Server port doenst get back to listen and is in closed_wait state , because of starting a daemon application from client

Question

Below is the basic client and server code . While iam trying to start an appliaction(which has to keep running in the machine once we send a msg to server) during connect call. using system(/bin/myApplication) in the client code (This basically is a simple c executable with an infinite loop,assuming my application has to be running); Once iam starting the application the server is in CLOSED_WAIT State and its coming back to listen only if i stop this application . The issue is with the server socket in CLOSED_WAIT state, Is there a way that this application keep running as a seperate process and server would be in listen state again. Client Code: There is a system command after read.and it starts an application(basically is an c executable with an infinite while loop)

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h> 

int main(int argc, char *argv[])
{
    int sockfd = 0, n = 0;
    char recvBuff[1024];
    struct sockaddr_in serv_addr; 

    if(argc != 2)
    {
        printf("\n Usage: %s <ip of server> \n",argv[0]);
        return 1;
    } 

    memset(recvBuff, '0',sizeof(recvBuff));
    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        printf("\n Error : Could not create socket \n");
        return 1;
    } 

    memset(&serv_addr, '0', sizeof(serv_addr)); 

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(5000); 

    if(inet_pton(AF_INET, argv[1], &serv_addr.sin_addr)<=0)
    {
        printf("\n inet_pton error occured\n");
        return 1;
    } 

    if( connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
    {
       printf("\n Error : Connect Failed \n");
       return 1;
    } 

    while ( (n = read(sockfd, recvBuff, sizeof(recvBuff)-1)) > 0)
    {
        recvBuff[n] = 0;
        if(fputs(recvBuff, stdout) == EOF)
        {
            printf("\n Error : Fputs error\n");
        }
    } 
    system(/bin/myApplication);
    if(n < 0)
    {
        printf("\n Read error \n");
    } 

    return 0;

Server Code:

int main(int argc, char *argv[])
{
    int listenfd = 0, connfd = 0;
    struct sockaddr_in serv_addr; 

    char sendBuff[1025];
    time_t ticks; 

    listenfd = socket(AF_INET, SOCK_STREAM, 0);
    memset(&serv_addr, '0', sizeof(serv_addr));
    memset(sendBuff, '0', sizeof(sendBuff)); 

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    serv_addr.sin_port = htons(5000); 

    bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr)); 

    listen(listenfd, 10); 

    while(1)
    {
        connfd = accept(listenfd, (struct sockaddr*)NULL, NULL); 

        ticks = time(NULL);
        snprintf(sendBuff, sizeof(sendBuff), "%.24s\r\n", ctime(&ticks));
        write(connfd, sendBuff, strlen(sendBuff)); 

        close(connfd);
        sleep(1);
     }
} 

My Sample C File , which iam trying to execute using system command in client code. with

gcc Sample.c -o myApplication

int main(int argc, char *argv[])
{

    while(1)

    {

    }
}

I have tried using fork , but still the socket is in same state , is there anyway that this application runs seperately and doesnt effect the socket nor does it depend on the parent process(Server here).

Answer 1

I added the below line before the system command

fcntl(sockfd, F_SETFD, fcntl(sockfd, F_GETFD) | FD_CLOEXEC);

And Changed my client code to

#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <fcntl.h>
int main(int argc, char *argv[])
{
    int sockfd = 0, n = 0;
    char recvBuff[1024];
    struct sockaddr_in serv_addr;

    if(argc != 2)
    {
        printf("\n Usage: %s <ip of server> \n",argv[0]);
        return 1;
    }

    memset(recvBuff, '0',sizeof(recvBuff));
    if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0)
    {
        printf("\n Error : Could not create socket \n");
        return 1;
    }

    memset(&serv_addr, '0', sizeof(serv_addr));

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(5000);

    if(inet_pton(AF_INET, argv[1], &serv_addr.sin_addr)<=0)
    {
        printf("\n inet_pton error occured\n");
        return 1;
    }

    if( connect(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
    {
       printf("\n Error : Connect Failed \n");
       return 1;
    }

    while ( (n = read(sockfd, recvBuff, sizeof(recvBuff)-1)) > 0)
    {
        recvBuff[n] = 0;
        if(fputs(recvBuff, stdout) == EOF)
          printf("\n Error : Fputs error\n");
        }

   }

        fcntl(sockfd, F_SETFD, fcntl(sockfd, F_GETFD) | FD_CLOEXEC);

         system("/software/itcsm/itcsm/refasset/bin/start_script.sh 1");

    if(n < 0)
    {
        printf("\n Read error \n");
    }
        printf("\n after system \n");

    return 0;
}

By default whenever we fork a process (which system command does), the child inherits all the parent's file descriptors. If the child doesn't need those descriptors, it SHOULD close them voluntarily on file descriptor using fcntl(sockfd, F_SETFD, fcntl(sockfd, F_GETFD) | FD_CLOEXEC);