文件描述符

在 POSIX 系统上，对于每个进程，内核维护一个当前打开的文件和资源表。 每个打开的文件都分配了一个简单的数字标识符，称为_文件描述符_。 在系统级，所有文件系统操作都使用这些文件描述符来识别和跟踪每个特定文件。 Windows 系统使用不同但概念上相似的机制来跟踪资源。 为了方便用户，Node.js 抽象了操作系统之间的差异，并为所有打开的文件分配了一个数字文件描述符。

基于回调的 fs.open() 和同步 fs.openSync() 方法打开一个文件并分配一个新的文件描述符。 分配后，文件描述符可用于从文件读取数据、向文件写入数据或请求有关文件的信息。

操作系统限制在任何给定时间可能打开的文件描述符的数量，因此在操作完成时关闭描述符至关重要。 否则将导致内存泄漏，最终导致应用程序崩溃。

import { open, close, fstat } from 'node:fs';

function closeFd(fd) {
  close(fd, (err) => {
    if (err) throw err;
  });
}

open('/open/some/file.txt', 'r', (err, fd) => {
  if (err) throw err;
  try {
    fstat(fd, (err, stat) => {
      if (err) {
        closeFd(fd);
        throw err;
      }

      // 使用统计

      closeFd(fd);
    });
  } catch (err) {
    closeFd(fd);
    throw err;
  }
});

基于 promise 的 API 使用 <FileHandle> 对象代替数字文件描述符。 这些对象由系统更好地管理，以确保资源不泄漏。 但是，仍然需要在操作完成时关闭它们：

import { open } from 'node:fs/promises';

let file;
try {
  file = await open('/open/some/file.txt', 'r');
  const stat = await file.stat();
  // 使用统计
} finally {
  await file.close();
}

On POSIX systems, for every process, the kernel maintains a table of currently open files and resources. Each open file is assigned a simple numeric identifier called a file descriptor. At the system-level, all file system operations use these file descriptors to identify and track each specific file. Windows systems use a different but conceptually similar mechanism for tracking resources. To simplify things for users, Node.js abstracts away the differences between operating systems and assigns all open files a numeric file descriptor.

The callback-based fs.open(), and synchronous fs.openSync() methods open a file and allocate a new file descriptor. Once allocated, the file descriptor may be used to read data from, write data to, or request information about the file.

Operating systems limit the number of file descriptors that may be open at any given time so it is critical to close the descriptor when operations are completed. Failure to do so will result in a memory leak that will eventually cause an application to crash.

import { open, close, fstat } from 'node:fs';

function closeFd(fd) {
  close(fd, (err) => {
    if (err) throw err;
  });
}

open('/open/some/file.txt', 'r', (err, fd) => {
  if (err) throw err;
  try {
    fstat(fd, (err, stat) => {
      if (err) {
        closeFd(fd);
        throw err;
      }

      // use stat

      closeFd(fd);
    });
  } catch (err) {
    closeFd(fd);
    throw err;
  }
});

The promise-based APIs use a <FileHandle> object in place of the numeric file descriptor. These objects are better managed by the system to ensure that resources are not leaked. However, it is still required that they are closed when operations are completed:

import { open } from 'node:fs/promises';

let file;
try {
  file = await open('/open/some/file.txt', 'r');
  const stat = await file.stat();
  // use stat
} finally {
  await file.close();
}