Receiving multiple TCP segments - c

Question

I have an assignment in which a TCP client sends data to the TCP server in the form of:
IP_address\0port\0message\n
Now, the server (IP address 10.0.2.15) receives the packet fine when I send some data through a terminal like this:
printf "127.0.0.1\0004444\000Some message\n" | nc -N 10.0.2.15 3333
However, the second part of the assignment is to read a packet that comes in multiple segments:
(printf "127.0.0.1"; sleep 0.3; printf "\0004444\000"; sleep 0.3; \
printf "It works"; sleep 0.5; printf "\n") | nc -N 10.0.2.15 3333
How should I implement the read function on the server so that, if possible, all the segments are stored into a buffer?

Answer 1

The number of bytes recv() returns can be as few as 1 byte up to as many bytes as requested. TCP is a byte stream, it has no concept of messages, that has to be handled in the application code instead.
The receiver must know how many bytes to expect, and then keep reading in a loop until it has read that many bytes, however many reads it takes.
However, in this situation, the receiver does not know the exact length of the message, because the sender is not sending the message length before sending the message itself, so the only option available is for the receiver to read from the socket byte-by-byte until it encounters the terminating \n.
For example:
int readLine(int socket, char **line)
{
int r, len = 0, cap = 256;
char b;
*line = NULL;
char *outline = (char*) malloc(cap);
if (!outline) return -2;
do
{
r = recv(socket, &b, 1, 0);
if (r <= 0)
{
free(outline);
return r;
}
if (b == '\n')
break;
if (len == cap)
{
cap += 256;
char *newline = (char*) realloc(outline, cap);
if (!newline)
{
free(outline);
return -2;
}
outline = newline;
}
outline[len] = b;
++len;
}
while (true);
if ((len > 0) && (line[len-1] == '\r'))
--len;
if (len == cap)
{
char *newline = (char*) realloc(outline, cap + 1);
if (!newline)
{
free(outline);
return -2;
}
outline = newline;
}
outline[len] = '\0';
*line = outline;
return 1;
}
char *line;
int r;
do
{
r = readLine(cliSock, &line);
if (r <= 0)
{
if (r == 0)
printf("client disconnected\n");
else if (r == -2)
printf("memory error\n");
else
printf("read error\n");
break;
}
// process line as needed...
free(line);
}
while (true);
Alternatively, you can use an intermediate buffer to help you cache data between reads and get data out of the socket more efficiently:
char *buffer;
int buflen, bufcap;
int readLine(int socket, char **line)
{
char *ptr;
int r, idx = 0;
*line = NULL;
do
{
ptr = memchr(buffer + idx, '\n', buflen - idx);
if (ptr)
{
int total = ((ptr + 1) - buffer);
int len = (total - 1);
if ((len > 0) && (buffer[len-1] == '\r'))
--len;
*line = (char*) malloc(len + 1);
if (*line == NULL)
return -2;
memcpy(*line, buffer, len);
(*line)[len] = '\0';
if (total < buflen)
memmove(buffer, buffer + total, buflen - total);
buflen -= total;
break;
}
if (buflen == bufcap)
{
int newcap = bufcap + 256;
char *newbuffer = (char*) realloc(buffer, newcap);
if (!newbuffer)
return -2;
buffer = newbuffer;
bufcap = newcap;
}
r = recv(socket, buffer + buflen, bufcap - buflen, 0);
if (r <= 0)
return r;
buflen += r;
}
while (true);
return 1;
}
buflen = 0;
bufcap = 256;
buffer = (char*) malloc(bufcap);
if (buffer)
{
char *line;
int r;
do
{
r = readLine(cliSock, &line);
if (r <= 0)
{
if (r == 0)
printf("client disconnected\n");
else if (r == -2)
printf("memory error\n");
else
printf("read error\n");
break;
}
// process line as needed...
free(line);
}
while (true);
free(buffer);
}