Linux TCP套接字：客户端已发送数据，但服务器仍在read（）上阻塞

我有一个在Linux上使用TCP套接字的简单客户端-服务器示例。服务器监听回送地址。客户端连接到服务器，并发送一些整数以及一个“ END”字符串以标记数据结束。服务器读取数字，将所有数字相加并返回总和。但是，read()即使客户端成功发送了所有数据，我的服务器有时还是处于打开状态。

这是代码：

server.c：

#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#define BACKLOG 5

int main(int argc, char *argv[]) {
    struct sockaddr_in addr;
    int down_flag = 0;
    int result = 0;
    int ret = 0;

    int sfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sfd < 0) {
        perror("Create server socket error: %s\n");
        return 0;
    }

    /* Bind socket to loopback address */
    memset((void *) &addr, 0, sizeof(struct sockaddr_in));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(8888);
    addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
    if (bind(sfd, (struct sockaddr *) &addr, sizeof(struct sockaddr_in)) == -1) {
        perror("Bind server socket failed");
        goto _exit;
    }

    if (listen(sfd, BACKLOG) == -1) {
        perror("Listen failed");
        goto _exit;
    }

    ssize_t num_rd = 0;
    char buf[100] = {0};
    for (;;)
    {
        printf("Waiting to accept a connection...\n");
        int cfd = accept(sfd, NULL, NULL);
        printf("Accepted socket fd = %d\n", cfd);
        result = 0;
        while ((num_rd = read(cfd, buf, sizeof(buf))) > 0) {
            /* Ensure the buffer is 0-terminated */
            buf[sizeof(buf) - 1] = 0;
            printf("Read data: %s\n", buf);

            /* Handle commands */
            if (!strncmp(buf, "DOWN", sizeof(buf))) {
                down_flag = 1;
                break;
            }
            if (!strncmp(buf, "END", sizeof(buf))) {
                break;
            }
            /* Add received summand */
            result += atoi(buf);
        }
        if (-1 == num_rd) {
            perror("Read error");
        }

        /* Send result */
        sprintf(buf, "%d", result);
        ret = write(cfd, buf, sizeof(buf));
        if (-1 == ret) {
            perror("Write error\n");
            goto _exit;
        }
        close(cfd);
        /* Quit on DOWN command */
        if (down_flag) {
            break;
        }
    }
_exit:
    close(sfd);
    return 0;
}

client.c：

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

int main(int argc, char *argv[]) {
    struct sockaddr_in addr;
    int ret;
    int data_socket;
    char buf[100] = {0};
    int i = 0;

    data_socket = socket(AF_INET, SOCK_STREAM, 0);
    if (-1 == data_socket) {
        perror("Create client socket error");
        exit(EXIT_FAILURE);
    }

    /* Connect to server socket */
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(8888);
    addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
    ret = connect(data_socket, (const struct sockaddr *) &addr, sizeof(addr));
    if (-1 == ret) {
        perror("Connect error");
        exit(EXIT_FAILURE);
    }

    /* Send arguments */
    for (i = 1; i < argc; i++) {
        ret = write(data_socket, argv[i], strlen(argv[i]) + 1);
        if (-1 == ret) {
            perror("Write error");
            break;
        }
    }
    strcpy(buf, "END");
    ret = write(data_socket, buf, strlen(buf) + 1);
    printf("write %s to socket, ret = %d\n", buf, ret);
    if (-1 == ret) {
        perror("Write to socket error");
        exit(EXIT_FAILURE);
    }
    /* Read the result */
    memset(buf, 0, sizeof(buf));
    ret = read(data_socket, buf, sizeof(buf));
    if (-1 == ret) {
        perror("Read from client socket error");
        exit(EXIT_FAILURE);
    }
    buf[sizeof(buf) - 1] = 0;
    printf("Result = %s\n", buf);
    close(data_socket);
    exit(EXIT_SUCCESS);
}

运行客户端几次，服务器将read()在某些时候阻止呼叫：

$ for i in {1..100}; do ./client 3 4 5 6; done
write END to socket, ret = 4
Result = 18
write END to socket, ret = 4

服务器输出：

$ ./server
Waiting to accept a connection...
Accepted socket fd = 4
Read data: 3
Read data: 4
Read data: 5
Read data: 6
Read data: END
Waiting to accept a connection...
Accepted socket fd = 4
Read data: 3

服务器阻塞while ((num_rd = read(cfd, buf, sizeof(buf))) > 0)在线。

编辑：我的问题是为什么read()块。AFAIKread()将阻塞，直到从套接字读取至少1个字节的数据为止。在这种情况下，客户端发送的数据多于服务器读取的数据，因此我认为有可用的数据要从套接字读取。那么为什么read()仍然会阻塞？