Why does Go\'s main function include an infinite loop and a seemingly pointless nil dereference?

Unraveling the Infinite Loop in Go's Main Function

Understanding the Issue

In the main function of Go's runtime library (src/runtime/proc.go), there exists a seemingly purposeless infinite for loop. This peculiar snippet of code:

<code class="go">    exit(0)
    for {
        var x *int32
        *x = 0
    }</code>

has puzzled many developers. Why does this loop exist and what purpose does it serve?

The Unexpected Behavior

In systems with a memory protection unit (MPU), assigning 0 to a protected memory region (such as (int)(nil) or *x in the code snippet above) causes a segmentation fault, effectively stopping the program. However, in systems without an MPU, writing 0 to a zero memory address results in no discernible effect.

Breaking the Loop

To address this issue, the developers of Go added the infinite for loop. Its primary purpose is to halt the program when the exit call fails. Since the exit function did not successfully terminate the program, it is possible that even a nil dereference may work. And in case that fails too, the loop ensures that the program remains in an idle state.

Unreachable Code

The infinite for loop is considered "unreachable code" in normal circumstances. It is intended to serve as a failsafe mechanism, triggered only when something unexpected occurs. The developers' remarks in the source code capture the essence of its purpose:

"If that loop is ever reached, something has gone badly wrong: the exit call should have caused the program to exit."

Panic and Segmentation Violation

Similar situations arise when a panic is called in Go. Inside src/runtime/panic.go, at the conclusion of func fatalpanic(msgs *_panic), another "unreachable code" snippet exists:

<code class="go">    systemstack(func() {
        exit(2)
    })

    *(*int)(nil) = 0 // not reached</code>

If a panic is triggered during compilation (as depicted in the provided source code), the fatalpanic function is invoked. However, the exit function may fail to terminate the program, prompting the execution of the "unreachable code" section. The nil dereference ((int)(nil) = 0) then triggers a segmentation violation, effectively halting the program.

Conclusion

The seemingly pointless infinite for loop and nil dereference in Go's runtime library serve as safety nets to manage exceptional scenarios where program termination through the exit call fails. They ensure that the program is brought to a standstill even when unexpected conditions arise, safeguarding system stability.

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