_exit

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Usually, this symbol is not handled in a special way. However, if it does not exist at all, it is redirected to the entry point of the program. The effect is that constructs of the form

.word _exit-entry_point
resolve to 0 if the symbol is undefined.

The kernel headers of the standard library reference this symbol as the program/library destructor. For kernel programs and libraries, it is called whenever the program exits or the library is unloaded.