What does `err.(*os.PathError)` mean in Go?

Understanding err.(*os.PathError)

While navigating the effective Go language documentation, a specific line caught your attention: err.(os.PathError). This line raises the question of what exactly err.(os.PathError) signifies in Go.

Explanation

err.(*os.PathError) is an example of using the error interface. The error interface defines a single method, Error(), which returns a string representation of the error. Many types in Go implement this interface.

In this particular context, err is the error returned by os.Create(), which implements the error interface. By using the type assertion err.(os.PathError), the code is attempting to convert err to the concrete type os.PathError. This allows access to additional information about the error beyond just the error message.

os.PathError is a specialized error type returned by the os package when encountering certain errors related to file paths. By converting to an os.PathError, you gain access to additional properties specific to file system errors, such as the system call error code (Err).

In the example provided, the code is handling the specific case of ENOSPC (no space left on the device) by checking if err is an *os.PathError and if the contained system call error code matches ENOSPC. If both conditions are met, the code triggers specific recovery actions.

Conclusion

Type assertions, like err.(*os.PathError), allow you to access and handle specific error types and their associated information. They provide a powerful mechanism for error handling and error-specific logic in Go applications.

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