Detailed explanation of socket programming send() and recv() functions

First send The function prototype

ssize_t send (int fd, const void *buf, size_t n, int flags);
ssize_t recv (int fd, void *buf, size_t n, int flags)

fd Indicates the socket to send to ,buf Pointer header indicating the data to be sent ,n Indicates that you want to send ( receive ) The length of ,flag Yes, the logo is generally set to 0.
1：send() recv() Behavioral overview and memset() Importance
  send() Your behavior is from buf Place selection n A byte of data is sent out ,recv() Then read n Byte to buf in . Here we need to pay attention to , They are wrong buf For any detection of the value in , Therefore, at the receiving end, it is best to first buffer Set as 0, And set it to be greater than that of each reception buffer The length of
Take a chestnut ：

// The sender 
char buffer[10] = "1234567";
send(fd,buffer,3,0);
// The receiving party 
char buffer[10]=“abcdef”;
recv(fd,buffer,3,0);

After the acceptance is completed buffer Change into "123def", There's no way to distinguish what's received ,
Standard practice ：

// The sender 
char buffer[10] = "1234567";
send(fd,buffer,3,0);
// The receiving party 
char buffer[10]=“abcdef”;
memset(buffer,0,sizeof(buffer));// Let's leave everything blank 
recv(fd,buffer,3,0);

2：send() function
about send function , The return value is equal to 0 Indicates that the connection is broken , Less than 0 Indicates an error , Greater than 0 Indicates the number of bytes actually sent successfully . That is, it is possible that the actual number of bytes sent is less than that in the function n（ This is a send The function is over , Start executing the next line of code ）, Make sure that if n All bytes sent out , Need to write while loop Judge the cumulative number of bytes sent each time （ Generally, all can be sent successfully ）.
The return value of blocking mode and non blocking mode has the same meaning .
Blocking mode ： If the length of the data to be sent is less than the size of the remaining buffer , Send it directly ; If the length of the data to be sent is greater than the remaining buffer , The block , Wait until the buffer is sent, and the data space becomes larger than the data to be sent several times before , Put it in the buffer , Returns the number of bytes successfully sent （ Maybe not all of them were sent successfully ）,0 Indicates that the connection is broken , Less than 0 Indicates an error .
Non-blocking mode ： If the length of the data to be sent is less than the size of the remaining buffer , Then send and return success data directly ; If the length of the data to be sent is greater than the remaining buffer, an error is returned （ Less than 0）,0 Indicates that the connection is broken , Less than 0 Indicates an error .
In other words, blocking and non blocking are just different modes , The meaning they return is the same .
3 recv() function
recv() Function and send() The return value of the function has the same meaning .
recv The function will be more complicated in practice ,send The function only needs to determine the size to send , Then send it （ Generally, the buffer is large enough , And it can be sent successfully at one time ）. But when recv When a function reads from a buffer, it's a little complicated .
In blocking mode ： If the buffer is empty , The block , If n Less than the data length of the buffer , Then read from the buffer n Bytes , The rest remains in the buffer , return n; If n If it is equal to the length of the buffer, all are read , Buffer is empty , return n, If n Greater than buffer length , Then read it all , Buffer is empty , Returns the actual read length .
In non-blocking mode ： If the buffer is empty , Returns an error , The rest are the same as the blocking mode .