1、阻塞I/O     2、非阻塞I/O     3 、I/O复用（select和poll）   4、信号驱动I/O    5、异步I/O

select函数I/O复用：

select Function

The select function determines the status of one or more sockets, waiting if necessary, to perform synchronous I/O.

int select(
  __in          int nfds,
  __in_out      fd_set* readfds,
  __in_out      fd_set* writefds,
  __in_out      fd_set* exceptfds,
  __in          const struct timeval* timeout
);

Parameters

nfds

Ignored. The nfds parameter is included only for compatibility with Berkeley sockets.

readfds

Optional pointer to a set of sockets to be checked for readability.

writefds

Optional pointer to a set of sockets to be checked for writability.

exceptfds

Optional pointer to a set of sockets to be checked for errors.

timeout

Maximum time for select to wait, provided in the form of a TIMEVAL structure. Set the timeout parameter to null for blocking operations.

Return Value

The select function returns the total number of socket handles that are ready and contained in thefd_set structures, zero if the time limit expired, or SOCKET_ERROR if an error occurred. If the return value is SOCKET_ERROR,WSAGetLastError can be used to retrieve a specific error code.

Remarks

The select function is used to determine the status of one or more sockets. For each socket, the caller can request information on read, write, or error status. The set of sockets for which a given status is requested is indicated by anfd_set structure. The sockets contained within thefd_set structures must be associated with a single service provider. For the purpose of this restriction, sockets are considered to be from the same service provider if theWSAPROTOCOL_INFO structures describing their protocols have the sameproviderId value. Upon return, the structures are updated to reflect the subset of these sockets that meet the specified condition. Theselect function returns the number of sockets meeting the conditions. A set of macros is provided for manipulating anfd_set structure. These macros are compatible with those used in the Berkeley software, but the underlying representation is completely different.

The parameter readfds identifies the sockets that are to be checked for readability. If the socket is currently in thelisten state, it will be marked as readable if an incoming connection request has been received such that anaccept is guaranteed to complete without blocking. For other sockets, readability means that queued data is available for reading such that a call torecv,WSARecv,WSARecvFrom, orrecvfrom is guaranteed not to block.

For connection-oriented sockets, readability can also indicate that a request to close the socket has been received from the peer. If the virtual circuit was closed gracefully, and all data was received, then arecv will return immediately with zero bytes read. If the virtual circuit was reset, then arecv will complete immediately with an error code such asWSAECONNRESET. The presence of OOB data will be checked if the socket option SO_OOBINLINE has been enabled (seesetsockopt).

The parameter writefds identifies the sockets that are to be checked for writability. If a socket is processing aconnect call (nonblocking), a socket is writeable if the connection establishment successfully completes. If the socket is not processing aconnect call, writability means asend, sendto, orWSASendto are guaranteed to succeed. However, they can block on a blocking socket if thelen parameter exceeds the amount of outgoing system buffer space available. It is not specified how long these guarantees can be assumed to be valid, particularly in a multithreaded environment.

The parameter exceptfds identifies the sockets that are to be checked for the presence of OOB data or any exceptional error conditions.

Note  Out-of-band data will only be reported in this way if the option SO_OOBINLINE is FALSE. If a socket is processing aconnect call (nonblocking), failure of the connect attempt is indicated inexceptfds (application must then call getsockopt SO_ERROR to determine the error value to describe why the failure occurred). This document does not define which other errors will be included.

Any two of the parameters, readfds, writefds, or exceptfds, can be given as null. At least one must be non-null, and any non-null descriptor set must contain at least one handle to a socket.

In summary, a socket will be identified in a particular set when select returns if:

readfds:

• If listen has been called and a connection is pending,accept will succeed.
• Data is available for reading (includes OOB data if SO_OOBINLINE is enabled).
• Connection has been closed/reset/terminated.

writefds:

• If processing a connect call (nonblocking), connection has succeeded.
• Data can be sent.

exceptfds:

• If processing a connect call (nonblocking), connection attempt failed.
• OOB data is available for reading (only if SO_OOBINLINE is disabled).

Four macros are defined in the header file Winsock2.h for manipulating and checking the descriptor sets. The variable FD_SETSIZE determines the maximum number of descriptors in a set. (The default value of FD_SETSIZE is 64, which can be modified by defining FD_SETSIZE to another value before including Winsock2.h.) Internally, socket handles in anfd_set structure are not represented as bit flags as in Berkeley Unix. Their data representation is opaque. Use of these macros will maintain software portability between different socket environments. The macros to manipulate and checkfd_set contents are:

FD_CLR(s, *set)

Removes the descriptor s from set.

FD_ISSET(s, *set)

Nonzero if s is a member of the set. Otherwise, zero.

FD_SET(s, *set)

Adds descriptor s to set.

FD_ZERO(*set)

Initializes the set to the null set.

The parameter time-out controls how long the select can take to complete. Iftime-out is a null pointer,select will block indefinitely until at least one descriptor meets the specified criteria. Otherwise,time-out points to aTIMEVAL structure that specifies the maximum time thatselect should wait before returning. Whenselect returns, the contents of theTIMEVAL structure are not altered. IfTIMEVAL is initialized to {0, 0},select will return immediately; this is used to poll the state of the selected sockets. Ifselect returns immediately, then theselect call is considered nonblocking and the standard assumptions for nonblocking calls apply. For example, the blocking hook will not be called, and Windows Sockets will not yield.

Note  The select function has no effect on the persistence of socket events registered withWSAAsyncSelect orWSAEventSelect.

Requirements

echocli.c

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

#define ERR_EXIT(m) \
        do \
        { \
                perror(m); \
                exit(EXIT_FAILURE); \
        } while(0)

ssize_t readn(int fd, void *buf, size_t count)
{
        size_t nleft = count;
        ssize_t nread;
        char *bufp = (char*)buf;

        while (nleft > 0)
        {
                if ((nread = read(fd, bufp, nleft)) < 0)
                {
                        if (errno == EINTR)
                                continue;
                        return -1;
                }
                else if (nread == 0)
                        return count - nleft;

                bufp += nread;
                nleft -= nread;
        }

        return count;
}

ssize_t writen(int fd, const void *buf, size_t count)
{
        size_t nleft = count;
        ssize_t nwritten;
        char *bufp = (char*)buf;

        while (nleft > 0)
        {
                if ((nwritten = write(fd, bufp, nleft)) < 0)
                {
                        if (errno == EINTR)
                                continue;
                        return -1;
                }
                else if (nwritten == 0)
                        continue;

                bufp += nwritten;
                nleft -= nwritten;
        }

        return count;
}

ssize_t recv_peek(int sockfd, void *buf, size_t len)
{
        while (1)
        {
                int ret = recv(sockfd, buf, len, MSG_PEEK);
                if (ret == -1 && errno == EINTR)
                        continue;
                return ret;
        }
}


ssize_t readline(int sockfd, void *buf, size_t maxline)
{
        int ret;
        int nread;
        char *bufp = buf;
        int nleft = maxline;
        while (1)
        {
                ret = recv_peek(sockfd, bufp, nleft);
                if (ret < 0)
                        return ret;
                else if (ret == 0)
                        return ret;

                nread = ret;
                int i;
                for (i=0; i<nread; i++)
                {
                        if (bufp[i] == ‘\n‘)
                        {
                                ret = readn(sockfd, bufp, i+1);
                                if (ret != i+1)
                                        exit(EXIT_FAILURE);

                                return ret;
                        }
                }

                if (nread > nleft)
                        exit(EXIT_FAILURE);

                nleft -= nread;
                ret = readn(sockfd, bufp, nread);
                if (ret != nread)
                        exit(EXIT_FAILURE);

                bufp += nread;
        }

        return -1;
}

void echo_cli(int sock)
{
/*
    char sendbuf[1024] = {0};
        char recvbuf[1024] = {0};
        while (fgets(sendbuf, sizeof(sendbuf), stdin) != NULL)
        {
                writen(sock, sendbuf, strlen(sendbuf));

                int ret = readline(sock, recvbuf, sizeof(recvbuf));
                if (ret == -1)
                        ERR_EXIT("readline");
                else if (ret == 0)
                {
                        printf("client close\n");
                        break;
                }

                fputs(recvbuf, stdout);
                memset(sendbuf, 0, sizeof(sendbuf));
                memset(recvbuf, 0, sizeof(recvbuf));
        }

        close(sock);
*/

    fd_set rset;
    FD_ZERO(&rset);

    int nready;
    int maxfd;
    int fd_stdin = fileno(stdin);
    if (fd_stdin > sock)
        maxfd = fd_stdin;
    else
        maxfd = sock;

    char sendbuf[1024] = {0};
        char recvbuf[1024] = {0};

    while (1)
    {
        FD_SET(fd_stdin, &rset);
        FD_SET(sock, &rset);
        nready = select(maxfd+1, &rset, NULL, NULL, NULL);
        if (nready == -1)
            ERR_EXIT("select");

        if (nready == 0)
            continue;

        if (FD_ISSET(sock, &rset))
        {
            int ret = readline(sock, recvbuf, sizeof(recvbuf));
                    if (ret == -1)
                            ERR_EXIT("readline");
                    else if (ret == 0)
                    {
                            printf("server close\n");
                            break;
                    }

                       fputs(recvbuf, stdout);
                    memset(recvbuf, 0, sizeof(recvbuf));
        }
        if (FD_ISSET(fd_stdin, &rset))
        {
            if (fgets(sendbuf, sizeof(sendbuf), stdin) == NULL)
                break;
            writen(sock, sendbuf, strlen(sendbuf));
            memset(sendbuf, 0, sizeof(sendbuf));
        }
    }

    close(sock);
}

void handle_sigpipe(int sig)
{
    printf("recv a sig=%d\n", sig);
}

int main(void)
{
/*
    signal(SIGPIPE, handle_sigpipe);
*/
    signal(SIGPIPE, SIG_IGN);
    int sock;
    if ((sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
        ERR_EXIT("socket");

    struct sockaddr_in servaddr;
    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(5188);
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");

    if (connect(sock, (struct sockaddr*)&servaddr, sizeof(servaddr)) < 0)
        ERR_EXIT("connect");

    struct sockaddr_in localaddr;
    socklen_t addrlen = sizeof(localaddr);
    if (getsockname(sock, (struct sockaddr*)&localaddr, &addrlen) < 0)
        ERR_EXIT("getsockname");

    printf("ip=%s port=%d\n", inet_ntoa(localaddr.sin_addr), ntohs(localaddr.sin_port));


    echo_cli(sock);

    return 0;
}

echosrv.c

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <signal.h>
#include <sys/wait.h>

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

#define ERR_EXIT(m) \
        do \
        { \
                perror(m); \
                exit(EXIT_FAILURE); \
        } while(0)

ssize_t readn(int fd, void *buf, size_t count)
{
    size_t nleft = count;
    ssize_t nread;
    char *bufp = (char*)buf;

    while (nleft > 0)
    {
        if ((nread = read(fd, bufp, nleft)) < 0)
        {
            if (errno == EINTR)
                continue;
            return -1;
        }
        else if (nread == 0)
            return count - nleft;

        bufp += nread;
        nleft -= nread;
    }

    return count;
}

ssize_t writen(int fd, const void *buf, size_t count)
{
    size_t nleft = count;
    ssize_t nwritten;
    char *bufp = (char*)buf;

    while (nleft > 0)
    {
        if ((nwritten = write(fd, bufp, nleft)) < 0)
        {
            if (errno == EINTR)
                continue;
            return -1;
        }
        else if (nwritten == 0)
            continue;

        bufp += nwritten;
        nleft -= nwritten;
    }

    return count;
}

ssize_t recv_peek(int sockfd, void *buf, size_t len)
{
    while (1)
    {
        int ret = recv(sockfd, buf, len, MSG_PEEK);
        if (ret == -1 && errno == EINTR)
            continue;
        return ret;
    }
}

ssize_t readline(int sockfd, void *buf, size_t maxline)
{
    int ret;
    int nread;
    char *bufp = buf;
    int nleft = maxline;
    while (1)
    {
        ret = recv_peek(sockfd, bufp, nleft);
        if (ret < 0)
            return ret;
        else if (ret == 0)
            return ret;

        nread = ret;
        int i;
        for (i=0; i<nread; i++)
        {
            if (bufp[i] == ‘\n‘)
            {
                ret = readn(sockfd, bufp, i+1);
                if (ret != i+1)
                    exit(EXIT_FAILURE);

                return ret;
            }
        }

        if (nread > nleft)
            exit(EXIT_FAILURE);

        nleft -= nread;
        ret = readn(sockfd, bufp, nread);
        if (ret != nread)
            exit(EXIT_FAILURE);

        bufp += nread;
    }

    return -1;
}

void echo_srv(int conn)
{
    char recvbuf[1024];
        while (1)
        {
                memset(recvbuf, 0, sizeof(recvbuf));
                int ret = readline(conn, recvbuf, 1024);
        if (ret == -1)
            ERR_EXIT("readline");
        if (ret == 0)
        {
            printf("client close\n");
            break;
        }
        
                fputs(recvbuf, stdout);
                writen(conn, recvbuf, strlen(recvbuf));
        }
}

void handle_sigchld(int sig)
{
/*    wait(NULL);*/
    while (waitpid(-1, NULL, WNOHANG) > 0)
        ;
}

int main(void)
{
/*    signal(SIGCHLD, SIG_IGN);*/
    signal(SIGCHLD, handle_sigchld);
    int listenfd;
    if ((listenfd = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0)
/*    if ((listenfd = socket(PF_INET, SOCK_STREAM, 0)) < 0)*/
        ERR_EXIT("socket");

    struct sockaddr_in servaddr;
    memset(&servaddr, 0, sizeof(servaddr));
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(5188);
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    /*servaddr.sin_addr.s_addr = inet_addr("127.0.0.1");*/
    /*inet_aton("127.0.0.1", &servaddr.sin_addr);*/

    int on = 1;
    if (setsockopt(listenfd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on)) < 0)
        ERR_EXIT("setsockopt");

    if (bind(listenfd, (struct sockaddr*)&servaddr, sizeof(servaddr)) < 0)
        ERR_EXIT("bind");
    if (listen(listenfd, SOMAXCONN) < 0)
        ERR_EXIT("listen");

    struct sockaddr_in peeraddr;
    socklen_t peerlen;    
    int conn;

/*
    pid_t pid;
    while (1)
    {
        if ((conn = accept(listenfd, (struct sockaddr*)&peeraddr, &peerlen)) < 0)
            ERR_EXIT("accept");

        printf("ip=%s port=%d\n", inet_ntoa(peeraddr.sin_addr), ntohs(peeraddr.sin_port));

        pid = fork();
        if (pid == -1)
            ERR_EXIT("fork");
        if (pid == 0)
        {
            close(listenfd);
            echo_srv(conn);
            exit(EXIT_SUCCESS);
        }
        else
            close(conn);
    }
*/

    int i;
    int client[FD_SETSIZE];
    int maxi = 0;

    for (i=0; i<FD_SETSIZE; i++)
        client[i] = -1;

    int nready;
    int maxfd = listenfd;
    fd_set rset;
    fd_set allset;
    FD_ZERO(&rset);
    FD_ZERO(&allset);
    FD_SET(listenfd, &allset);
    while (1)
    {
        rset = allset;
        nready = select(maxfd+1, &rset, NULL, NULL, NULL);
        if (nready == -1)
        {
            if (errno == EINTR)
                continue;
            
            ERR_EXIT("select");
        }
        if (nready == 0)
            continue;

        if (FD_ISSET(listenfd, &rset))
        {
            peerlen = sizeof(peeraddr);
            conn = accept(listenfd, (struct sockaddr*)&peeraddr, &peerlen);
            if (conn == -1)
                ERR_EXIT("accept");

            for (i=0; i<FD_SETSIZE; i++)
            {
                if (client[i] < 0)
                {
                    client[i] = conn;
                    if (i > maxi)
                        maxi = i;
                    break;
                }
            }

            if (i == FD_SETSIZE)
            {
                fprintf(stderr, "too many clients\n");
                exit(EXIT_FAILURE);
            }
            printf("ip=%s port=%d\n", inet_ntoa(peeraddr.sin_addr), ntohs(peeraddr.sin_port));
            
            FD_SET(conn, &allset);
            if (conn > maxfd)
                maxfd = conn;

            if (--nready <= 0)
                continue;

        }

        for (i=0; i<=maxi; i++)
        {
            conn = client[i];
            if (conn == -1)
                continue;

            if (FD_ISSET(conn, &rset))
            {
                char recvbuf[1024] = {0};
                int ret = readline(conn, recvbuf, 1024);
                        if (ret == -1)
                                ERR_EXIT("readline");
                        if (ret == 0)
                        {
                                printf("client close\n");
                    FD_CLR(conn, &allset);
                    client[i] = -1;
                        }

                        fputs(recvbuf, stdout);
                        writen(conn, recvbuf, strlen(recvbuf));

                if (--nready <= 0)
                    break;
                
            }
        }
    }    
    return 0;
}

makefile:

.PHONY:clean all
CC=gcc
CFLAGS=-Wall -g
BIN=echosrv echocli
all:$(BIN)
%.o:%.c
    $(CC) $(CFLAGS) -c $< -o $@
clean:
    rm -f *.o $(BIN)