Static library vs shared library

1. Static library
   Before the shared library appeared , Public functions exist in the form of static libraries , Package multiple object files of the general function module , The program that uses it only needs to specify this library file when linking , The linker will extract the functional code used from this library and copy it to the target program , There is no need to recompile these general function codes every time .
   Static library embodies a good idea of modularization , But with the development of computer industry , The static library gradually exposes two serious problems

• Large disk and memory space . Although the static library accelerates the compilation speed , It improves the efficiency of collaboration between different departments , But in the program linked by each static library , Will save a copy of the referenced general function code , And at runtime , Each copy will occupy the corresponding physical memory .
• Upgrading the version of the library is very troublesome , Once the public library is modified , Every program that references him needs to be relinked with the new version of the Library . In a scenario where libraries and applications are maintained by different companies or organizations , The upgrade work has become extremely complex , If there is bug Repair , All applications using this library need to be upgraded separately

2. Shared library
   To solve these two problems , Shared library technology came into being .
   First , Applications that use shared libraries when compiling links , Do not copy the functions in the library to the target file , Only one reference is recorded in the target file , Mark the library function referred to , Until the program runs Only dynamic connector can locate the position of function code , Therefore, the volume of the generated executable program is significantly reduced .
   secondly , Each shared library has only one copy in physical memory , Multiple applications will map the same executable file in their virtual address space , Therefore, it can save objective memory space .
   This working mode of shared library greatly facilitates the upgrading of the Library , When the shared library releases a new version , Users only need to upgrade this shared library , All applications that use this library can automatically get the features in the new library or bug Repair , Instead of upgrading each application separately .