Not receiving separate values on the server-side client-server TCP - c

Question

I have a problem with receiving the correct values on the server-side of my client-server program.
header file included in both server and client:
#define CHUNK_SIZE 1024
#define ARR_LEN 3
client:
int uids[ARR_LEN] = {1994, 2423, 1222};
unsigned int uidlen = 0;
char uidbuffer[CHUNK_SIZE] = {0};
for(int i = 0; i < ARLL; i++)
{
uidlen = strlen(uids[i])+1;
snprintf(uidbuffer, uidlen, "%s", uids[i]);
if(send(socket, uidbuffer, strlen(uidbuffer), 0) < 0)
DIE("Write Error");
if(recv(socket, uidbuffer, sizeof(uidbuffer), 0) < 0)
DIE("Acknowledge Error");
memset(uidbuffer, 0, sizeof(uidbuffer));
}
server:
char uid_buff[CHUNK_SIZE];
for(int i = 0; i < ARR_LEN; i++)
{
memset(uid_buff, 0, sizeof(uid_buff));
// receiving the UID and storing it directly
if(recv(client_sock, uid_buff, sizeof(uid_buff), 0) < 0)
DIE("Receive Error");
printf("buffer content: %s\n", uid_buff);
uid_str = uid_buff;
uids[i] = (uid_t)strtol(uid_str, (char **)NULL, 10);
if(send(client_sock, uid_buff, sizeof(uid_buff), 0) < 0)
DIE("Acknowledge Error");
}
These are only parts of my program. I tried to only include the relevant parts. The output is this:
buffer content: 1994
buffer content: 24231222
buffer content:
While I want it to be:
buffer content: 1994
buffer content: 2423
buffer content: 1222
What could be the problem? I know that it's not so easy and that server–client communication is carried out in a stream of bytes rather than messages, but I want to mimmic that functionality by acknowledging every received "message". Could you please clue me what to do? I'm getting desperate.

Answer 1

You will need a protocol.
For example, you define that each message in your application has following format:
xx | message
this means first two bytes (mind the endianness) you receive indicate the length of the message that follows. Now you should first receive first two bytes - check the length - and then receive exactly that number of bytes. After that you know you have successfully received that message. And then you can proceed to other messages (which could/should have similar format: length + message itself).
Example:
Say you want to send three messages:
char s1[]="message1";
char s2[]="message2";
char s3[]="message3";
//You do this(client side):
int x1 = strlen(s1); // length of message1
int intsize = 4; // just size of integer -we'll need in next call
sendall(socket, &x1, &intsize); // send length of first message
sendall(socket, s1, &x1); // Now send the message
//On server:
int x = 0;
int y = 4; //size of integer most probably
receiveall(socket,&x,&y);//get length first; y=4 because that is size of integer
receiveall(socket, buffer, &x); // now we know how many bytes to expect - x - so request that number of bytes only
you can repeat this logic for other messages too.
Finally, you want to use such functions (here) instead of send and receive (because send and receive might not send/receive the number of bytes you tell it to):
int sendall(int s, char *buf, int *len)
{
int total = 0; // how many bytes we've sent
int bytesleft = *len; // how many we have left to send
int n;
while(total < *len) {
n = send(s, buf+total, bytesleft, 0);
if (n == -1) { break; }
total += n;
bytesleft -= n;
}
*len = total; // return number actually sent here
return n==-1?-1:0; // return -1 on failure, 0 on success
}
you will need a similar receiveall function.

Answer 2

You do not show the code that establishes the connection.
If you are using UDP sockets, each message is stand alone, you will receive a separate message for each message sent, but not necessarily in the same order.
If you are using TCP or Unix sockets, data may be received in chunks of different sizes than those used for sending. Only the sequence of bytes is preserved, not the chunk sizes. You need to specify a protocol: unless you specify a fixed number of bytes per message, you need to send some sort of separator to allow the server to tell where each message ends.