Tcp server only taking one command. Need to flush recv buffer? - sockets

Question

I am able to send the command "insert data" to the tcp server and it will do what is suppose to. I would like the server to take multiple commands one after the other. At the moment if i send "insert data" and then hit enter and then send "bob" which should not do anything the server responds as if i sent "insert data" again. If you think i should post full source code up let me know in comments.. Screenshot: http://imgur.com/UNRFb5n
#define buf 2000
void *connection_handler(void *socket_desc)
{
//Get the socket descriptor
int sock = *(int*)socket_desc;
ssize_t read_size;
char *message , client_message[buf];
//char *contents;
//contents = "hello";
//strcpy(mess,contents);
//Send some messages to the client
message = "Greetings! I am your connection handler\n";
write(sock , message , strlen(message));
message = "Now type something and i shall repeat what you type \n";
write(sock , message , strlen(message));
//Receive a message from client
while( (read_size = recv(sock , client_message , buf , 0 )) > 0 )
{
//write(sock , client_message , strlen(client_message));
char start_char[] = "start";
char insert_demo_char[] = "insert_demo";
char *inserting = "Inserting Data\n";
char *complete = "Task Complete\n";
if(strcmp(message, start_char))
{
printf("Starting...\n");
//start();
//printf("it works");
//fflush( stdout );
}
if(strcmp(message, insert_demo_char))
{
write(sock , inserting , strlen(inserting));
printf("Inserting data\n");
insert_demo();
write(sock, complete, strlen(complete));
printf("Finished Inserting Data\n");
}
}
if(read_size == 0)
{
puts("Client disconnected");
fflush(stdout);
}
else if(read_size == -1)
{
perror("recv failed");
}
//Free the socket pointer
free(socket_desc);
return 0;
}

Answer 1

while( (read_size = recv(sock , client_message , buf , 0 )) > 0 )
{
[...]
if(strcmp(message, start_char))
After you receive data into client_message, you are checking the buffer named message instead. Since you didn't recv() into that buffer, of course it has not changed.
Also note that strcmp() returns 0 if the two strings are equal, and non-zero if the two strings are different; you may have that backwards in your if(strcmp()) tests (I'm not sure what behavior you intended).

Answer 2

Since TCP is an octet streaming service, and cannot send application level messages longer than one byte, sending 'insert data' from the client may result in the recv() call loading the buffer with any of:
i
in
ins
inse
inser
insert
insert
insert d
insert da
insert dat
insert data
In the cases of incomplete application-level messages, more calls to recv() will be required to receive the remaining bytes of the message.
Note that sending 'insert data\0' will only result in a null-terminated char array in the buffer if recv() happens to return all that data in one call, which is why the 'read_size' returned by recv() is the ONLY way to determine how many bytes were loaded when transferring binary data: using strXXX calls on such a buffer is UB. You can use 'read_size' when transferring text to ensure that the text is null terminated, so preventing UB if the null terminator is not in the buffer:
while( (read_size = recv(sock , client_message , buf-1 , 0 )) > 0 )
client_message[read_size]:=\0;
..will at least give you a 'client_message' that is guaranteed null-terminated, though it will not help with strcmp() failing to identify partial application-level messages in the buffer.
To transfer application messages larger than one byte, you need a protocol on top of TCP that can parse the messages out from the byte stream.