#include <unistd.h>
#include <stdio.h>

static bool switch_to_user(uid_t user_id, gid_t gp_id)
{
    if ((user_id == 0) && (gp_id == 0))
    {
        return false;
    }

    gid_t gid = getgid();
    uid_t uid = getuid();
    if (((gid != 0) || (uid != 0)) && ((gid != gp_id) || (uid != user_id)))
    {
        return false;
    }

    if (uid != 0)
    {
        return true;
    }

    if ((setgid(gp_id) < 0) || (setuid(user_id) < 0))
    {
        return false;
    }

    return true;
}

int main()
{
    printf("uid:%d  gid:%d\n", getuid(), getgid());

    switch_to_user(1000, 1000);
    printf("uid:%d  gid:%d\n", getuid(), getgid());

    return 0;
}

#include <unistd.h>
#include <stdio.h>

int main()
{
    uid_t uid = getuid();
    uid_t euid = geteuid();
    printf( "userid is %d, effective userid is: %d\n", uid, euid );
    return 0;
}

#include <assert.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>

int main(int argc, char* argv[])
{
    int filefd = open("/opt/test.txt", O_CREAT | O_WRONLY | O_TRUNC, 0666);
    assert(filefd > 0);

    int pipefd_stdout[2];
    int ret = pipe(pipefd_stdout);
    assert(ret != -1);

    int pipefd_file[2];
    ret = pipe(pipefd_file);
    assert(ret != -1);

    //close( STDIN_FILENO );
    // dup2( pipefd_stdout[1], STDIN_FILENO );
    //write( pipefd_stdout[1], "abc\n", 4 );
    ret = splice(STDIN_FILENO, NULL, pipefd_stdout[1], NULL, 32768, SPLICE_F_MORE | SPLICE_F_MOVE);
    assert(ret != -1);

    // 两个管道之间复制数据(零拷贝)
    ret = tee(pipefd_stdout[0], pipefd_file[1], 32768, SPLICE_F_NONBLOCK);
    assert(ret != -1);
    ret = splice(pipefd_file[0], NULL, filefd, NULL, 32768, SPLICE_F_MORE | SPLICE_F_MOVE);
    assert(ret != -1);
    ret = splice(pipefd_stdout[0], NULL, STDOUT_FILENO, NULL, 32768, SPLICE_F_MORE | SPLICE_F_MOVE);
    assert(ret != -1);

    close(filefd);
    close(pipefd_stdout[0]);
    close(pipefd_stdout[1]);
    close(pipefd_file[0]);
    close(pipefd_file[1]);
    return 0;
}

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

int main(int argc, char* argv[])
{
    if (argc <= 2)
    {
        printf("usage: %s ip_address port_number\n", basename(argv[0]));
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi(argv[2]);

    struct sockaddr_in address = {0};
    address.sin_family = AF_INET;
    inet_pton(AF_INET, ip, &address.sin_addr);
    address.sin_port = htons(port);

    int sock = socket(PF_INET, SOCK_STREAM, 0);
    assert(sock >= 0);

    int ret = bind(sock, (struct sockaddr*)&address, sizeof(address));
    assert(ret != -1);

    ret = listen(sock, 5);
    assert(ret != -1);

    struct sockaddr_in client;
    socklen_t client_addrlength = sizeof(client);
    int connfd = accept(sock, (struct sockaddr*)&client, &client_addrlength);
    if (connfd < 0)
    {
        printf("errno is: %d\n", errno);
    }
    else
    {
        // 关闭标准文件输入
        close(STDOUT_FILENO);

        // 把标准输出重定向到一个网络连接
        dup(connfd);
        printf("abcd\n");
        close(connfd);
    }

    close(sock);
    return 0;
}

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

int main(int argc, char* argv[])
{
    if (argc <= 2)
    {
        printf("usage: %s ip_address port_number\n", basename(argv[0]));
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi(argv[2]);

    int sock = socket(PF_INET, SOCK_STREAM, 0);
    assert(sock >= 0);
    int reuse = 1;
    setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse));

    struct sockaddr_in address = {0};
    address.sin_family = AF_INET;
    inet_pton(AF_INET, ip, &address.sin_addr);
    address.sin_port = htons(port);
    int ret = bind(sock, (struct sockaddr*)&address, sizeof(address));
    assert(ret != -1);

    ret = listen(sock, 5);
    assert(ret != -1);

    struct sockaddr_in client;
    socklen_t client_addrlength = sizeof(client);
    int connfd = accept(sock, (struct sockaddr*)&client, &client_addrlength);
    if (connfd < 0)
    {
        printf("errno is: %d\n", errno);
    }
    else
    {
        char remote[INET_ADDRSTRLEN];
        printf("connected with ip: %s and port: %d\n",
            inet_ntop(AF_INET, &client.sin_addr, remote, INET_ADDRSTRLEN), ntohs(client.sin_port));
        close(connfd);
    }

    close(sock);
    return 0;
}

#include <stdio.h>
void byteorder()
{
    union
    {
        short value;
        char union_bytes[sizeof(short)];
    } test;
    test.value = 0x0102;
    if ((test.union_bytes[0] == 1) && (test.union_bytes[1] == 2))
    {
        printf("big endian\n");
    }
    else if ((test.union_bytes[0] == 2) && (test.union_bytes[1] == 1))
    {
        printf("little endian\n");
    }
    else
    {
        printf("unknown...\n");
    }
}

int main(int argc, char* argv[])
{
    byteorder();
    return 0;
}

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


int main(int argc, char *argv[])
{
    // 命令hostname获取本地计算机名
    char* host = "localhost.localdomain";
    struct hostent* hostinfo = gethostbyname(host);

#if 1
    struct in_addr addr;
    addr.s_addr = inet_addr("10.12.129.189");

    // 16, AF_INET6 IPV6 16字节
    // 4, AF_INET   IPV4 4字节
    hostinfo = gethostbyaddr(&addr, 4, AF_INET);
#else
    hostinfo = gethostbyname(host);
#endif

    assert(hostinfo);
    struct servent* servinfo = getservbyname("daytime", "tcp");
    assert(servinfo);
    printf("daytime port is %d\n", ntohs(servinfo->s_port));

    struct sockaddr_in address = { 0 };
    address.sin_family = AF_INET;
    address.sin_port = servinfo->s_port;
    address.sin_addr = *(struct in_addr*)*hostinfo->h_addr_list;

    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    int result = connect(sockfd, (struct sockaddr*)&address, sizeof(address));
    assert(result != -1);

    char buffer[128] = { 0 };
    result = read(sockfd, buffer, sizeof(buffer));
    assert(result > 0);
    buffer[result] = '\0';
    printf("the day tiem is: %s", buffer);
    close(sockfd);
    return 0;
}

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

#define BUFFER_SIZE 1024

int main(int argc, char* argv[])
{
    if (argc <= 3)
    {
        printf("usage: %s ip_address port_number receive_buffer_size\n", basename(argv[0]));
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi(argv[2]);

    struct sockaddr_in address = {0};
    address.sin_family = AF_INET;
    inet_pton(AF_INET, ip, &address.sin_addr);
    address.sin_port = htons(port);

    int sock = socket(PF_INET, SOCK_STREAM, 0);
    assert(sock >= 0);
    int recvbuf = atoi(argv[3]);
    int len = sizeof(recvbuf);
    setsockopt(sock, SOL_SOCKET, SO_RCVBUF, &recvbuf, sizeof(recvbuf));
    getsockopt(sock, SOL_SOCKET, SO_RCVBUF, &recvbuf, (socklen_t*)&len);
    printf("the receive buffer size after settting is %d\n", recvbuf);

    int ret = bind(sock, (struct sockaddr*)&address, sizeof(address));
    assert(ret != -1);

    ret = listen(sock, 5);
    assert(ret != -1);

    struct sockaddr_in client;
    socklen_t client_addrlength = sizeof(client);
    int connfd = accept(sock, (struct sockaddr*)&client, &client_addrlength);
    if (connfd < 0)
    {
        printf("errno is: %d\n", errno);
    }
    else
    {
        char buffer[BUFFER_SIZE];
        memset(buffer, '\0', BUFFER_SIZE);
        while (recv(connfd, buffer, BUFFER_SIZE - 1, 0) > 0) {}
        close(connfd);
    }

    close(sock);
    return 0;
}

#include <sys/socket.h>
#include <arpa/inet.h>
#include <assert.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>

#define BUFFER_SIZE 512

int main(int argc, char* argv[])
{
    if (argc <= 3)
    {
        printf("usage: %s ip_address port_number send_bufer_size\n", basename(argv[0]));
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi(argv[2]);

    struct sockaddr_in server_address = {0};
    server_address.sin_family = AF_INET;
    inet_pton(AF_INET, ip, &server_address.sin_addr);
    server_address.sin_port = htons(port);

    int sock = socket(PF_INET, SOCK_STREAM, 0);
    assert(sock >= 0);

    int sendbuf = atoi(argv[3]);
    int len = sizeof(sendbuf);
    setsockopt(sock, SOL_SOCKET, SO_SNDBUF, &sendbuf, sizeof(sendbuf));
    getsockopt(sock, SOL_SOCKET, SO_SNDBUF, &sendbuf, (socklen_t*)&len);
    printf("the tcp send buffer size after setting is %d\n", sendbuf);

    if (connect(sock, (struct sockaddr*)&server_address, sizeof(server_address)) != -1)
    {
        char buffer[BUFFER_SIZE];
        memset(buffer, 'a', BUFFER_SIZE);
        send(sock, buffer, BUFFER_SIZE, 0);
    }

    close(sock);
    return 0;
}

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

int main(int argc, char* argv[])
{
    if (argc <= 2)
    {
        printf("usage: %s ip_address port_number\n", basename(argv[0]));
        return 1;
    }
    const char* ip = argv[1];
    int port = atoi(argv[2]);

    struct sockaddr_in server_address = {0};
    server_address.sin_family = AF_INET;
    inet_pton(AF_INET, ip, &server_address.sin_addr);
    server_address.sin_port = htons(port);

    int sockfd = socket(PF_INET, SOCK_STREAM, 0);
    assert(sockfd >= 0);
    if (connect(sockfd, (struct sockaddr*)&server_address, sizeof(server_address)) < 0)
    {
        printf("connection failed\n");
    }
    else
    {
        printf("send oob data out\n");
        const char* oob_data = "abc";
        const char* normal_data = "123";
        send(sockfd, normal_data, strlen(normal_data), 0);
        send(sockfd, oob_data, strlen(oob_data), MSG_OOB);
        send(sockfd, normal_data, strlen(normal_data), 0);
    }

    close(sockfd);
    return 0;
}