当你打开一个文件时到底会发生什么，这取决于操作系统。下面我将描述在Linux中发生的事情，因为它可以让您了解当您打开一个文件时会发生什么，如果您对更详细的内容感兴趣，您可以检查源代码。我没有涉及权限，因为这会使这个答案太长。

In Linux every file is recognised by a structure called inode. Each structure has an unique number and every file only gets one inode number. This structure stores meta data for a file, for example file-size, file-permissions, time stamps and pointer to disk blocks, however, not the actual file name itself. Each file (and directory) contains a file name entry and the inode number for lookup. When you open a file, assuming you have the relevant permissions, a file descriptor is created using the unique inode number associated with file name. As many processes/applications can point to the same file, inode has a link field that maintains the total count of links to the file. If a file is present in a directory, its link count is one, if it has a hard link its link count will be two and if a file is opened by a process, the link count will be incremented by 1.

记账,主要是。这包括各种检查，如“文件是否存在?”和“我是否有权限打开此文件进行写入?”。

但这些都是内核的东西——除非你正在实现自己的玩具操作系统，没有太多的东西需要深入研究(如果你是，玩得开心——这是一个很好的学习经历)。当然，您仍然应该了解在打开文件时可能收到的所有错误代码，以便正确地处理它们——但这些通常都是不错的小抽象。

代码级别上最重要的部分是，它为您提供了打开文件的句柄，您可以将其用于对文件进行的所有其他操作。难道不能使用文件名来代替这个任意的句柄吗?当然，但使用手柄也有一些好处:

The system can keep track of all the files that are currently open, and prevent them from being deleted (for example). Modern OSs are built around handles - there's tons of useful things you can do with handles, and all the different kinds of handles behave almost identically. For example, when an asynchronous I/O operation completes on a Windows file handle, the handle is signalled - this allows you to block on the handle until it's signalled, or to complete the operation entirely asynchronously. Waiting on a file handle is exactly the same as waiting on a thread handle (signalled e.g. when the thread ends), a process handle (again, signalled when the process ends), or a socket (when some asynchronous operation completes). Just as importantly, handles are owned by their respective processes, so when a process is terminated unexpectedly (or the application is poorly written), the OS knows what handles it can release. Most operations are positional - you read from the last position in your file. By using a handle to identify a particular "opening" of a file, you can have multiple concurrent handles to the same file, each reading from their own places. In a way, the handle acts as a moveable window into the file (and a way to issue asynchronous I/O requests, which are very handy). Handles are much smaller than file names. A handle is usually the size of a pointer, typically 4 or 8 bytes. On the other hand, filenames can have hundreds of bytes. Handles allow the OS to move the file, even though applications have it open - the handle is still valid, and it still points to the same file, even though the file name has changed.

您还可以使用其他一些技巧(例如，在进程之间共享句柄，从而在不使用物理文件的情况下拥有通信通道;在unix系统上，文件也用于设备和各种其他虚拟通道，所以这不是严格必要的)，但它们并没有真正绑定到open操作本身，所以我不打算深入研究这一点。

我建议您通过open()系统调用的简化版本来了解本指南。它使用下面的代码片段，它代表了打开文件时在幕后发生的事情。

0  int sys_open(const char *filename, int flags, int mode) {
1      char *tmp = getname(filename);
2      int fd = get_unused_fd();
3      struct file *f = filp_open(tmp, flags, mode);
4      fd_install(fd, f);
5      putname(tmp);
6      return fd;
7  }

简单地说，下面是代码逐行执行的操作:

Allocate a block of kernel-controlled memory and copy the filename into it from user-controlled memory. Pick an unused file descriptor, which you can think of as an integer index into a growable list of currently open files. Each process has its own such list, though it's maintained by the kernel; your code can't access it directly. An entry in the list contains whatever information the underlying filesystem will use to pull bytes off the disk, such as inode number, process permissions, open flags, and so on. The filp_open function has the implementation struct file *filp_open(const char *filename, int flags, int mode) { struct nameidata nd; open_namei(filename, flags, mode, &nd); return dentry_open(nd.dentry, nd.mnt, flags); } which does two things: Use the filesystem to look up the inode (or more generally, whatever sort of internal identifier the filesystem uses) corresponding to the filename or path that was passed in. Create a struct file with the essential information about the inode and return it. This struct becomes the entry in that list of open files that I mentioned earlier. Store ("install") the returned struct into the process's list of open files. Free the allocated block of kernel-controlled memory. Return the file descriptor, which can then be passed to file operation functions like read(), write(), and close(). Each of these will hand off control to the kernel, which can use the file descriptor to look up the corresponding file pointer in the process's list, and use the information in that file pointer to actually perform the reading, writing, or closing.

如果您有雄心壮志，可以将这个简化的示例与Linux内核中open()系统调用的实现进行比较，这是一个名为do_sys_open()的函数。你应该不难找到相似之处。

当然，这只是调用open()时发生的事情的“顶层”——或者更准确地说，它是在打开文件的过程中调用的内核代码的最高级别部分。高级编程语言可能会在此基础上添加额外的层。有很多事情发生在较低的层次上。(感谢Ruslan和pjc50的解释。)大致从上到下:

open_namei() and dentry_open() invoke filesystem code, which is also part of the kernel, to access metadata and content for files and directories. The filesystem reads raw bytes from the disk and interprets those byte patterns as a tree of files and directories. The filesystem uses the block device layer, again part of the kernel, to obtain those raw bytes from the drive. (Fun fact: Linux lets you access raw data from the block device layer using /dev/sda and the like.) The block device layer invokes a storage device driver, which is also kernel code, to translate from a medium-level instruction like "read sector X" to individual input/output instructions in machine code. There are several types of storage device drivers, including IDE, (S)ATA, SCSI, Firewire, and so on, corresponding to the different communication standards that a drive could use. (Note that the naming is a mess.) The I/O instructions use the built-in capabilities of the processor chip and the motherboard controller to send and receive electrical signals on the wire going to the physical drive. This is hardware, not software. On the other end of the wire, the disk's firmware (embedded control code) interprets the electrical signals to spin the platters and move the heads (HDD), or read a flash ROM cell (SSD), or whatever is necessary to access data on that type of storage device.

由于缓存，这也可能有些不正确。严肃地说，我漏掉了很多细节——一个人(不是我)可以写好几本书来描述整个过程是如何工作的。但这应该能给你一个概念。

Basically, a call to open needs to find the file, and then record whatever it needs to so that later I/O operations can find it again. That's quite vague, but it will be true on all the operating systems I can immediately think of. The specifics vary from platform to platform. Many answers already on here talk about modern-day desktop operating systems. I've done a little programming on CP/M, so I will offer my knowledge about how it works on CP/M (MS-DOS probably works in the same way, but for security reasons, it is not normally done like this today).

On CP/M you have a thing called the FCB (as you mentioned C, you could call it a struct; it really is a 35-byte contiguous area in RAM containing various fields). The FCB has fields to write the file-name and a (4-bit) integer identifying the disk drive. Then, when you call the kernel's Open File, you pass a pointer to this struct by placing it in one of the CPU's registers. Some time later, the operating system returns with the struct slightly changed. Whatever I/O you do to this file, you pass a pointer to this struct to the system call.

What does CP/M do with this FCB? It reserves certain fields for its own use, and uses these to keep track of the file, so you had better not ever touch them from inside your program. The Open File operation searches through the table at the start of the disk for a file with the same name as what's in the FCB (the '?' wildcard character matches any character). If it finds a file, it copies some information into the FCB, including the file's physical location(s) on the disk, so that subsequent I/O calls ultimately call the BIOS which may pass these locations to the disk driver. At this level, specifics vary.

你想谈论的任何文件系统或操作系统我都可以。好了!

在ZX Spectrum上，初始化LOAD命令将使系统进入一个紧密的循环，读取音频。

数据开始由一个常量音调表示，之后是长/短脉冲序列，其中短脉冲表示二进制0，较长脉冲表示二进制1 (https://en.wikipedia.org/wiki/ZX_Spectrum_software)。紧加载循环收集位，直到它填满一个字节(8位)，将其存储到内存中，增加内存指针，然后循环回来扫描更多的位。

通常，加载器会读取的第一件事是一个短的、固定格式的头，它至少指示了预期的字节数，以及可能的附加信息，如文件名、文件类型和加载地址。在读取这个短报头后，程序可以决定是继续加载数据的主要部分，还是退出加载例程并为用户显示适当的消息。

可以通过接收任意数量的字节来识别文件结束状态(可以是固定数量的字节，在软件中是硬连接的，也可以是在头文件中指出的可变数量)。如果加载循环在一定时间内没有接收到预期频率范围内的脉冲，则抛出错误。

关于这个答案有一点背景知识

所描述的过程是从普通磁带中加载数据——因此需要扫描audio In(它与磁带录音机的标准插头连接)。LOAD命令在技术上与打开文件相同——但它在物理上与实际加载文件绑定。这是因为磁带录音机不是由计算机控制的，你不能(成功地)打开一个文件而不加载它。

The "tight loop" is mentioned because (1) the CPU, a Z80-A (if memory serves), was really slow: 3.5 MHz, and (2) the Spectrum had no internal clock! That means that it had to accurately keep count of the T-states (instruction times) for every. single. instruction. inside that loop, just to maintain the accurate beep timing. Fortunately, that low CPU speed had the distinct advantage that you could calculate the number of cycles on a piece of paper, and thus the real world time that they would take.

在几乎所有高级语言中，打开文件的函数都是对应内核系统调用的包装器。它也可以做其他奇特的事情，但是在当代的操作系统中，打开一个文件必须总是通过内核。

这就是为什么fopen库函数或Python的open函数的实参非常类似于open(2)系统调用的实参。

除了打开文件，这些函数通常还会设置一个缓冲区，用于读/写操作。这个缓冲区的目的是确保每当您想读取N个字节时，相应的库调用将返回N个字节，而不管对底层系统调用的调用是否返回更少。

我对实现我自己的功能不感兴趣;只是为了理解到底发生了什么……“超越语言”，如果你喜欢的话。

在类unix操作系统中，成功调用open将返回一个“文件描述符”，它只是用户进程上下文中的一个整数。因此，该描述符将被传递给与打开的文件交互的任何调用，并且在对其调用close之后，该描述符将失效。

重要的是要注意，对open的调用就像一个进行各种检查的验证点。如果不是所有条件都满足，则调用失败，返回-1而不是描述符，错误类型在errno中指出。基本检查包括:

文件是否存在; 调用进程是否有权限以指定的方式打开该文件。这是通过将文件权限、所有者ID和组ID与调用进程的ID相匹配来确定的。

在内核上下文中，进程的文件描述符和物理打开的文件之间必须存在某种映射。映射到描述符的内部数据结构可能包含另一个处理基于块的设备的缓冲区，或者指向当前读/写位置的内部指针。