дофига открытых сокетов

$ cat server.cpp 
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdint.h>
#include <unistd.h>

int main()
{
        int rc;


        rc = socket(AF_INET, SOCK_STREAM, 0);
        if (rc == -1)
        {
                printf("error socket: %s\n", strerror(errno));
                return 1;
        }
        int sock = rc;


        sockaddr_in addr;
        addr.sin_family      = AF_INET;
        addr.sin_port        = htons(30000);
        addr.sin_addr.s_addr = inet_addr("127.0.0.1");


        rc = bind(sock, (struct sockaddr *)&addr, sizeof(addr));
        if (rc == -1)
        {
                printf("error bind: %s\n", strerror(errno));
                return 1;
        }


        rc = listen(sock, 1024);
        if (rc == -1)
        {
                printf("error listen: %s\n", strerror(errno));
                return 1;
        }


        uint64_t count = 1;
        for (;;)
        {
                rc = accept(sock, NULL, NULL);
                if (rc == -1)
                {
                        printf("error accept: %s\n", strerror(errno));
                        return 1;
                }

                printf("server count:%lu\n", count++);
        }


        return 0;
}

$ cat client.cpp 
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <stdint.h>
#include <unistd.h>

int main()
{
        int rc;


        uint64_t count = 1;
        for (;;)
        {
                rc = socket(AF_INET, SOCK_STREAM, 0);
                if (rc == -1)
                {
                        printf("error socket: %s\n", strerror(errno));
                        return 1;
                }
                int sock = rc;


                sockaddr_in addr;
                addr.sin_family      = AF_INET;
                addr.sin_port        = htons(30000);
                addr.sin_addr.s_addr = inet_addr("127.0.0.1");


                rc = connect(sock, (struct sockaddr *)&addr, sizeof(addr));
                if (rc == -1)
                {
                        printf("error connect: %s\n", strerror(errno));
                        break;
                }

                printf("client count:%lu\n", count++);
        }


        pause();


        return 0;
}

// c.cc

#include <cstring>
#include <cerrno>

#include <string>
#include <iostream>

#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>

#define CHECK(call) \
    if ((call) == -1) throw std::string(strerror(errno));

int main(int, char **argv) {

    try {

        for (size_t count = 1;; ++count) {

            int sock;
            CHECK(sock = socket(AF_INET, SOCK_STREAM, 0));

            sockaddr_in laddr;
            laddr.sin_family = AF_INET;
            laddr.sin_addr.s_addr = inet_addr(argv[1]);
            laddr.sin_port = 0;
            CHECK(bind(sock, (sockaddr*)&laddr, sizeof laddr));

            sockaddr_in raddr;
            raddr.sin_family = AF_INET;
            raddr.sin_port = htons(30000);
            raddr.sin_addr.s_addr = inet_addr(argv[2]);
            CHECK(connect(sock, (sockaddr*)&raddr, sizeof raddr));

            std::cout << '[' << getpid() << ']'
                      << " client count: " << count << std::endl;

        }

    } catch (std::string const& error) {

        std::cout << error << std::endl;

    }

}

// s.cc

#include <cstring>
#include <cerrno>

#include <string>
#include <iostream>

#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>

#define CHECK(call) \
    if ((call) == -1) throw std::string(strerror(errno));

int main() {

    try {

        int sock;
        CHECK(sock = socket(AF_INET, SOCK_STREAM, 0));

        int optval = 1;
        CHECK(setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof optval));

        sockaddr_in addr;
        addr.sin_family = AF_INET;
        addr.sin_port = htons(30000);
        addr.sin_addr.s_addr = inet_addr("127.0.0.1"); // htonl(INADDR_ANY);
        CHECK(bind(sock, (sockaddr*)&addr, sizeof addr));

        CHECK(listen(sock, 1024));

        for (size_t count = 1;; ++count) {
            CHECK(accept(sock, 0, 0));
            std::cout << "server count: " << count << std::endl;
        }

    } catch (std::string const& error) {

        std::cout << error << std::endl;

    }

}

# echo 1000000 > /proc/sys/fs/file-max
# ulimit -Hn 1000000; ulimit -Sn 1000000
# c++ c.cc -o c; c++ s.cc -o s
[1] # ./c 127.0.0.1 127.0.0.1 & ./c 127.0.0.2 127.0.0.1 &./c 127.0.0.3 127.0.0.1 &./c 127.0.0.4 127.0.0.1 &./c 127.0.0.5 127.0.0.1 & ./c 127.0.0.6 127.0.0.1 & ./c 127.0.0.7 127.0.0.1 &./c 127.0.0.8 127.0.0.1 &./c 127.0.0.9 127.0.0.1 &./c 127.0.0.10 127.0.0.1 &
...
[5421] client count: 28072
[5418] client count: 28233
Address already in use
[5421] client count: 28073
[5421] client count: 28233
Address already in use

[1]   Done                    ./c 127.0.0.1 127.0.0.1
[2]   Done                    ./c 127.0.0.2 127.0.0.1
[3]   Done                    ./c 127.0.0.3 127.0.0.1
[4]   Done                    ./c 127.0.0.4 127.0.0.1
[5]   Done                    ./c 127.0.0.5 127.0.0.1
[6]   Done                    ./c 127.0.0.6 127.0.0.1
[7]   Done                    ./c 127.0.0.7 127.0.0.1
[8]   Done                    ./c 127.0.0.8 127.0.0.1
[9]-  Done                    ./c 127.0.0.9 127.0.0.1
[10]+  Done                    ./c 127.0.0.10 127.0.0.1
[2] # ./s
...
server count: 282328
server count: 282329
server count: 282330
^C