I know that I should close opened files. I know that if I don't do that the file descriptors will leak. I also know that file descriptor is just an integer. With that integer os associates some resources. And here is the question. What are those resources? What makes it difficult to create infinite (a lot) file descriptors? Why can't os detect those leakages? And why os doesn't give the same file descriptors for the same file opening?
What are those resources?
This link posted by codeforester contains some material about.
Anyway, those file descriptors are simply handles to complex data the kernel holds for a program. They could be opaque pointers, but using simple numbers has its advantages (stdin, stdout, stderr have a well-known number, for example). What kind and amount of data is a kernel thing, and a program should not, and doesn't need, to know. So, nor you and me. But, just to speak, for example some buffer is needed. Then, the kernel must know in any moment which files are opened, otherwise, for example, you could unmount a filesystem with open files and leave programs dangling.
What makes it difficult to create infinite (a lot) file descriptors?
Because file descriptors cost ram (and CPU also), which is a finite resource, and nobody wants a kernel crash because some (stupid) programmer wastes file descriptors... :-). So, the kernel reserves a finite amount of resources for file descriptors (which are not always simple files). Kernels are not all equal, each can have its policy and often some way for users to manage relevant settings.
Why can't os detect those leakages?
Because it can not. The kernel can not tell the difference between a poor written program, which leaks resources, and a program which legitimately allocates many resources. Moreover, it is not the duty of a kernel to try to distinguish good programs from bad ones. A kernel must supply services, fast and efficiently -- all the rest is responsibility of programmers.
And why os doesn't give the same file descriptors for the same file opening?
Because it is legitimate to open the same file twice or more. Two programs can open the same file, two threads can, or even a single thread. And the kernel must always respect the "contract" its API claims, always in the same manner: again, it is the programmer who must know what he is doing.
In all programming languages (that I use at least), you must open a file before you can read or write to it.
But what does this open operation actually do?
Manual pages for typical functions dont actually tell you anything other than it 'opens a file for reading/writing':
http://www.cplusplus.com/reference/cstdio/fopen/
https://docs.python.org/3/library/functions.html#open
Obviously, through usage of the function you can tell it involves creation of some kind of object which facilitates accessing a file.
Another way of putting this would be, if I were to implement an open function, what would it need to do on Linux?
In almost every high-level language, the function that opens a file is a wrapper around the corresponding kernel system call. It may do other fancy stuff as well, but in contemporary operating systems, opening a file must always go through the kernel.
This is why the arguments of the fopen library function, or Python's open closely resemble the arguments of the open(2) system call.
In addition to opening the file, these functions usually set up a buffer that will be consequently used with the read/write operations. The purpose of this buffer is to ensure that whenever you want to read N bytes, the corresponding library call will return N bytes, regardless of whether the calls to the underlying system calls return less.
I am not actually interested in implementing my own function; just in understanding what the hell is going on...'beyond the language' if you like.
In Unix-like operating systems, a successful call to open returns a "file descriptor" which is merely an integer in the context of the user process. This descriptor is consequently passed to any call that interacts with the opened file, and after calling close on it, the descriptor becomes invalid.
It is important to note that the call to open acts like a validation point at which various checks are made. If not all of the conditions are met, the call fails by returning -1 instead of the descriptor, and the kind of error is indicated in errno. The essential checks are:
Whether the file exists;
Whether the calling process is privileged to open this file in the specified mode. This is determined by matching the file permissions, owner ID and group ID to the respective ID's of the calling process.
In the context of the kernel, there has to be some kind of mapping between the process' file descriptors and the physically opened files. The internal data structure that is mapped to the descriptor may contain yet another buffer that deals with block-based devices, or an internal pointer that points to the current read/write position.
I'd suggest you take a look at this guide through a simplified version of the open() system call. It uses the following code snippet, which is representative of what happens behind the scenes when you open a file.
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 }
Briefly, here's what that code does, line by line:
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.
If you're feeling ambitious, you can compare this simplified example to the implementation of the open() system call in the Linux kernel, a function called do_sys_open(). You shouldn't have any trouble finding the similarities.
Of course, this is only the "top layer" of what happens when you call open() - or more precisely, it's the highest-level piece of kernel code that gets invoked in the process of opening a file. A high-level programming language might add additional layers on top of this. There's a lot that goes on at lower levels. (Thanks to Ruslan and pjc50 for explaining.) Roughly, from top to bottom:
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.
This may also be somewhat incorrect due to caching. :-P Seriously though, there are many details that I've left out - a person (not me) could write multiple books describing how this whole process works. But that should give you an idea.
Any file system or operating system you want to talk about is fine by me. Nice!
On a ZX Spectrum, initializing a LOAD command will put the system into a tight loop, reading the Audio In line.
Start-of-data is indicated by a constant tone, and after that a sequence of long/short pulses follow, where a short pulse is for a binary 0 and a longer one for a binary 1 (https://en.wikipedia.org/wiki/ZX_Spectrum_software). The tight load loop gathers bits until it fills a byte (8 bits), stores this into memory, increases the memory pointer, then loops back to scan for more bits.
Typically, the first thing a loader would read is a short, fixed format header, indicating at least the number of bytes to expect, and possibly additional information such as file name, file type and loading address. After reading this short header, the program could decide whether to continue loading the main bulk of the data, or exit the loading routine and display an appropriate message for the user.
An End-of-file state could be recognized by receiving as many bytes as expected (either a fixed number of bytes, hardwired in the software, or a variable number such as indicated in a header). An error was thrown if the loading loop did not receive a pulse in the expected frequency range for a certain amount of time.
A little background on this answer
The procedure described loads data from a regular audio tape - hence the need to scan Audio In (it connected with a standard plug to tape recorders). A LOAD command is technically the same as open a file - but it's physically tied to actually loading the file. This is because the tape recorder is not controlled by the computer, and you cannot (successfully) open a file but not load it.
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.
It depends on the operating system what exactly happens when you open a file. Below I describe what happens in Linux as it gives you an idea what happens when you open a file and you could check the source code if you are interested in more detail. I am not covering permissions as it would make this answer too long.
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.
Bookkeeping, mostly. This includes various checks like "Does the file exist?" and "Do I have the permissions to open this file for writing?".
But that's all kernel stuff - unless you're implementing your own toy OS, there isn't much to delve into (if you are, have fun - it's a great learning experience). Of course, you should still learn all the possible error codes you can receive while opening a file, so that you can handle them properly - but those are usually nice little abstractions.
The most important part on the code level is that it gives you a handle to the open file, which you use for all of the other operations you do with a file. Couldn't you use the filename instead of this arbitrary handle? Well, sure - but using a handle gives you some advantages:
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.
There's also some other tricks you can do (for example, share handles between processes to have a communication channel without using a physical file; on unix systems, files are also used for devices and various other virtual channels, so this isn't strictly necessary), but they aren't really tied to the open operation itself, so I'm not going to delve into that.
At the core of it when opening for reading nothing fancy actually needs to happen. All it needs to do is check the file exists and the application has enough privileges to read it and create a handle on which you can issue read commands to the file.
It's on those commands that actual reading will get dispatched.
The OS will often get a head start on reading by starting a read operation to fill the buffer associated with the handle. Then when you actually do the read it can return the contents of the buffer immediately rather then needing to wait on disk IO.
For opening a new file for write the OS will need to add a entry in the directory for the new (currently empty) file. And again a handle is created on which you can issue the write commands.
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.
In simple terms, when you open a file you are actually requesting the operating system to load the desired file ( copy the contents of file ) from the secondary storage to ram for processing. And the reason behind this ( Loading a file ) is because you cannot process the file directly from the Hard-disk because of its extremely slow speed compared to Ram.
The open command will generate a system call which in turn copies the contents of the file from the secondary storage ( Hard-disk ) to Primary storage ( Ram ).
And we 'Close' a file because the modified contents of the file has to be reflected to the original file which is in the hard-disk. :)
Hope that helps.
Ideally, I want to have a directory that is not visible in the filesystem and that will be automatically removed when it's last open file descriptor is closed. It's contents would only be accessible through openat(), fstatat(), etc.
For regular files, this behaviour is achieved by giving the O_TMPFILE flag to open(). However, mkdir() doesnt have a flags parameter.
Assuming I have the latest linux kernel available, is this possible?
I'm not aware of any way to do this, and don't expect it to be possible. Unlike files, which can have zero or more pathnames (due to hard links and unlinked files), directories have exactly one pathname, and it would probably break some valid application usage if the OS did not meet this expectation.
I've just recently learned how to check if a file exists, in C, without opening it with open(). What I wanted to ask is if there is an option or a flag or other instruction that checks if a file exists, but blocks the process until the file exists. Something like this:
while(access("socket", F_OK) !=0);
But without all this processing cycle... Something like select but to check if a file exists.
There's no way to do this portably, and on some filesystems (e.g. certain network filesystems) there simply isn't any way to do it at all without periodically checking for the file's existence.
That said, there are nonportable approaches which can cover the majority of platforms in wide use:
OS X: FSEvents
Linux: inotify
*BSD: kqueue
Windows: ReadDirectoryChanges
No, there is no standard function that does what you're asking for.
The way you're checking is poor form in that it will consume as much CPU time as it possibly can while checking. At a minimum, you want to add a delay in your loop, such as:
while(access("socket", F_OK) !=0) sleep(1);
A better solution is to monitor a directory for changes. There isn't any standard function to do this but there are various operating system specific methods. See Monitor directory for new files only for some possibilities.
The flag O_DIRECTORY can be used with the syscalls open(2) and openat(2) to avoid denial-of-service vulnerabilities when opening directories. However: How can I avoid the same kind of race conditions for regular files?
Some background information: I am trying to develop some kind of backup tool. The programs walks over a directory tree, reads all regular files and only stats other files. If I first call fstatat(2) for each directory entry, test the result for regular files and open them with openat(2), then there is a race condition between the syscalls. An attacker could replace the regular file with a FIFO, and my program would hang on the FIFO.
How can I avoid this race condition? For directories, there is O_DIRECTORY, for symbolic links, O_PATH can be used. However, I have found no solution for regular files. I only need a solution that works on recent Linux versions.
If your only concern is fifos, O_NONBLOCK will prevent blocking and allow you to open a fifo even if it has a no writers (see http://pubs.opengroup.org/onlinepubs/9699919799/functions/open.html for where this is specified). However, there are also a few other concerns:
Device nodes
Fake files in Linux /proc with bad properties
...
Since these normally can't be created in arbitrary locations by non-root users, O_NOFOLLOW should be sufficient to avoid following symlinks to them.
With that said, on modern Linux there is an even safer solution: perform the initial open with O_PATH|O_NOFOLLOW, then perform stat on /proc/self/fd/%d to check the file type. You can then open /proc/self/fd/%d and be completely certain it corresponds to the same file you just stat'd.
Note that on sufficiently new Linux, you don't need to use /proc/self/fd/%d to reach the file to which you obtained an inode handle with O_PATH. You can use fstat and openat on it directly to "stat" it and get a descriptor to a real open file description, respectively. However O_PATH file descriptors had a lot of broken/unimplemented corner cases like this in the range of late 2.6.x (when they were first added) to 3.8 or so, and I find the /proc method the most reliable. Of course you could always try the direct method and fallback to /proc if it fails.
Open with O_RDONLY|O_NONBLOCK, check that the result isn't -1, then do an fstat() on the resulting file descriptor and compare st_mode (and possibly st_dev and st_ino) with what you expected.
Remember to set the AT_SYMLINK_NOFOLLOW flag on your fstatat.