Difference between polling and interrupt method for serial communication

Polling Method

1. CPU Usage: Polling involves continuously checking the status of the serial port to see if data is available. This constant checking uses CPU cycles, even when no data is present, leading to higher CPU utilization.

2. Predictability: Polling provides predictable timing since checks are done at regular intervals. However, this predictability comes at the cost of constant CPU engagement.

3. Suitability: Polling can be resource-efficient in systems with low data rates or where the communication interface is always expected to have data when checked.

轮询方法

1. CPU 使用率：轮询涉及不断检查串行端口的状态以查看是否有数据可用。即使没有数据，这种持续检查也会使用 CPU 周期，导致较高的 CPU 利用率。

2. 可预测性：轮询提供可预测的时间，因为检查是定期进行的。然而，这种可预测性是以持续 CPU 使用为代价的。

3. 适用性：在数据速率低或通信接口始终在检查时都有数据的系统中，轮询可以是资源高效的方法。

Interrupt-Driven Method

1. CPU Efficiency: In an interrupt-driven approach, the CPU is free to perform other tasks and is only interrupted when necessary (e.g., when new data arrives or needs to be sent). This leads to more efficient CPU usage, as it isn't occupied with continuous checking.

2. Responsiveness: Interrupts can provide quicker responsiveness to events compared to polling, as the CPU can immediately respond to incoming data.

3. Complexity: The interrupt-driven method might introduce more complexity in terms of setup and handling, but this is often offset by the gains in efficiency and responsiveness.

4. Suitability: This method is preferable in multi-tasking systems, real-time applications, or scenarios where data transmission is sporadic or unpredictable.

中断驱动方法

1. CPU 效率：在中断驱动方法中，CPU 可以自由地执行其他任务，并且只在必要时（例如，当有新数据到达或需要发送时）中断。这导致更高效的 CPU 使用，因为它不需要持续检查。

2. 响应能力：与轮询相比，中断可以更快地响应事件，因为 CPU 可以立即响应传入的数据。

3. 复杂性：中断驱动方法可能会引入更多设置和处理的复杂性，但这往往可以通过效率和响应能力的显著提升来抵消。

4. 适用性：在多任务系统、实时应用程序或数据传输不规律或不可预测的情况下，这种方法更为合适。

Resource Consumption Summary

• CPU Resource: Interrupts generally consume less CPU resources compared to polling, as they don’t require the CPU to continuously check for data.

• System Responsiveness: Interrupts can improve overall system responsiveness, allowing the CPU to manage tasks more efficiently.

• Software Complexity: While interrupts may require more complex software handling, this complexity is often justified by the significant gains in efficiency and performance.

资源消耗总结

• CPU 资源：与轮询相比，中断通常消耗较少的 CPU 资源，因为它不需要 CPU 持续检查数据。

• 系统响应能力：中断可以提高整体系统的响应能力，使 CPU 更有效地管理任务。

• 软件复杂性：虽然中断可能需要更复杂的软件处理，但这种复杂性往往可以通过效率和性能上的显著提升来证明其合理性。

Conclusion

结论

附录

Appendix

Difference and behavior between "O_EXCL" and "O_SYNC" in serial open in C++

在 C++ 中的串行打开中 "O_EXCL" 和 "O_SYNC" 的差异和行为

1. O_EXCL:
• Stands for "Exclusive".
• When combined with O_CREAT, it ensures that the caller must create the file. If the file already exists, the open call will fail.
• This is particularly useful for ensuring that a file is not accidentally overwritten and for implementing locking mechanisms where a file's presence signifies a lock.
• Example use case: Ensuring that a temporary file or a lock file is created only once and not overwritten if it already exists.

2. O_EXCL：
• 代表 "独占"。
• 当与 O_CREAT 结合使用时，它确保调用者必须创建文件。如果文件已经存在，则 open 调用将失败。
• 这在确保文件不会被意外覆盖和实现文件锁定机制（文件的存在表示锁定）方面特别有用。
• 示例用例：确保临时文件或锁文件仅创建一次，如果已经存在则不覆盖。

3. O_SYNC:
• Stands for "Synchronous I/O".
• This flag ensures that all data and metadata (file attributes) modifications are immediately written to the underlying hardware before the open call returns control to the process.
• This is used for applications that require a guarantee that data is physically stored, which is crucial for data integrity and consistency.
• Example use case: Database systems or file systems that need to ensure data is not lost in the event of a power failure or system crash.

4. O_SYNC：
• 代表 "同步 I/O"。
• 此标志确保所有数据和元数据（文件属性）修改在 open 调用返回控制权给进程之前立即写入底层硬件。
• 这用于需要保证数据物理存储的应用程序，这对于数据完整性和一致性至关重要。
• 示例用例：需要确保在断电或系统崩溃时数据不会丢失的数据库系统或文件系统。

1. Using O_EXCL:
• The O_EXCL flag is typically used in conjunction with O_CREAT for file operations, primarily for ensuring that a new file is created. If the file already exists, the open operation fails.
• In the context of opening a serial port, using O_EXCL without O_CREAT would be inappropriate and might cause the open function to fail if the serial device file already exists, which is usually the case.
• If the open function fails due to the inappropriate use of O_EXCL, your program wouldn't have a valid file descriptor for the serial port, leading to a scenario where no data can be received from the serial port.

2. 使用 O_EXCL：
• O_EXCL 标志通常与 O_CREAT 结合使用进行文件操作，主要用于确保创建新文件。如果文件已经存在，则 open 操作将失败。
• 在打开串行端口的上下文中，使用不带 O_CREAT 的 O_EXCL 是不合适的，如果串行设备文件已经存在，可能会导致 open 函数失败，而这通常是情况。
• 如果 open 函数由于不适当使用 O_EXCL 而失败，您的程序将没有有效的串行端口文件描述符，导致无法从串行端口接收任何数据。

3. Switching to O_SYNC:
• The O_SYNC flag is used to ensure synchronous I/O for data integrity. When writing data, this flag ensures that data is physically stored to the medium before moving forward.
• In the context of reading from a serial port, O_SYNC would not have a significant effect because it's more relevant to writing operations. However, using O_SYNC would not cause the open operation to fail, unlike the misuse of O_EXCL.
• When you switch to O_SYNC, since it doesn't cause the open call to fail (as it’s a valid flag for opening serial ports for both read and write operations), you end up with a valid file descriptor. Therefore, you can receive data normally from the serial port.

4. 切换到 O_SYNC：
• O_SYNC 标志用于确保数据完整性的同步 I/O。在写入数据时，此标志确保数据在继续之前已经物理存储到介质中。
• 在从串行端口读取的上下文中，O_SYNC 不会产生重大影响，因为它更相关于写操作。然而，使用 O_SYNC 不会导致 open 操作失败，不像错误使用 O_EXCL。
• 当切换到 O_SYNC 时，由于它不会导致 open 调用失败（因为它是用于打开串行端口进行读写操作的有效标志），因此您会获得有效的文件描述符。因此，您可以正常从串行端口接收数据。