How is this tcp socket code handling the rx buffer? - c

Question

I came across this tcp server example, provided with the Altera Nios II processor, and I'm not getting the section on handling the rx_buffer.
server.h
typedef struct SSS_SOCKET {
enum {
READY, COMPLETE, CLOSE
} state;
int fd;
int close;
INT8U rx_buffer[SSS_RX_BUF_SIZE];
INT8U *rx_rd_pos; /* position we've read up to */
INT8U *rx_wr_pos; /* position we've written up to */
} SSSConn;
server.c
int data_used = 0, rx_code = 0;
INT8U *lf_addr;
conn->rx_rd_pos = conn->rx_buffer;
conn->rx_wr_pos = conn->rx_buffer;
printf("[sss_handle_receive] processing RX data\n");
while (conn->state != CLOSE) {
/* Find the Carriage return which marks the end of the header */
lf_addr = strchr(conn->rx_buffer, '\n');
if (lf_addr) {
/* go off and do whatever the user wanted us to do */
sss_exec_command(conn);
}
/* No newline received? Then ask the socket for data */
else {
rx_code = recv(conn->fd, conn->rx_wr_pos,
SSS_RX_BUF_SIZE - (conn->rx_wr_pos - conn->rx_buffer) -1, 0);
if (rx_code > 0) {
conn->rx_wr_pos += rx_code;
/* Zero terminate so we can use string functions */
*(conn->rx_wr_pos + 1) = 0;
}
}
/*
* When the quit command is received, update our connection state so that
* we can exit the while() loop and close the connection
*/
conn->state = conn->close ? CLOSE : READY;
/* Manage buffer */
data_used = conn->rx_rd_pos - conn->rx_buffer;
memmove(conn->rx_buffer, conn->rx_rd_pos,
conn->rx_wr_pos - conn->rx_rd_pos);
conn->rx_rd_pos = conn->rx_buffer;
conn->rx_wr_pos -= data_used;
memset(conn->rx_wr_pos, 0, data_used);
}
Specifically, I don't see the purpose of the data_used variable. rx_rd_pos is pointing to rx_buffer and there doesn't appear to be an operation on either, so how will they be different? In fact, the only thing that seems to happen under Manage buffer is the copying of data into rx_buffer. I'm sure I'm missing something simple, but I can't seem to see it.
Thanks for any help in advance.
Edit: Here's the sss_exec_command() function.
void sss_exec_command(SSSConn* conn) {
int bytes_to_process = conn->rx_wr_pos - conn->rx_rd_pos;
INT8U tx_buf[SSS_TX_BUF_SIZE];
INT8U *tx_wr_pos = tx_buf;
INT8U error_code;
/*
* "SSSCommand" is declared static so that the data will reside
* in the BSS segment. This is done because a pointer to the data in
* SSSCommand
* will be passed via SSSLedCommandQ to the LEDManagementTask.
* Therefore SSSCommand cannot be placed on the stack of the
* SSSSimpleSocketServerTask, since the LEDManagementTask does not
* have access to the stack of the SSSSimpleSocketServerTask.
*/
static INT32U SSSCommand;
SSSCommand = CMD_LEDS_BIT_0_TOGGLE;
while (bytes_to_process--) {
SSSCommand = toupper(*(conn->rx_rd_pos++));
if (SSSCommand >= ' ' && SSSCommand <= '~') {
tx_wr_pos += sprintf(tx_wr_pos,
"--> Simple Socket Server Command %c.\n",
(char) SSSCommand);
if (SSSCommand == CMD_QUIT) {
tx_wr_pos += sprintf(tx_wr_pos,
"Terminating connection.\n\n\r");
conn->close = 1;
} else {
error_code = OSQPost(SSSLEDCommandQ, (void *) SSSCommand);
alt_SSSErrorHandler(error_code, 0);
}
}
}
send(conn->fd, tx_buf, tx_wr_pos - tx_buf, 0);
return;
}
Answers below are correct. I missed the pointer arithmetic on rx_rd in the command function :P

Answer 1

That section removes data from the buffer once it has been processed. The code you posted never uses the data stores in the buffer, but the sss_exec_command function will, after a newline is received. That function is passed the connection, so it can increment the read position by however much it uses.
After data is used, the buffer management section reclaims the space. The amount of data left in the buffer is the difference between the write and read positions. This much data is moved from the write position to the start of the buffer, then the read and write pointer are updated to their new positions. The read position is set to the start of the buffer, and the write position is decremented by data_used, which is the original difference between the start of the buffer and the read pointer, i.e. the amount of data used.

Answer 2

Assuming the code actually works, then data_used = conn->rx_rd_pos - conn->rx_buffer implies rx_rd_pos is being changed; this would be being changed when the code has consumed the data written into the buffer (it's written in at rx_wr_pos and consumed from rx_rd_pos). This would imply that sss_exec_command(conn) is adjusting conn. Is that the case?