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A program that is configured to use a library can use either static-linking or dynamic-linking.Historically, libraries could only be static. [4] For static-linking (), the library is effectively embedded into the programs executable file, while for dynamic-linking the library can be loaded at runtime from a shared location, such as system files.
Like static libraries, import libraries for DLLs are noted by the .lib file extension. For example, kernel32.dll, the primary dynamic library for Windows's base functions such as file creation and memory management, is linked via kernel32.lib. The usual way to tell an import library from a proper static library is by size: the import library is ...
Static avoids DLL Hell or more generally dependency hell and therefore can simplify development, distribution and installation. Another trade-off is memory used to load the library. With static linking, a smart linker only includes the code that is actually used, but for a dynamic library, the entire library is loaded into memory.
Therefore, if a library is called upon many times by different programs, the same functions in that library are duplicated in several places in the system's memory. Using shared, dynamic libraries means that, instead of linking each file to its own copy of a library at compilation time and potentially wasting memory space, only one copy of the ...
Dynamic linking offers three advantages: Often-used libraries (for example the standard system library) need to be stored in only one location, not duplicated in every single binary. If a library is upgraded or replaced, all programs using it dynamically will immediately benefit from the corrections. Static builds would have to be re-linked first.
Dynamic loading is a mechanism by which a computer program can, at run time, load a library (or other binary) into memory, retrieve the addresses of functions and variables contained in the library, execute those functions or access those variables, and unload the library from memory.
A filename typically starts with lib, and ends with .a for a static library or .so for a shared object (dynamically linked library). For example, libfoo.a and libfoo.so . Often, symbolic link files are used to manage versioning of a library by providing a link file named without a version that links to a file named with a version.
Some global variables (e.g. arrays of string literals, virtual function tables) are expected to contain an address of an object in data section respectively in code section of the dynamic library; therefore, the stored address in the global variable must be updated to reflect the address where the DLL was loaded to.