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In Unix operating systems, a soname is a field of data in a shared object file. The soname is a string (typically the filename of the library) which is used as a "logical name" describing the functionality of the object. This is often used to provide version backwards-compatibility information. For instance, if versions 1.0 through 1.9 of the shared library libx provide identical interface, they would all have the same soname, e.g. libx.so.1. If the system only includes version 1.3 of that shared object, libx.so.1.3, the soname field of the shared object tells the system that it can be used to fill the dependency for a binary which was originally compiled using version 1.2.