Linkage and storage duration for objects in C and C++. storage class at file scope (C) or specifier namespace scope (C++) auto extern at block scope no linkage; automatic storage normally external linkage, but possibly internal linkage;* static storage same as auto no linkage; static storage same as auto at class scope (only in C++ ) invalid invalid register static none invalid normally external linkage, but possibly internal linkage;* static storage invalid internal linkage; static storage normally external linkage, except that const objects in C++ have internal linkage; static storage invalid external linkage; static storage no linkage; same storage duration as enclosing object *The name has internal linkage if previously declared with internal linkage in a visible declaration. Table 2 With many C compilers, you can place this declaration in a header, include that header in several source ﬁles, compile each source ﬁle, and link them together. The linker will treat one of those tentative deﬁnitions as a deﬁnition and all the other tentative deﬁnitions as declarations referring to that deﬁnition. This behavior is a common extension to the C standard, but may not be portable to all C environments, and doesn’t work in C++. Using C++, you’ll get a link error complaining about a multiply-deﬁned symbol. If you want the code to compile and link in C++ as well as C, you must declare n with the extern speciﬁer, as in: extern int n; both parameter v and local object n have no linkage. You can’t write a declaration for an integer v elsewhere and get it to link to foo’s parameter v. Likewise for n. Thus, the two objects named n in this example are distinct objects. If you add the static storage class speciﬁer to the declaration of n at block scope, you change its storage duration but not its linkage. For example: static int n; int foo(int v) { static int n; } // still internal linkage // still no linkage As before, n has external linkage, but now the declaration is not a deﬁnition—not even a tentative one in C. You can place this declaration in a header, include that header in several source ﬁles, compile each source ﬁle, and link them together. But you’ll still get a link error, unless there’s exactly one deﬁnition for all those declarations to reference. Just one of the source ﬁles must provide a deﬁnition for n. Any one of the following deﬁnitions will do: int n; int n = 0; extern int n = 0; defines two distinct objects named n, both with static storage duration. The n at block scope still has no linkage. There’s more to say about linkage, but this is enough for now. ■ ENDNOTES: 1. Saks, Dan, “Scope regions in C and C++,” Embedded Systems Design, November, 2007, p. 15. Available online at www.embedded.com/columns/programmingpointers/ 202600398. Saks, Dan, “Storage class speciﬁers and storage duration,” Embedded Systems Design, January, 2008, p. 9. Available online at www.embedded.com/columns/programmingpointers/ 205203843. An object declared at block scope without a storage class speciﬁer has automatic storage duration and no linkage. For example, in: static int n; int foo(int v) { int n; } // internal linkage 2. ACKNOWLEDGMENTS: // no linkage My thanks to Steve Adamczyk of the Edison Design Group (www.edg.com), Tom Plum of Plum Hall (www.plumhall.com), and Joel Saks (www.joelsaks.com) for their valuable assistance with this article. 14 MARCH 2008 | embedded systems design | www.embedded.com http://www.embedded.com/columns/programmingpointers/202600398 http://www.embedded.com/columns/programmingpointers/205203843 http://www.embedded.com/columns/programmingpointers/205203843 http://www.plumhall.com http://www.edg.com http://www.joelsaks.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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.