FIFO：一个进程从不从 pipe 读取

Question

我正在关注 Linux 管道的这个TutorialsPoint 指南，我特别需要使用 FIFO。

但是，该代码根本不适用于服务器端。

服务器文件要么无限期挂起，要么什么也不读取，而客户端则写入 FIFO 并立即读取它刚刚写入的内容。

这是两个文件的完整代码，以防您不想通过 TutorialsPoint go：

fifoserver_twoway.cpp

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

#define FIFO_FILE "/tmp/fifo_twoway"

void reverse_string(char *);
int main() {
   int fd;
   char readbuf[80];
   char end[10];
   int to_end;
   int read_bytes;

   /* Create the FIFO if it does not exist */
   mkfifo(FIFO_FILE, S_IFIFO|0640);
   strcpy(end, "end");
   fd = open(FIFO_FILE, O_RDWR);
   while(1) {
      read_bytes = read(fd, readbuf, sizeof(readbuf));
      readbuf[read_bytes] = '\0';
      printf("FIFOSERVER: Received string: \"%s\" and length is %d\n", readbuf, (int)strlen(readbuf));
      to_end = strcmp(readbuf, end);

      if (to_end == 0) {
         close(fd);
         break;
      }
      reverse_string(readbuf);
      printf("FIFOSERVER: Sending Reversed String: \"%s\" and length is %d\n", readbuf, (int) strlen(readbuf));
      write(fd, readbuf, strlen(readbuf));
      /*
      sleep - This is to make sure other process reads this, otherwise this
      process would retrieve the message
      */
      sleep(2);
   }
   return 0;
}

void reverse_string(char *str) {
   int last, limit, first;
   char temp;
   last = strlen(str) - 1;
   limit = last/2;
   first = 0;

   while (first < last) {
      temp = str[first];
      str[first] = str[last];
      str[last] = temp;
      first++;
      last--;
   }
   return;
}

fifoclient_twoway.cpp

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

#define FIFO_FILE "/tmp/fifo_twoway"
int main() {
   int fd;
   int end_process;
   int stringlen;
   int read_bytes;
   char readbuf[80];
   char end_str[5];
   printf("FIFO_CLIENT: Send messages, infinitely, to end enter \"end\"\n");
   fd = open(FIFO_FILE, O_CREAT|O_RDWR);
   strcpy(end_str, "end");

   while (1) {
      printf("Enter string: ");
      fgets(readbuf, sizeof(readbuf), stdin);
      stringlen = strlen(readbuf);
      readbuf[stringlen - 1] = '\0';
      end_process = strcmp(readbuf, end_str);

      //printf("end_process is %d\n", end_process);
      if (end_process != 0) {
         write(fd, readbuf, strlen(readbuf));
         printf("FIFOCLIENT: Sent string: \"%s\" and string length is %d\n", readbuf, (int)strlen(readbuf));
         read_bytes = read(fd, readbuf, sizeof(readbuf));
         readbuf[read_bytes] = '\0';
         printf("FIFOCLIENT: Received string: \"%s\" and length is %d\n", readbuf, (int)strlen(readbuf));
      } else {
         write(fd, readbuf, strlen(readbuf));
         printf("FIFOCLIENT: Sent string: \"%s\" and string length is %d\n", readbuf, (int)strlen(readbuf));
         close(fd);
         break;
      }
   }
   return 0;
}

当我运行这两个进程时，这就是我得到的：

./fifoserver_twoway

FIFOSERVER: Received string: "" and length is 0
FIFOSERVER: Sending Reversed String: "" and length is 0
FIFOSERVER: Received string: "" and length is 0
FIFOSERVER: Sending Reversed String: "" and length is 0

./fifoclient_twoway

FIFOCLIENT: Sent string: "ciao" and string length is 4
FIFOCLIENT: Received string: "ciao" and length is 4
Enter string: why won't you reverse?
FIFOCLIENT: Sent string: "why won't you reverse?" and string length is 29
FIFOCLIENT: Received string: "why won't you reverse?" and length is 29

还值得注意的是，在开始写这个问题之前，服务器的行为是完全不同的：不像你在这里看到的那样接收和打印，它会在“读取”之后无限期挂起（而且我没有改变一点代码, 除了改变 FIFO_FILE 路径）

Answer 1

你让服务器在写完之后休眠——而不是客户端。 这样，客户端仍然可以在服务器获取之前读取它自己的 output。 所以至少你应该在两次写入之后添加一个睡眠，让服务器睡眠更长一点，以确保客户端首先醒来以读取服务器 output。

同时访问未命名管道的同一端（通过pipe 函数创建）是未定义的行为。 虽然不确定命名管道，但我认为那里也几乎相同。 通过简单的延迟（ sleep ， usleep ）同步对这些目的的并发访问可能会奏效，但这是一种非常不安全的方法。

我宁愿推荐两个单独的管道（正如Tony Tannous已经建议的那样），每个方向一个（根据需要打开各自的末端RDONLY或WRONLY ），然后您将获得全双工通信而不是半双工，并且您不需要进一步同步（在最简单的变体中延迟）：

// server
int fd_cs = open(FIFO_FILE_CS, O_RDONLY);
int fd_sc = open(FIFO_FILE_SC, O_WRONLY);
    read(fd_cs, ...);
    write(fd_sc, ...);

// client
int fd_cs = open(FIFO_FILE_CS, O_WRONLY);
int fd_sc = open(FIFO_FILE_SC, O_RDONLY);
    write(fd_cs, ...);
    read(fd_sc, ...);