Difference Between Dynamic Loading and Dynamic Linking

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Dynamic loading is used to obtain better memory-space utilization. With dynamic loading a routine is not loaded until it is called. All routines are kept on disk in a re-locatable load format. The main program is loaded into memory and is executed. When a routine needs to call another routine, the calling routine first checks whether the other routine has been loaded or not. The re-allocatable linking loader is called to load the desired routine into memory and to update the program's address tables to reflect this change. Then, control is passed to the newly loaded routine. Loading with bad time dynamic link libraries takes a little longer since, the loader has more work to do, but it saves a lot of disk space that would have been wasted with duplicate copies of every module.

The concept of dynamic linking is similar to that of dynamic loading. Rather than loading is being postponed until execution time, linking is postponed. This feature is usually used with system libraries, such as language subroutine libraries. Without this facility, all programs on a system need to have a copy of their language library (or at least the routines referenced by the program) included in the executable image. This requirement wastes both disk space and main memory. With dynamic linking, a stub is included in the image for each library-routine reference. This stub is a small piece of code that indicates how to locate the appropriate memory-resident library routine, or how to load the library if the routine is not already present.

When this stub is executed, it checks whether the needed routine is already in memory. If the routine is not in memory, the program loads it into memory. Either way, the stub replaces itself with the address of the routine, and executes the routine. Thus, the next time that code segment is reached, the library routine is executed directly, incurring on cost for dynamic linking. Under this scheme, all processes that use a language library execute only one copy of the library code. 

Sai Spandana. Kotturi

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