Linux Client Server Socket Program

Today let's see how to create an effective socket program with client and server communication capability. This program is based on TCP protocol. First, server program should be launched. You can specify a port in server for listening client requests. When the server get a  client request, server needs to create a separate process for that client. When running client program, client should know IP address and the server port.

Server.c

[sourcecode language="c"]
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <linux/in.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <stdbool.h>
#include <math.h>

float stof(const char* s);
float calculator(char *calculation);

extern int errno;

int main(int argc,char **argv)
{
int clientaddrlen, listenfd, connectfd, bytes_rcvd, listen_queue_size=1;
short int port_no;
char readBuff[1000];
struct sockaddr_in servaddr, clientaddr;
pid_t  childpid;
int status;
char sendBuff[1025];

if(argc!=3){
printf("Usage Format: ./server -p <PortNumber>\n");
printf("Sample Run: ./server -p 2000\n");
exit(1);
}
port_no = atoi(argv[argc-1]);
printf("Server running at port #%d\n", port_no);

// Create server socket.
if ( (listenfd = socket(AF_INET, SOCK_STREAM, 0)) < 0){
fprintf(stderr, "Cannot create server socket! errno %i: %s\n",errno,strerror(errno));
exit(1);
}
printf("Server socket created\n");
// Bind (attach) this process to the server socket.
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(port_no);
errno = bind(listenfd, (struct sockaddr *) &servaddr, sizeof(servaddr));
if(errno < 0){
printf("Server bind failure errno %i: %s\n",errno,strerror(errno));
exit(1);
}
printf("Server socket is bound to port #%d\n", port_no);
// Turn 'listenfd' to a listening socket. Listen queue size is 1.
errno=listen(listenfd,listen_queue_size);
if(errno < 0){
printf("Server listen failure errno %i: %s\n",errno,strerror(errno));
exit(1);
}
printf("Server listening with a queue of size %d. \n", listen_queue_size);
// Wait for connection(s) from client(s).
while (1){
clientaddrlen = sizeof(clientaddr);
connectfd = accept(listenfd, (struct sockaddr *) &clientaddr, &clientaddrlen);
if(connectfd<0){
printf("Server accept failure errno %d: %s\n",errno,strerror(errno));
exit(1);
}
printf("A connection received from a client. Creating a child to serve the client.\n");
if((childpid = fork()) == 0) { /* child process */
close(listenfd); /* close listening socket */
printf("Child process serving the client.\n");
if (recv(connectfd, readBuff, sizeof(readBuff), 0 ) > 0){
printf("Received message: %s\n", readBuff);
float answer = calculator(readBuff);
char msg[60];
char chAns[30]="";
sprintf(chAns,"%f",answer);
strcat(msg,chAns);
write(connectfd,msg, strlen(readBuff));
}
//close(connectfd);   /* parent closes connected socket */
exit(1);
}
else if (childpid <0){ /* failed to fork */
printf("Failed to fork\n");
printf("Fork error errno %d: %s\n",errno,strerror(errno));
}
else if(childpid != 0){  /* parent process */
close(connectfd);   /* parent closes connected socket */
childpid = wait(&status);
}
}
return 0;
}

float calculator(char calculation[256])
{

char *token;

char *val1;
char *val2;

float  v1;
float  v2;
float  ans=-9999.0f;

bool validCal = true;

printf("****\n");

char tempCalculation[256];

strcpy(tempCalculation,calculation);

printf("****\n");

token = strtok(tempCalculation,"+-*//^");

int c = 0;
while(token != NULL)
{
c++;
printf("%d\n",c);

if(c == 1)
val1 = token;

if(c == 2)
val2 = token;

printf("%s\n",token);
token = strtok(NULL,"+-*//^");

}

if(c == 2)
{

int i1;
const int lenVal1 = strlen(val1);
for(i1=0; i1<lenVal1 ; i1++)
{
if(!isdigit(val1[i1]))
break;
}
if(i1 != lenVal1)
validCal = false;

int i2;
const int lenVal2 = strlen(val2)-1;
for(i2=0; i2<lenVal2 ; i2++)
{
if(!isdigit(val2[i2]))
break;
}
if(i2 != lenVal2)
validCal = false;

printf("operand1:%soperand2:%s\nlen1:%dlen2%d\n",val1,val2,lenVal1,lenVal2);

if(validCal)
{
char operator = calculation[lenVal1];

printf("Operator%c\n",operator);
printf("Calculation:%s\n",calculation);

v1 = stof(val1);
v2 = stof(val2);

printf("v1=%f\n",v1);
printf("v2=%f\n",v2);

float fv1 = stof(val1);
printf("fv1=%f\n",fv1);

//float f1 = 56.4f;
//float f2 = 2.4f;

if(operator == '+')
{
printf("Addition\n");
ans = v1 + v2;
}
else if(operator == '-')
{
printf("Substraction\n");
ans = v1 - v2;
}
else if(operator == '*')
{
printf("Multiplication\n");
ans = v1 * v2;
}
else if(operator == '/')
{
printf("Devision\n");
ans = v1 / v2;
}
else if(operator == '^')
{
printf("Power\n");

}

else
{

}

printf("Answer:%f\n",ans);

}
else
{
printf("The values are not numeric!\n");

}

return ans;

}
else
{
printf("Two operands are required for calculation\n");
validCal = false;

return ans;

}

};

float stof(const char* s){
float rez = 0, fact = 1;
if (*s == '-'){
s++;
fact = -1;
};

int point_seen;
for (point_seen = 0; *s; s++){
if (*s == '.'){
point_seen = 1;
continue;
};
int d = *s - '0';
if (d >= 0 && d <= 9){
if (point_seen) fact /= 10.0f;
rez = rez * 10.0f + (float)d;
};
};
return rez * fact;
};

[/sourcecode]

Client.c

[sourcecode language="c"]
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <linux/in.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
int main(int argc,char **argv)
{
int clientfd;
short int port_no;
char msg[1000];
char buff[1000];

struct sockaddr_in servaddr;
char *server_ip_address;
if(argc!=5){
printf("Usage Format: ./client -a <IPAddress> -p <PortNumber>\n");
printf("Sample Run: ./client -a 127.0.0.1 -p 2000\n");
exit(1);
}
server_ip_address=(char *)argv[2];
port_no = atoi(argv[4]);
printf("Client will connect to the server at IP %s, port #%d\n", server_ip_address, port_no);

// Create client socket.
if((clientfd = socket(AF_INET, SOCK_STREAM, 0))<0){
printf("Socket could not be created\n");
printf("errno %i: %s\n",errno,strerror(errno));
exit(1);
}
printf("Client socket created\n");
errno=0;
// Connect to the server client
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(port_no);
if((inet_pton(AF_INET, server_ip_address, &servaddr.sin_addr))<=0){
printf("inet_pton error for %s\n",server_ip_address);
printf("errno %d: %s\n",errno,strerror(errno));
exit(1);
}
errno=0;
if((connect(clientfd, (struct sockaddr *) &servaddr, sizeof(servaddr)))<0){
printf("Connect error\n");
printf("errno %d: %s\n",errno,strerror(errno));
exit(1);
}
printf("Client socket connected\n");
// Read one line of message from the input and send it to the server.
while(1)
{
printf("\nEnter the message to be sent to the server: ");
scanf("%s", msg);
send(clientfd, msg, strlen(msg)+1, 0);
//close(clientfd);

if (recv(clientfd, buff, sizeof(buff), 0 ) > 0){
printf("Server Response: %s\n", buff);

}
}
return 0;
}

[/sourcecode]

Run server :

gcc -oa server.c

./a -p 2000

Run client:

gcc  -ob client.c

./b  -a 127.0.0.1  -p 2000