Creating and Using Libraries

Switching from Archive to Shared Libraries

More commonly, programmers may take advantage of undocumented linker behavior to minimize the size of routines copied into the a.out files from archive libraries. This is no longer necessary if all libraries are shared.

Although it is impossible to characterize the new resolution rules exactly, the following rules always apply:

•If a symbol is defined in two shared libraries, the definition used at run time is the one that appeared first, regardless of where the reference was.

•The linker treats shared libraries more like object files.

As a consequence of the second rule, programs that call wrapper libraries may become larger. (A wrapper library is a library that contains alternate versions of C library functions, each of which performs some bookkeeping and then calls the actual C function. For example, each function in the wrapper library might update a counter of how many times the actual C routine is called.) With archive libraries, if the program references only one routine in the wrapper library, then only the wrapper routine and the corresponding routine from the C library are copied into the a.out file. If, on the other hand, a shared wrapper library and archive C library are specified, in that order, then all routines that can be referenced by any routine in the wrapper library are copied from the C library. To avoid this, link with archive or shared versions for both the wrapper library and C library, or use an archive version of the wrapper library and a shared version of the C library.

Absolute Virtual Addresses

Writing code that relies on the linker to locate a symbol in a particular location or in a particular order in relation to other symbols is known as making an implicit address dependency. Because of the nature of shared libraries, the linker cannot always preserve the exact ordering of symbols declared in shared libraries. In particular, variables declared in a shared library may be located far from the main program's virtual address space, and they may not reside in the same relative order within the library as they were linked. Therefore, code that has implicit address dependencies may not work as expected with shared libraries.

An example of an implicit address dependency is a function that assumes that two global variables that were defined adjacently in the source code will actually be adjacent in virtual memory. Since the linker may

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HP 9000 specifications

The Kenwood HP 9000 is a cutting-edge home appliance designed for culinary enthusiasts seeking to elevate their cooking experience. This powerful kitchen machine stands out for its multi-functionality, combining the roles of a food processor, blender, and mixer into one sleek unit.

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