linux TCP multiclient echo server in c - c

Question

I am currently studying TCP multiclient echo server and client using fork, thread, multiplexing IO and so on.
Below are the simple server and client using fork().
server_fork.c
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <arpa/inet.h>
static const int BUFSIZE = 1024;
int readn(int fd, char *buf, short n);
int main(void)
{
int cnt = 0;
int listenFD, connectFD;
struct sockaddr_in listenSocket, connectSocket;
char buffer [BUFSIZE];
if ((listenFD = socket(PF_INET, SOCK_STREAM, 0)) < 0) {
perror("socket() error\n");
exit(0);
}
if (setsockopt(listenFD, SOL_SOCKET, SO_REUSEADDR, &(int){ 1 }, sizeof(int)) < 0) {
perror("sockopt error\n");
exit(0);
}
memset(&listenSocket, 0, sizeof(listenSocket));
listenSocket.sin_family = AF_INET;
listenSocket.sin_addr.s_addr = inet_addr("0.0.0.0");
listenSocket.sin_port = htons(7777);
if (bind(listenFD, (struct sockaddr *)&listenSocket, sizeof(listenSocket)) < 0) {
perror("bind() error\n");
exit(0);
}
if (listen(listenFD, 1) < 0) {
perror("listen() error\n");
exit(0);
}
signal(SIGCHLD, SIG_IGN);
int connectSocketLen;
short readLen;
pid_t pid;
while (1) {
connectSocketLen = sizeof(connectSocket);
if ((connectFD = accept(listenFD, (struct sockaddr *)&connectSocket,
&connectSocketLen)) < 0) {
perror("accept() error\n");
exit(0);
}
pid = fork();
cnt++;
if (pid == 0) {
close(listenFD);
while (1) {
memset(buffer, 0, BUFSIZE);
if (readn(connectFD, buffer, 2) == 0) {
break;
}
readLen = (*(short *)&buffer);
if(readLen != 12)
printf("[%d] : %d\n", cnt, readLen);
if (readn(connectFD, buffer, readLen) == 0) {
break;
}
buffer[readLen] = 0;
int n;
if ((n = write(connectFD, buffer, readLen)) <= 0) {
perror("!!");
}
sleep(0);
}
close(connectFD);
exit(0);
}
else if (pid > 0) {
close(connectFD);
}
else {
perror("fork() error\n");
exit(0);
}
}
close(listenFD);
return 0;
}
int readn(int fd, char *buf, short n)
{
short sp = 0, readed;
while (n) {
readed = read(fd, buf + sp, n);
if (readed <= 0) {
return 0;
}
n -= readed;
sp += readed;
}
return 1;
}
client.c
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <stdio.h>
#include <signal.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <time.h>
static const int bufSize = 1024;
int main(int argc,char *argv[])
{
signal(SIGCHLD, SIG_IGN);
fork();
fork();
fork();
fork();
fork();
fork();
fork();
fork();
//fork();
//fork();
char length[2], recvBuf[bufSize];
char buf[]="hello, world\0";
short len = strlen(buf);
sprintf(length,"%c",len);
int client_sockfd, size, i, n, state;
uint64_t delta_us = 0;
struct sockaddr_in server_addr;
struct timespec start, end;
client_sockfd = socket(PF_INET, SOCK_STREAM, 0);
memset(&server_addr, 0, sizeof server_addr);
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(7777);
inet_pton(AF_INET, "127.0.0.1", &server_addr.sin_addr.s_addr);
state = connect(client_sockfd, (struct sockaddr *)&server_addr,
sizeof server_addr);
if (state < 0) {
perror("connect err");
exit(1);
}
for (i=0;i<10;i++) {
clock_gettime(CLOCK_MONOTONIC_RAW, &start);
n = write(client_sockfd, length, sizeof length);
if (n<=0) {
perror("write err");
exit(1);
}
n = write(client_sockfd, buf, *((short *)&length));
if (n<=0) {
perror("write err");
exit(1);
}
n = read(client_sockfd, recvBuf, *((short *)&length));
if (n<=0) {
perror("read err");
exit(1);
}
clock_gettime(CLOCK_MONOTONIC_RAW, &end);
delta_us += (end.tv_sec - start.tv_sec) * 1000000 +
(end.tv_nsec - start.tv_nsec)/1000;
printf("%lu\n", delta_us);
sleep(1);
}
return 0;
}
The client first transmits the length of the message represented by 2 bytes.
Then client sends a "hello, world" message to the server in buf.
The server first reads 2 bytes through readn () and reads the message as much.
But in all cases (fork, thread, multiplexing IO... whatever), I have a common difficulty.
The problem is that : In the above source code, the length of the message("hello, world" is 12. so it is expected to read 12 in the first readn () of the server.
In fact, when the number of clients is small (the number of forks on the client is 7 or less), it works fine.
However, if the number of clients increases, the value of readLen will be 25960 on some connections. 25960 is the value that represents "he" in hex.
Why is this problem happening when there are many clients?
I wonder if it is the same as sending a TCP packet at one time and dividing it two times.
Below is a tcpdump capture of the above problem situation.
I'm sorry that I could not upload the image right away.
tcpdump

Answer 1

On the client side, you have multiple problems surrounding how you send the message length. Starting here:
char length[2], recvBuf[bufSize];
char buf[]="hello, world\0";
short len = strlen(buf);
sprintf(length,"%c",len);
Your sprintf format promises that the third argument will be of type char (promoted to int), and instructs it to output the corresponding character into the string. In fact, the argument is a short (promoted to int), and this mismatch produces undefined behavior.
In practice, the overall sprintf call is probably equivalent to this:
length[0] = (char)(int)len;
length[1] = '\0';
That has implementation-defined characteristics if char is a signed type, but in any event, it cannot capture a length greater than the maximum value that can be represented by an unsigned char.
The client goes on to do this:
n = write(client_sockfd, length, sizeof length);
That's not inherently wrong, but it does fail to accommodate the possibility of a short write. Moreover, the server does not interpret this part of the message in a manner consistent with the way it was prepared:
if (readn(connectFD, buffer, 2) == 0) {
break;
}
readLen = (*(short *)&buffer);
As it turns out, that combination might happen to work if the server uses a 16-bit, little-endian representation for type short (subject to the restriction I already described on representable message length) and an execution character set compatible with the client's, but those are not safe assumptions for network software in general.
In part, you seem to be missing an important point about read() and write() and char pointers: a char * can be used to read the representation of an object of any type, so you do not need to move your data into a char array in order to send or receive it.
Overall, this would be a more appropriate approach:
// Client:
uint16_t len = strlen(buf); // risk: buf may be too long
uint16_t len_n = htons(len);
int n = writen(client_sockfd, &len_n, 2); // a write() analog of readn()
// ... handle possible error ...
// Sever:
uint16_t len_n;
int n = readn(connectFD, &len_n, 2);
// ... possible handle error ...
uint16_t readLen = ntohs(len_n);
Note that there is no need to copy the length into a separate char array to send it, nor to receive it into a char array. On the other hand, note also the use of a specified-size data type (uint16_t) on both sides, and the use of htons() and ntohs() to ensure that the client and server interpret the bytes of the data the same way. Furthermore, note the use of a write analog of readn() to send the data, which otherwise could be sent incompletely.
By the same token the client and server should both use the (hypothetical) writen() function to send the text of the message, and just like the server, the client should use readn() to read it. Failing to account for the possibility of short reads and writes is a significant flaw, especially in a protocol such as yours whose message boundaries are not easily distinguishable from data. An unnoticed short read or write will cause your client and server to fall out of sync, with no easy way to recover.

Answer 2

Apart from the problems already noticed by #JohnBollinger, you only use a listen window of 1 in server.c when all your forked client processes try to connect at the same time. It caused plenty of connect : conn reset by peer errors in my tests. I had to use a window greater than 64 on my FreeBSD (no errors at 256) to get rid of them :
if (listen(listenFD, 256) < 0) {
perror("listen() error\n");
exit(0);
}