Undefined symbols on Runtime that cause no Error on Compile/Link Time

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http://stackoverflow.com – I have been searching for this a bit on Google, and there are also some topics on StackOverFlow but they don't answer the main question: Why sometimes, under GCC on LINUX (and I mean Linux since on MinGW it doesn't happen with the SAME code), an application compiles and links correctly with other libraries, without any errors and then, on execution time, undefined symbols are thrown? Why wasn't this undefined symbol thrown during compilation/linking time? What is the reason for the linker to accept the libraries and the code even though that such function has not been defined? Why only durin (HowTos)