sockets - Is there a connection limitation using LWIP？ -

i'm writing tcp server using lwip protocol, server can connect multiple tcp clients. have problem , server can't connect more 3 active connections. 4th or more clients not connect unknown reasons. looked around forums , tried increasing number limit memp_num_tcp_pcb in opt.h (lwip-x86_64\include\lwip\opt.h) didn't help.

following code, , questions whether there max connection limitation using lwip ? maximum number of connections 3 or more?

//server code  #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <errno.h> #include <string.h> #include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <arpa/inet.h> #define myport 1234    // port users connecting #define backlog 2// how many pending connections queue hold #define buf_size 1024 int fd_a[backlog];    // accepted connection fd int conn_amount;      // current connection amount void showclient() {     int i;     printf("-----> client amount: %d\n", conn_amount);     (i = 0; < backlog; i++) {     printf("backlog%d ---> fd = %d\n", i, fd_a[i]);     }     printf("\n\n"); } int main(void) {     sleep(1);     printf("start server\n");     int sock_fd, new_fd;             // listen on sock_fd, new connection on new_fd     struct sockaddr_in server_addr;  // server address information     struct sockaddr_in client_addr;  // connector's address information     socklen_t sin_size;     int yes = 1;     char buf[buf_size];     int ret;     int i;     if ((sock_fd = socket(af_inet, sock_stream, 0)) == -1) {     perror("socket");     exit(1);     }     //if (setsockopt(sock_fd, sol_socket, so_reuseaddr, &yes, sizeof(int)) == -1) {     /*if (setsockopt(sock_fd, sol_socket, so_reuseaddr, &yes, sizeof(int)) == -1) {     //perror("setsockopt");     //exit(1);     }     if (fcntl(sock_fd, f_setfl, o_nonblock) == -1) {     printf("set server socket nonblock failed\n");     exit(1);     }*/     memset(&server_addr, 0, sizeof(server_addr));     server_addr.sin_family = af_inet;         // host byte order     server_addr.sin_port = htons(myport);     // short, network byte order     server_addr.sin_addr.s_addr = htonl(inaddr_any); // automatically fill ip     //memset(server_addr.sin_zero, '0', sizeof(server_addr.sin_zero));     if (bind(sock_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) == -1) {     perror("bind");     exit(1);     }     printf("listen...\n");     if (listen(sock_fd, backlog) == -1) {     perror("listen");     exit(1);     }     printf("listen port %d\n", myport);     fd_set fdsr, wfds, efds;     int maxsock;     struct timeval tv;     conn_amount = 0;     sin_size = sizeof(client_addr);     maxsock = sock_fd;     while (1)      {     sleep(1);  // it's necessary     // initialize file descriptor set     fd_zero(&fdsr);     fd_zero(&wfds);     //fd_zero(&efds);     fd_set(sock_fd, &fdsr);  // add fd     fd_set(sock_fd, &wfds);  // add fd     //fd_set(sock_fd, &efds);  // add fd     // timeout setting     tv.tv_sec = 30;     tv.tv_usec = 0;     // add active connection fd set     (i = 0; < backlog; i++) {         if (fd_a[i] != 0) {             fd_set(fd_a[i], &fdsr);         }     }     //printf("before select!!!!!!!!!!!! ret = %d\n", ret);     if ((select(maxsock + 1, &fdsr, &wfds, (fd_set*) 0, (struct timeval*) 0)) < 0) {         perror("select");         break;     }       // check every fd in set     (i = 0; < conn_amount; i++)      {         if (fd_isset(fd_a[i], &fdsr)) // check fd ready         {             ret = recv(fd_a[i], buf, sizeof(buf), 0);             if (ret <= 0)              {        // client close                 printf("ret : %d , client[%d] close\n", ret, i);                 close(fd_a[i]);                 fd_clr(fd_a[i], &fdsr);  // delete fd                  fd_a[i] = 0;                 conn_amount--;             }             else              {        // receive data                 if (ret < buf_size)                     memset(&buf[ret], '\0', 1); // add null('/0')                 printf("client[%d] send:%s\n", i, buf);             }         }     }     // check whether new connection comes     if (fd_isset(sock_fd, &fdsr))  // accept new connection      {         new_fd = accept(sock_fd, (struct sockaddr *)&client_addr, &sin_size);         if (new_fd <= 0)          {             perror("accept");             continue;         }         // add fd queue         if (conn_amount < backlog)          {             fd_a[conn_amount++] = new_fd;             printf("------> new connection client[%d] %s:%d\n", conn_amount,                     inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));             if (new_fd > maxsock)  // update maxsock fd select function                 maxsock = new_fd;         }         else          {             printf("max connections arrive, exit\n");             send(new_fd, "bye", 4, 0);             close(new_fd);             break;            }     }     //showclient();     }     // close other connections     (i = 0; < backlog; i++)      {     if (fd_a[i] != 0)      {         close(fd_a[i]);     }     }     exit(0); }  //client code #include   <sys/stat.h>    #include   <sys/types.h>    #include   <sys/socket.h>    #include   <stdio.h>    #include   <malloc.h>    #include   <netdb.h>    #include   <fcntl.h> #include   <unistd.h> #include   <netinet/in.h> #include   <arpa/inet.h> #include   <string.h> #define    res_length  10240  int     connect_socket(char * server,int serverport); int     send_msg(int sockfd,char * sendbuff); char *  recv_msg(int sockfd); int     close_socket(int sockfd); int main(int argc, char ** argv) {     int   sockfd[1024] = {0};     char  sendmsg[30]="zhangchengfei\r\n\r";     char* res;     int   port = 1234;     char  ip[128] = {0};     strncpy(ip, "10.107.19.62", 128);     if(argc > 2)     {     strncpy(ip, argv[1], 128);     port = atoi(argv[2]);     printf("input ip: %s, port : %d\n", ip, port);     }     else if(argc > 1)     {        //port = atoi(argv[1]);     //printf("input port : %d\n", port);     //     int num_client = atoi(argv[1]);     int = 0;     (i = 0; <= num_client; i++) {     printf("start connect %d\n", + 1);                sockfd[i]=connect_socket(ip, port);     printf("connect %d ok\n", + 1);        getchar();     }     (i = 0; <= num_client; i++) {     send_msg(sockfd[i], sendmsg);     /* res=recv_msg(sockfd); */      printf("client %d send msg = %s\n", + 1, sendmsg);     //printf(res);     //free(res);     getchar();     }     (i = 0; <= num_client; i++) {     close_socket(sockfd[i]);     printf("close socket %d\n", + 1);     }     }     return 0; }  int    connect_socket(char * server,int serverport){     int    sockfd=0;     struct    sockaddr_in    addr;     struct    hostent        * phost;      if((sockfd=socket(af_inet,sock_stream,0))<0){     herror("init socket error!");     return -1;     }     bzero(&addr,sizeof(addr));     addr.sin_family = af_inet;     addr.sin_port = htons(serverport);     addr.sin_addr.s_addr = inet_addr(server);      if(addr.sin_addr.s_addr == inaddr_none){     phost = (struct hostent*)gethostbyname(server);     if(phost==null){         herror("init socket s_addr error!");         return -1;     }     addr.sin_addr.s_addr =((struct in_addr*)phost->h_addr)->s_addr;     }     if(connect(sockfd,(struct sockaddr*)&addr, sizeof(addr))<0)     {     perror("connect server fail!");     return -1;      }     else     return sockfd; }   int send_msg(int sockfd,char * sendbuff) {     int sendsize=0;     if((sendsize=send(sockfd,sendbuff,strlen(sendbuff),0))<=0){     herror("send msg error!");     return -1;     }else     return sendsize; }   char* recv_msg(int sockfd){     char * response;     int  flag=0,reclenth=0;     response=(char *)malloc(res_length);     memset(response,0,res_length);      for(flag=0;;)     {     printf("======recv data:\n");     if(( reclenth=recv(sockfd,response+flag,res_length-flag,0))==-1 )     {         free(response);         printf("return value : %d\n", reclenth);         perror("recv msg error : ");         return null;     }     else if(reclenth==0)         break;     else     {         printf("%d char recieved data : %s.\n", reclenth, response+flag);         flag+=reclenth;         reclenth=0;     }     }     printf("return value : %d\n", reclenth);     response[flag]='0';     return response; }   int close_socket(int sockfd) {     close(sockfd);     return 0; } 

i have solved problem caused makefile bug in ./stubdom, after executing make crossclean , found .o files still existed in lwip-x86_64 folder. add command find . -name "*.o" | xargs rm -f in makefile , ok.

the reason lwip configure given macro definition, macro definition have been replaced @ pre-compilation stage, if modify lwip configure , did not delete .o files, there no effect. following macro definitions have modified in opt.h.

    memp_num_tcp_pcb 100     memp_num_tcp_pcb_listen 100     memp_num_netconn 100 

now server can break through connection limitations , more 3 active tcp connections can established.