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我正在学习 FIFO，并编写了一个迭代服务器来接收来自多个客户端的请求。客户端通过写入服务器众所周知的 fifo 来请求文件。服务器从它的 FIFO 中读取，并将请求文件的内容放入客户端读取的新 FIFO 中。我运行服务器。当我第一次运行客户端时，一切正常，客户端读取文件的内容。当我第二次运行客户端时，来自客户端的消息以退格字符为前缀。我不知道这个退格是从哪里来的。有任何想法吗？这是服务器代码：

#include<stdio.h>
#include<string.h>
#include<unistd.h>
#include<errno.h>
#include<sys/types.h>
#include<fcntl.h>
#include<stdlib.h>
#include"fifo.h"

#define SERVFIFO "/tmp/fifo.serv"
#define FILE_MODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)

int main(int argc, char** argv) {
    int readfifo, dummywrite, filefd, writefd, n;
    int clientpid;
    char buff[MAXLINE], *spaceptr, fifoname[MAXLINE];

    if (mkfifo(SERVFIFO, FILE_MODE) < 0 && errno != EEXIST) {
        printf("Can't create %s", SERVFIFO);
    }

    readfifo = open(SERVFIFO, O_RDONLY, 0);
    dummywrite = open(SERVFIFO, O_WRONLY, 0);

    while ((n = Readline(readfifo, buff, MAXLINE)) > 0) {
        printf("Read data from the fifo:%s and the length is:%d and the character is:%d\n", buff, strlen(buff), buff[0]);

        if (buff[n - 1] == '\n') {
            n--;
            printf("I am also omitting the newline\n");
        }

        buff[n] = '\0';
        printf("Buff just after read is:%s and length is %d", buff, strlen(buff));

        if ((spaceptr = strchr(buff, ' ')) == NULL) {
            printf("Bad request from client");
            continue;
        }

        printf("Found the space:%c\n", *(spaceptr + 1));
        *spaceptr++ = '\0';
        printf("The value of buffer now is:%s and the length of buffer is:%d and the culprit is %d\n", buff, strlen(buff), *(buff + 0));
        clientpid = atol(buff);
        printf("The client pid is %ld\n", clientpid);
        snprintf(fifoname, sizeof(fifoname), "/tmp/fifoname.%ld", clientpid);

        if (mkfifo(fifoname, FILE_MODE) < 0 && errno != EEXIST) {
            perror("Can't create the fifo");
            continue;
        }

        printf("Successfully created fifo %s for client\n", fifoname);

        if ((writefd = open(fifoname, O_WRONLY, 0)) < 0) {
            printf("Cannot open %s", fifoname);
            continue;
        }

        if ((filefd = open(spaceptr, O_RDONLY, 0)) < 0) {
            printf("Error opening file\n");
            continue;
        }
        else {
            while ((n = read(filefd, buff, MAXLINE))) {
                write(writefd, buff, n);
            }

            close(filefd);
            close(writefd);
        }
    }
}

这是客户端代码：

#include<stdio.h>
#include<string.h>
#include<unistd.h>
#include<errno.h>
#include<sys/types.h>
#include<fcntl.h>
#include<stdlib.h>

#define SERVER_FIFO "/tmp/fifo.serv"
#define MAXLINE 100
#define MSG "%ld sup.c"
#define READFIFO "/tmp/fifoname.%ld"
int main(int argc, char** argv) {
    int writefifo, readfifo, n;
    char buff[MAXLINE];
    pid_t self_pid = getpid();
    printf("Started client with PID:%ld\n", self_pid);
    writefifo = open(SERVER_FIFO, O_WRONLY, 0);
    snprintf(buff, sizeof(buff), MSG, self_pid);
    printf("The message to be written to the server is:%s and the length of the message is %d\n", buff, strlen(buff));

    if ((n = write(writefifo, buff, sizeof(buff))) != sizeof(buff)) {
        perror("Unable to write to server fifo");
        exit(0);
    }

    printf("Message written to the server; Waiting to read contents\n");
    snprintf(buff, sizeof(buff), READFIFO, self_pid);
    readfifo = open(buff, O_RDONLY, 0);

    while ((n = read(readfifo, buff, MAXLINE))) {
        write(STDOUT_FILENO, buff, n);
    }

    close(readfifo);
    close(writefifo);
    unlink(buff);
}
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1 回答 1

2

也许这是您真实程序的切碎版本，因为这不会显示您描述的行为。有几件事：

(1) 在服务器中

readfifo = open(SERVFIFO, O_RDONLY, 0);
dummywrite = open(SERVFIFO, O_WRONLY, 0);

我假设您知道，如果您打开著名的 fifo，而另一端没有任何东西，则 open 将阻塞。这仍然会阻止 O_RONLY 打开，因为在正常情况下，您将在任何客户端之前运行服务器。如果您想避免阻塞，只需将其以读/写方式打开一次。

(2) 在客户端

snprintf(buff, sizeof(buff), READFIFO, self_pid);
readfifo = open(buff, O_RDONLY, 0);

这不是你想的那样。您的客户端向服务器的 fifo 写入一条消息，然后服务器使用mkfifo. 由于客户端首先运行，因此执行此操作时将不存在 fifo open。实际上，您正在尝试以只读方式打开一个普通文件（不存在）。您无法读取不存在的文件，因此open失败，但是由于您不检查打开状态，因此您不知道它。

(3) 完成后取消链接 FIFO。

于 2013-08-29T05:10:17.110 回答