How to implement simple TCP Client and Server Program in C?

This is a C Program that illustrates a simple TCP (Transmission Control Protocol) server that accepts incoming client connections. Once a client connection is established, a thread is spawned to read data from the client and echo it back (if the echo option is not disabled).

You can use any C/C++ Compilers in windows to compile and run this program as it uses winsock2 windows network API.

How to Compile the TCP Client/Server program?

The following TCP Client Server program is divided into two parts, the server and the client. Please compile each program separately as per the following instructions.

Server:

To compile the server program, use the following command line. TCP server accepts incoming client connections. Once a client connection is established, a thread is spawned to read data from the client and echo it back (if the echo option is not disabled).

Compile:

cl -o Server Server.c ws2_32.lib

Command line options:
server [-p:x] [-i:IP] [-o]
-p:x Port number to listen on
-i:str Interface to listen on
-o Receive only, don’t echo the data back

Client:

This sample is the echo client. It connects to the TCP server, sends data, and reads data back from the server.

Compile:
cl -o Client Client.c ws2_32.lib

Command Line Options:

client [-p:x] [-s:IP] [-n:x] [-o]
-p:x Remote port to send to
-s:IP Server’s IP address or hostname
-n:x Number of times to send message
-o Send messages only; don’t receive

Client Source Code

// Module Name: Client.c
//
// Description:
// This sample is the echo client. It connects to the TCP server,
// sends data, and reads data back from the server.
//
// Compile:
// cl -o Client Client.c ws2_32.lib
//
// Command Line Options:
// client [-p:x] [-s:IP] [-n:x] [-o]
// -p:x Remote port to send to
// -s:IP Server's IP address or hostname
// -n:x Number of times to send message
// -o Send messages only; don't receive
//
#include <winsock2.h>
#include <stdio.h>
#include <stdlib.h>

#define DEFAULT_COUNT 20
#define DEFAULT_PORT 5150
#define DEFAULT_BUFFER 2048
#define DEFAULT_MESSAGE "This is a test of the emergency \
broadcasting system"

char szServer[128], // Server to connect to
szMessage[1024]; // Message to send to sever
int iPort = DEFAULT_PORT; // Port on server to connect to
DWORD dwCount = DEFAULT_COUNT; // Number of times to send message
BOOL bSendOnly = FALSE; // Send data only; don't receive

//
// Function: usage:
//
// Description:
// Print usage information and exit
//
void usage()
{
	printf("usage: client [-p:x] [-s:IP] [-n:x] [-o]\n\n");
	printf(" -p:x Remote port to send to\n");
	printf(" -s:IP Server's IP address or hostname\n");
	printf(" -n:x Number of times to send message\n");
	printf(" -o Send messages only; don't receive\n");
	ExitProcess(1);
}

//
// Function: ValidateArgs
//
// Description:
// Parse the command line arguments, and set some global flags
// to indicate what actions to perform
//
void ValidateArgs(int argc, char** argv)
{
	int i;

	for (i = 1; i < argc; i++)
	{
		if ((argv[i][0] == '-') || (argv[i][0] == '/'))
		{
			switch (tolower(argv[i][1]))
			{
			case 'p': // Remote port
				if (strlen(argv[i]) > 3)
					iPort = atoi(&amp;argv[i][3]);
				break;
			case 's': // Server
				if (strlen(argv[i]) > 3)
					strcpy(szServer, &amp;argv[i][3]);
				break;
			case 'n': // Number of times to send message
				if (strlen(argv[i]) > 3)
					dwCount = atol(&amp;argv[i][3]);
				break;
			case 'o': // Only send message; don't receive
				bSendOnly = TRUE;
				break;
			default:
				usage();
				break;
			}
		}
	}
}

//
// Function: main
//
// Description:
// Main thread of execution. Initialize Winsock, parse the
// command line arguments, create a socket, connect to the
// server, and then send and receive data.
//
int main(int argc, char** argv)
{
	WSADATA wsd;
	SOCKET sClient;
	char szBuffer[DEFAULT_BUFFER];
	int ret,
		i;
	struct sockaddr_in server;
	struct hostent* host = NULL;

	// Parse the command line and load Winsock
	//
	ValidateArgs(argc, argv);
	if (WSAStartup(MAKEWORD(2, 2), &amp;wsd) != 0)
	{
		printf("Failed to load Winsock library!\n");
		return 1;
	}
	strcpy(szMessage, DEFAULT_MESSAGE);
	//
	// Create the socket, and attempt to connect to the server
	//
	sClient = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
	if (sClient == INVALID_SOCKET)
	{
		printf("socket() failed: %d\n", WSAGetLastError());
		return 1;
	}
	server.sin_family = AF_INET;
	server.sin_port = htons(iPort);
	server.sin_addr.s_addr = inet_addr(szServer);
	//
	// If the supplied server address wasn't in the form
	// "aaa.bbb.ccc.ddd" it's a hostname, so try to resolve it
	//
	if (server.sin_addr.s_addr == INADDR_NONE)
	{
		host = gethostbyname(szServer);
		if (host == NULL)
		{
			printf("Unable to resolve server: %s\n", szServer);
			return 1;
		}
		CopyMemory(&amp;server.sin_addr, host->h_addr_list[0],
			host->h_length);
	}
	if (connect(sClient, (struct sockaddr*)&amp;server,
		sizeof(server)) == SOCKET_ERROR)
	{
		printf("connect() failed: %d\n", WSAGetLastError());
		return 1;
	}
	// Send and receive data
	//
	for (i = 0; i < dwCount; i++)
	{
		ret = send(sClient, szMessage, strlen(szMessage), 0);
		if (ret == 0)
			break;
		else if (ret == SOCKET_ERROR)
		{
			printf("send() failed: %d\n", WSAGetLastError());
			break;
		}
		printf("Send %d bytes\n", ret);
		if (!bSendOnly)
		{
			ret = recv(sClient, szBuffer, DEFAULT_BUFFER, 0);
			if (ret == 0) // Graceful close
				break;
			else if (ret == SOCKET_ERROR)
			{
				printf("recv() failed: %d\n", WSAGetLastError());
				break;
			}
			szBuffer[ret] = '\0';
			printf("RECV [%d bytes]: '%s'\n", ret, szBuffer);
		}
	}
	closesocket(sClient);

	WSACleanup();
	return 0;
}

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// Module Name: Client.c

// Description:

// This sample is the echo client. It connects to the TCP server,

// sends data, and reads data back from the server.

// Compile:

// cl -o Client Client.c ws2_32.lib

// Command Line Options:

// client [-p:x] [-s:IP] [-n:x] [-o]

// -p:x Remote port to send to

// -s:IP Server's IP address or hostname

// -n:x Number of times to send message

// -o Send messages only; don't receive

#include <winsock2.h>

#include <stdio.h>

#include <stdlib.h>

#define DEFAULT_COUNT 20

#define DEFAULT_PORT 5150

#define DEFAULT_BUFFER 2048

#define DEFAULT_MESSAGE "This is a test of the emergency \

broadcasting system"

char szServer[128], // Server to connect to

szMessage[1024]; // Message to send to sever

int iPort = DEFAULT_PORT; // Port on server to connect to

DWORD dwCount = DEFAULT_COUNT; // Number of times to send message

BOOL bSendOnly = FALSE; // Send data only; don't receive

// Function: usage:

// Description:

// Print usage information and exit

void usage()

{

printf("usage: client [-p:x] [-s:IP] [-n:x] [-o]\n\n");

printf(" -p:x Remote port to send to\n");

printf(" -s:IP Server's IP address or hostname\n");

printf(" -n:x Number of times to send message\n");

printf(" -o Send messages only; don't receive\n");

ExitProcess(1);

}

// Function: ValidateArgs

// Description:

// Parse the command line arguments, and set some global flags

// to indicate what actions to perform

void ValidateArgs(int argc, char** argv)

{

int i;

for (i = 1; i < argc; i++)

{

if ((argv[i][0] == '-') || (argv[i][0] == '/'))

{

switch (tolower(argv[i][1]))

{

case 'p': // Remote port

if (strlen(argv[i]) > 3)

iPort = atoi(&argv[i][3]);

break;

case 's': // Server

if (strlen(argv[i]) > 3)

strcpy(szServer, &argv[i][3]);

break;

case 'n': // Number of times to send message

if (strlen(argv[i]) > 3)

dwCount = atol(&argv[i][3]);

break;

case 'o': // Only send message; don't receive

bSendOnly = TRUE;

break;

default:

usage();

break;

}

// Function: main

// Description:

// Main thread of execution. Initialize Winsock, parse the

// command line arguments, create a socket, connect to the

// server, and then send and receive data.

int main(int argc, char** argv)

{

WSADATA wsd;

SOCKET sClient;

char szBuffer[DEFAULT_BUFFER];

int ret,

struct sockaddr_in server;

struct hostent* host = NULL;

// Parse the command line and load Winsock

ValidateArgs(argc, argv);

if (WSAStartup(MAKEWORD(2, 2), &wsd) != 0)

{

printf("Failed to load Winsock library!\n");

return 1;

}

strcpy(szMessage, DEFAULT_MESSAGE);

// Create the socket, and attempt to connect to the server

sClient = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

if (sClient == INVALID_SOCKET)

{

printf("socket() failed: %d\n", WSAGetLastError());

return 1;

}

server.sin_family = AF_INET;

server.sin_port = htons(iPort);

server.sin_addr.s_addr = inet_addr(szServer);

// If the supplied server address wasn't in the form

// "aaa.bbb.ccc.ddd" it's a hostname, so try to resolve it

if (server.sin_addr.s_addr == INADDR_NONE)

{

host = gethostbyname(szServer);

if (host == NULL)

{

printf("Unable to resolve server: %s\n", szServer);

return 1;

}

CopyMemory(&server.sin_addr, host->h_addr_list[0],

host->h_length);

}

if (connect(sClient, (struct sockaddr*)&server,

sizeof(server)) == SOCKET_ERROR)

{

printf("connect() failed: %d\n", WSAGetLastError());

return 1;

}

// Send and receive data

for (i = 0; i < dwCount; i++)

{

ret = send(sClient, szMessage, strlen(szMessage), 0);

if (ret == 0)

break;

else if (ret == SOCKET_ERROR)

{

printf("send() failed: %d\n", WSAGetLastError());

break;

}

printf("Send %d bytes\n", ret);

if (!bSendOnly)

{

ret = recv(sClient, szBuffer, DEFAULT_BUFFER, 0);

if (ret == 0) // Graceful close

break;

else if (ret == SOCKET_ERROR)

{

printf("recv() failed: %d\n", WSAGetLastError());

break;

}

szBuffer[ret] = '\0';

printf("RECV [%d bytes]: '%s'\n", ret, szBuffer);

}

closesocket(sClient);

WSACleanup();

return 0;

}

Server Source Code

// Module Name: Server.c
//
// Description:
// This example illustrates a simple TCP server that accepts
// incoming client connections. Once a client connection is
// established, a thread is spawned to read data from the
// client and echo it back (if the echo option is not
// disabled).
//
// Compile:
// cl -o Server Server.c ws2_32.lib
//
// Command line options:
// server [-p:x] [-i:IP] [-o]
// -p:x Port number to listen on
// -i:str Interface to listen on
// -o Receive only, don't echo the data back
//
#include <winsock2.h>

#include <stdio.h>
#include <stdlib.h>

#define DEFAULT_PORT 5150
#define DEFAULT_BUFFER 4096

int iPort = DEFAULT_PORT; // Port to listen for clients on
BOOL bInterface = FALSE, // Listen on the specified interface
bRecvOnly = FALSE; // Receive data only; don't echo back
char szAddress[128]; // Interface to listen for clients on

//
// Function: usage
//
// Description:
// Print usage information and exit
//
void usage()
{
	printf("usage: server [-p:x] [-i:IP] [-o]\n\n");
	printf(" -p:x Port number to listen on\n");
	printf(" -i:str Interface to listen on\n");
	printf(" -o Don't echo the data back\n\n");
	ExitProcess(1);
}

//
// Function: ValidateArgs
//
// Description:
// Parse the command line arguments, and set some global flags
// to indicate what actions to perform
//
void ValidateArgs(int argc, char** argv)
{
	int i;

	for (i = 1; i < argc; i++)
	{
		if ((argv[i][0] == '-') || (argv[i][0] == '/'))
		{
			switch (tolower(argv[i][1]))
			{
			case 'p':
				iPort = atoi(&amp;argv[i][3]);
				break;
			case 'i':
				bInterface = TRUE;
				if (strlen(argv[i]) > 3)
					strcpy(szAddress, &amp;argv[i][3]);
				break;
			case 'o':
				bRecvOnly = TRUE;
				break;
			default:
				usage();
				break;
			}
		}
	}
}

//
// Function: ClientThread
//
// Description:
// This function is called as a thread, and it handles a given
// client connection. The parameter passed in is the socket
// handle returned from an accept() call. This function reads
// data from the client and writes it back.
//
DWORD WINAPI ClientThread(LPVOID lpParam)
{
	SOCKET sock = (SOCKET)lpParam;
	char szBuff[DEFAULT_BUFFER];
	int ret,
		nLeft,
		idx;

	while (1)
	{
		// Perform a blocking recv() call
		//
		ret = recv(sock, szBuff, DEFAULT_BUFFER, 0);
		if (ret == 0) // Graceful close
			break;
		else if (ret == SOCKET_ERROR)
		{
			printf("recv() failed: %d\n", WSAGetLastError());
			break;
		}
		szBuff[ret] = '\0';
		printf("RECV: '%s'\n", szBuff);
		//
		// If we selected to echo the data back, do it
		//
		if (!bRecvOnly)
		{
			nLeft = ret;
			idx = 0;
			//
			// Make sure we write all the data
			//
			while (nLeft > 0)
			{
				ret = send(sock, &amp;szBuff[idx], nLeft, 0);
				if (ret == 0)
					break;
				else if (ret == SOCKET_ERROR)
				{
					printf("send() failed: %d\n",
						WSAGetLastError());
					break;
				}
				nLeft -= ret;
				idx += ret;
			}
		}
	}
	return 0;
}

//
// Function: main
//
// Description:
// Main thread of execution. Initialize Winsock, parse the
// command line arguments, create the listening socket, bind
// to the local address, and wait for client connections.
//
int main(int argc, char** argv)
{
	WSADATA wsd;
	SOCKET sListen,
		sClient;
	int iAddrSize;
	HANDLE hThread;
	DWORD dwThreadId;
	struct sockaddr_in local,
		client;

	ValidateArgs(argc, argv);
	if (WSAStartup(MAKEWORD(2, 2), &amp;wsd) != 0)
	{
		printf("Failed to load Winsock!\n");
		return 1;
	}
	// Create our listening socket
	//
	sListen = socket(AF_INET, SOCK_STREAM, IPPROTO_IP);
	if (sListen == SOCKET_ERROR)
	{
		printf("socket() failed: %d\n", WSAGetLastError());
		return 1;
	}
	// Select the local interface and bind to it
	//
	if (bInterface)
	{
		local.sin_addr.s_addr = inet_addr(szAddress);
		if (local.sin_addr.s_addr == INADDR_NONE)
			usage();
	}
	else
		local.sin_addr.s_addr = htonl(INADDR_ANY);
	local.sin_family = AF_INET;
	local.sin_port = htons(iPort);

	if (bind(sListen, (struct sockaddr*)&amp;local,
		sizeof(local)) == SOCKET_ERROR)
	{
		printf("bind() failed: %d\n", WSAGetLastError());
		return 1;
	}
	listen(sListen, 8);
	//
	// In a continous loop, wait for incoming clients. Once one
	// is detected, create a thread and pass the handle off to it.
	//
	while (1)
	{
		iAddrSize = sizeof(client);
		sClient = accept(sListen, (struct sockaddr*)&amp;client,
			&amp;iAddrSize);
		if (sClient == INVALID_SOCKET)
		{
			printf("accept() failed: %d\n", WSAGetLastError());
			break;
		}
		printf("Accepted client: %s:%d\n",
			inet_ntoa(client.sin_addr), ntohs(client.sin_port));

		hThread = CreateThread(NULL, 0, ClientThread,
			(LPVOID)sClient, 0, &amp;dwThreadId);
		if (hThread == NULL)
		{
			printf("CreateThread() failed: %d\n", GetLastError());
			break;
		}
		CloseHandle(hThread);
	}
	closesocket(sListen);

	WSACleanup();
	return 0;
}

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// Module Name: Server.c

// Description:

// This example illustrates a simple TCP server that accepts

// incoming client connections. Once a client connection is

// established, a thread is spawned to read data from the

// client and echo it back (if the echo option is not

// disabled).

// Compile:

// cl -o Server Server.c ws2_32.lib

// Command line options:

// server [-p:x] [-i:IP] [-o]

// -p:x Port number to listen on

// -i:str Interface to listen on

// -o Receive only, don't echo the data back

#include <winsock2.h>

#include <stdio.h>

#include <stdlib.h>

#define DEFAULT_PORT 5150

#define DEFAULT_BUFFER 4096

int iPort = DEFAULT_PORT; // Port to listen for clients on

BOOL bInterface = FALSE, // Listen on the specified interface

bRecvOnly = FALSE; // Receive data only; don't echo back

char szAddress[128]; // Interface to listen for clients on

// Function: usage

// Description:

// Print usage information and exit

void usage()

{

printf("usage: server [-p:x] [-i:IP] [-o]\n\n");

printf(" -p:x Port number to listen on\n");

printf(" -i:str Interface to listen on\n");

printf(" -o Don't echo the data back\n\n");

ExitProcess(1);

}

// Function: ValidateArgs

// Description:

// Parse the command line arguments, and set some global flags

// to indicate what actions to perform

void ValidateArgs(int argc, char** argv)

{

int i;

for (i = 1; i < argc; i++)

{

if ((argv[i][0] == '-') || (argv[i][0] == '/'))

{

switch (tolower(argv[i][1]))

{

case 'p':

iPort = atoi(&argv[i][3]);

break;

case 'i':

bInterface = TRUE;

if (strlen(argv[i]) > 3)

strcpy(szAddress, &argv[i][3]);

break;

case 'o':

bRecvOnly = TRUE;

break;

default:

usage();

break;

}

// Function: ClientThread

// Description:

// This function is called as a thread, and it handles a given

// client connection. The parameter passed in is the socket

// handle returned from an accept() call. This function reads

// data from the client and writes it back.

DWORD WINAPI ClientThread(LPVOID lpParam)

{

SOCKET sock = (SOCKET)lpParam;

char szBuff[DEFAULT_BUFFER];

int ret,

nLeft,

idx;

while (1)

{

// Perform a blocking recv() call

ret = recv(sock, szBuff, DEFAULT_BUFFER, 0);

if (ret == 0) // Graceful close

break;

else if (ret == SOCKET_ERROR)

{

printf("recv() failed: %d\n", WSAGetLastError());

break;

}

szBuff[ret] = '\0';

printf("RECV: '%s'\n", szBuff);

// If we selected to echo the data back, do it

if (!bRecvOnly)

{

nLeft = ret;

idx = 0;

// Make sure we write all the data

while (nLeft > 0)

{

ret = send(sock, &szBuff[idx], nLeft, 0);

if (ret == 0)

break;

else if (ret == SOCKET_ERROR)

{

printf("send() failed: %d\n",

WSAGetLastError());

break;

}

nLeft -= ret;

idx += ret;

}

return 0;

}

// Function: main

// Description:

// Main thread of execution. Initialize Winsock, parse the

// command line arguments, create the listening socket, bind

// to the local address, and wait for client connections.

int main(int argc, char** argv)

{

WSADATA wsd;

SOCKET sListen,

sClient;

int iAddrSize;

HANDLE hThread;

DWORD dwThreadId;

struct sockaddr_in local,

client;

ValidateArgs(argc, argv);

if (WSAStartup(MAKEWORD(2, 2), &wsd) != 0)

{

printf("Failed to load Winsock!\n");

return 1;

}

// Create our listening socket

sListen = socket(AF_INET, SOCK_STREAM, IPPROTO_IP);

if (sListen == SOCKET_ERROR)

{

printf("socket() failed: %d\n", WSAGetLastError());

return 1;

}

// Select the local interface and bind to it

if (bInterface)

{

local.sin_addr.s_addr = inet_addr(szAddress);

if (local.sin_addr.s_addr == INADDR_NONE)

usage();

}

else

local.sin_addr.s_addr = htonl(INADDR_ANY);

local.sin_family = AF_INET;

local.sin_port = htons(iPort);

if (bind(sListen, (struct sockaddr*)&local,

sizeof(local)) == SOCKET_ERROR)

{

printf("bind() failed: %d\n", WSAGetLastError());

return 1;

}

listen(sListen, 8);

// In a continous loop, wait for incoming clients. Once one

// is detected, create a thread and pass the handle off to it.

while (1)

{

iAddrSize = sizeof(client);

sClient = accept(sListen, (struct sockaddr*)&client,

&iAddrSize);

if (sClient == INVALID_SOCKET)

{

printf("accept() failed: %d\n", WSAGetLastError());

break;

}

printf("Accepted client: %s:%d\n",

inet_ntoa(client.sin_addr), ntohs(client.sin_port));

hThread = CreateThread(NULL, 0, ClientThread,

(LPVOID)sClient, 0, &dwThreadId);

if (hThread == NULL)

{

printf("CreateThread() failed: %d\n", GetLastError());

break;

}

CloseHandle(hThread);

}

closesocket(sListen);

WSACleanup();

return 0;

}

How to implement simple TCP Client and Server Program in C?

How to Compile the TCP Client/Server program?

Client Source Code

Server Source Code

About The Author

M. Saqib

How to implement simple TCP Client and Server Program in C?

How to Compile the TCP Client/Server program?

Client Source Code

Server Source Code

About The Author

M. Saqib

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