为什么poll（）立即在常规文件上返回并在fifo上阻止？

Question

我多次检查此代码，不明白为什么poll（）立即返回？

这里文件已打开以供读取，应等待事件。如何使其等待输入？

#include <iostream>

#include <poll.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>


using namespace std;

ssize_t read_out_to_the_end(int fd){
   char chunk[1024];
   ssize_t ret = 0, n;
   while((n = ::read(fd, chunk, sizeof chunk)) > 0){
      ret += n;
      cerr << "read chunk: " << n << " | ";
      cerr.write(chunk, n);
      cerr << endl;
   }
   if (n < 0) {
       cerr << "err in read" << endl;
   }
   else if (ret == 0){
      cerr << "nothing to read" << endl;
   }
   return ret;
}

int main() {
   int bininfd = open("bin-in", O_RDONLY | O_CREAT);//, 0644/*S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH*/);
   if (bininfd < 0) {
      perror("err in open(binin)");
      return -1;
   }

   struct pollfd pollfds[] = {
         {bininfd, POLLIN, 0},
   };
   auto&[pfd] = pollfds;

   while (1) {
      pfd.revents = 0;  // cleanup, shouldn't it be redundant
      int pollret = poll(pollfds, 1, -1);
      if (pollret > 0) {
         if (pfd.revents & POLLIN) {
            cerr << "(pfd.revents & POLLIN)" << endl;
            read_out_to_the_end(pfd.fd);
         }
      } else if (pollret == 0) {
         cerr << "poll timed out" << endl;
         continue;
      } else {
         cerr << "check for error" << endl;
         continue;
      }
   }
}

输出为

(pfd.revents & POLLIN)
nothing to read
(pfd.revents & POLLIN)
nothing to read
(pfd.revents & POLLIN)
nothing to read
(pfd.revents & POLLIN)
nothing to read
(pfd.revents & POLLIN)
nothing to read
(pfd.revents & POLLIN)
nothing to read
(pfd.revents & POLLIN)
nothing to read
............... etc ....................

live example

更新：1. read_out_to_the_end（）已修复。感谢@RemyLebeau2.它适用于fifos，但不适用于常规文件

Answer 1

read_out_to_the_end()有几个问题：

• ret未初始化。

• while循环在应分配n时递增。但是，如果while循环达到EOF，则即使在达到EOF之前实际读取了数据，if( n == 0)也将为true。

• chunk可以为空值终止，但是根据输入数据，它也可能也接收空值。因此，不应使用cerr将其写入cout（为什么不使用operator<<？），请改用cerr.write()，以便您可以将读取的实际字节数传递给它。

尝试以下方法：

ssize_t read_out_to_the_end(int fd){
   char chunk[1024];
   ssize_t ret = 0, n;
   while((n = ::read(fd, chunk, sizeof chunk)) > 0){
      ret += n;
      cerr << "read chunk: " << n << " | ";
      cerr.write(chunk, n);
      cerr << endl;
   }
   if (n < 0) {
       cerr << "err in read" << endl;
   }
   else if (ret == 0){
      cerr << "nothing to read" << endl;
   }
   return ret;
}

int main() {
   int bininfd = open("bin-in", O_RDONLY | O_CREAT);//, 0644/*S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH*/);
   if (bininfd < 0) {
      perror("err in open(binin)");
      return -1;
   }

   pollfd pfd = {};
   pfd.fd = bininfd;
   pfd.events = POLLIN;

   while (true) {
      pfd.revents = 0;  // cleanup, shouldn't it be redundant
      int pollret = poll(&pfd, 1, -1);
      if (pollret > 0) {
         if (pfd.revents & POLLIN) {
            cerr << "(pfd.revents & POLLIN)" << endl;
            read_out_to_the_end(pfd.fd);
         }
      } else if (pollret == 0) {
         cerr << "poll timed out" << endl;
         continue;
      } else {
         cerr << "poll error " << errno << endl;
         break;
      }
   }
}

此外，open() documentation说：

open()自变量指定在创建新文件时应用的文件模式位。 [mode中指定O_CREAT或O_TMPFILE时必须提供此参数]；如果既未指定O_CREAT也未指定O_TMPFILE，则忽略flags。有效模式由进程的mode以通常的方式修改：在没有默认ACL的情况下，创建的文件的模式为umask。请注意，此模式仅适用于将来对新创建文件的访问。创建只读文件的(mode & ~umask)调用很可能会返回读/写文件描述符。