In the go language, channel is a very important concept. Channels provide a way to safely pass data between different goroutines. By using channels, we can avoid multiple goroutines to securely access the shared memory space, thereby reducing the probability of a program's race condition.
We know that when using a channel, the sender needs to write the data to the channel, and the receiver then reads the data from the channel. However, when the data in the channel has been received, how to ensure the normal closing of the channel? In this article, we will discuss how to close a channel and what to consider when closing it.
1 Why do you need to close the channel
When using a channel, we usually use a for range loop to iterate over elements in the channel. For example, the following is an example of reading data in a channel:
func readData(ch chan int){
for data:= range ch{
(data)
}
}
In the above code, we use a for range loop to iterate over elements in the channel. But what if the channel has no data to read? In this case, the for range loop will keep blocking and waiting for the data to arrive, which will cause the program to fail to exit normally.
Therefore, we need a way to close the channel so that the program exits normally after reading all data. In addition, closing the channel can also remind the receiver that the channel has no data available, thereby preventing some unnecessary blocking or deadlocking.
2 How to close the channel
In Go, the built-in close function can be used to close the channel. The signature of the close function is as follows:
func close(ch chan<- Type)
In the above signature, the <- symbol indicates the direction of the channel. ch chan<- Type means that ch is a write-only channel and can only be used to send data. therefore,Close function can only be used to close channels that can send data。
Here is an example of closing a channel using the close function:
func main() {
ch := make(chan int)
go func() {
for i := 0; i < 10; i++ {
ch <- i
}
close(ch)
}()
for data := range ch {
(data)
}
}
In the above code, we first create an integer channel ch. Then, we use a Goroutine to continuously send data to the channel until after sending 10 numbers, we call the close function to close the channel.
Next, we use a for range loop to iterate through the elements in the channel and print it out. After all data in the channel has been read, the for range loop will automatically exit.
3 Turn off unbuffered and buffered channels
In the example above, we demonstrate how to use the close function to close the unbuffered channel.When closing the unbuffered channel, all data must be read, otherwise blockage will occur.If there are Goroutines that block the channel until it is closed, they will get a signal of the channel closing and therefore exit.
And when we turn off the buffered channel, there may be some data that has not been read yet. When closing the buffered channel, all data in the channel will be read first, and then a shutdown signal will be sent to all Goroutines.
We can demonstrate how to close a buffered channel by modifying the example above:
func main() {
ch := make(chan int, 10)
go func() {
for i := 0; i < 10; i++ {
ch <- i
}
close(ch)
}()
for data := range ch {
(data)
}
}
In the above code, we declare the channel ch as a buffered channel and set the buffered length to 10. We use a Goroutine to continuously send data into the channel. The same as closing an unbuffered channel, we use the close function to close the channel after sending 10 numbers. In the main Goroutine, we use a for range loop to iterate through the data in the channel and output it.
If there are Goroutines that keep blocking the channel after the channel is closed, they will get a signal of the channel closing and therefore exit. In addition, if there is still unread data in the channel, the data will be automatically discarded after closing the channel.
4 Safely close the channel
When closing the channel, we need to pay attention to some things to ensure the normal operation of the program:
4.1 Do not close the channel during concurrent read and write operations
If the same channel is read and write in multiple Goroutines at the same time, and the close function is called in one of the Goroutines, it may cause exceptions to read and write operations on the channel for other Goroutines.
Therefore, when using channels, we should avoid reading and writing the same channel at the same time in multiple Goroutines as much as possible. If the same channel must be read and written at the same time, we should use locks or other synchronization primitives to ensure the security of concurrency.
4.2 Do not close the channel repeatedly
If we try to close the same channel multiple times, it will cause the panic exception to occur. Therefore, before closing the channel, we should make sure that the channel has not been closed yet.
You can use the ok-idit value to check a given channel:
If the channel has been closed, the ok value will be false. We can use this method to check if the channel is closed before reading it.
4.3 Do not close the channel in the Goroutine of the receiving channel
Normally, we should use the close function in the Goroutine where the data is sent to close the channel. Close the channel in the Goroutine receiving data may cause the above problems, such as abnormal read and write operations of other Goroutines on the channel.
Therefore, when using channels, we should make sure the channels are used correctly to avoid similar problems.
5 Summary
In this article, we discuss how to close a channel and what to consider when closing it. As a very important concurrent primitive in Go language, using the correct posture of the channel can effectively avoid problems such as race conditions and deadlocks of the program.
Therefore, when writing code, we should fully understand the principles and usage methods of channels, and use them reasonably to implement concurrent operations. When closing a channel, we need to pay attention to problems such as concurrent operations of the channel in multiple Goroutines and repeated closing of the channel to ensure the correct operation of the program.
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