Socket problem record

This article mainly records socket Code problems .

recv(size) Larger than buffer size What's going to happen

The problem is that the author wants to send a large file , But the pass was slow , So increase the buffer size , Although the donkey is not right , But I thought about it as he thought , That should be no different from normal , This problem is similar to that the data in the buffer is 10, In your app size by 100, What you receive is 10 Well .
The following code

char msgbuffer[100];
int size;

if((size = recv(sock, msgbuffer, sizeof(msgbuffer), 0)) < 0){
    	// Read buffer data to msgbuffer in 
	cout << " Read error ！ The reason for the error  errno:" << errno << endl;		
	close(sock);
	return 0;

}
else if(size == 0){
    
	cout << " The reader is turned off " << endl;		
	close(sock);
	return 0;

}
else
	cout << " Read successful :" << msgbuffer << endl << " Read byte :" << size << endl;

Set the threshold

Read threshold can be set , You can't write , I don't know why
This is because the online article writing, reading and writing thresholds are 1 and 2048, But I checked , All are 1, And the read threshold can be modified , It is not allowed to write the threshold .

int val;
socklen_t valLen = sizeof(int);

cout << " Socket read buffer " << endl;
getsockopt(sock, SOL_SOCKET, SO_RCVBUF, &val, &valLen);
cout << " Default size :" << val << endl;
getsockopt(sock, SOL_SOCKET, SO_RCVLOWAT, &val, &valLen);
cout << " Default water level :" << val << endl;

cout << " Socket write buffer " << endl;
getsockopt(sock, SOL_SOCKET, SO_SNDBUF, &val, &valLen);
cout << " Default size :" << val << endl;
getsockopt(sock, SOL_SOCKET, SO_SNDLOWAT, &val, &valLen);
cout << " Default water level :" << val << endl;

val = 1000;
setsockopt(sock, SOL_SOCKET, SO_RCVLOWAT, &val, sizeof(val));
getsockopt(sock, SOL_SOCKET, SO_RCVLOWAT, &val, &valLen);
cout << " The reading water level is modified to :" << val << endl;

val = 1000;
setsockopt(sock, SOL_SOCKET, SO_SNDLOWAT, &val, sizeof(val));
getsockopt(sock, SOL_SOCKET, SO_SNDLOWAT, &val, &valLen);
cout << " Write the water level as :" << val << endl;

You will find that the writing water level is 1 And can't modify .

Read / write and modify buffer size

When reading data > Read side buffer
When writing data > Write side buffer , What's going to happen .

When reading data > Read side buffer
   Nothing will happen . Reading end-to-end data is actually the sending data of the opposite end . When connecting, the sending end will set the sending window according to the buffer size of the reading end , Therefore, it will not overflow . The following code

// Modify the read end size to the minimum : The minimum size of the read end is 2304
val = 1;
setsockopt(sock, SOL_SOCKET, SO_RCVBUF, &val, sizeof(int));
getsockopt(sock, SOL_SOCKET, SO_RCVBUF, &val, &valLen);
cout << " Read end size  :" << val << endl;

// Assume that the peer transmission is greater than 2304 data . Set to 20000

//recv  Read from buffer 
char msgbuffer[1000000];
int size;
int sum = 0;

while(true){
    
	size = recv(sock, msgbuffer, sizeof(msgbuffer), 0);
	cout << " Read bytes :" << size << endl;
	
	sum += size;
	if(sum == 20000){
    
		cout << " Read sum :" << sum << endl;
		return 0;
	}
}

We will find that the data in the buffer does not exceed the buffer size .

// The code above msgbuffer It is amended as follows 100
char msgbuffer[100];

You can see read-only 100. This is because msgbuffer by 100, But that doesn't mean the buffer is just 100 The data of .

When writing data > Write side buffer
  rwqrq
rqwrqw

// Modify the write end size to the minimum : The minimum write end size is 4608
val = 1;
setsockopt(sock, SOL_SOCKET, SO_SNDBUF, &val, sizeof(int));
getsockopt(sock, SOL_SOCKET, SO_SNDBUF, &val, &valLen);
cout << " Write end size  :" << val << endl;

// Assume that the peer transmission is greater than 2304 data . Set to 20000

// write in 
char msgbuffer[20000];
int size;	

size = send(sock, msgbuffer, sizeof(msgbuffer), 0);
cout << " Write Bytes :" << size << endl;

send(sock, msgbuffer, 1000000, 0); To write 100w
recv(sock, msgbuffer, 1000000, 0); Read 100w
Consider two things separately
1.100w And buffer space
2.100w And the number of buffers
100w < Space
   Write a successful
100w = Space
   Write a successful
100w > Space
   Writing failure , Returns the number of writes .
100w < Number
   Read successfully
100w = Number
   Read successfully
100w > Number
   Read failed , Returns the number of reads .
let me put it another way
When it comes to writing > Written in , Describe buffer space < There is only so much to write
When it comes to reading > Read the , Specify the number of buffers < There is only so much to read

recv(sock, msgbuffer, 1000000, 0); Read 100w
return < 100w : The read buffer has < 100w The data of
return = 100w : The read buffer has >=100w The data of
return > 100w : It's impossible to return > 100w

Sending buffer size modification is useless , Read buffer can . I don't understand why .