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I created a program that monitors for events.
I want to log these events "in the right way".
Currently I have a string array, log[500][100].
Each line is a string of characters (up to 100) that report something about the event.
I have it set up so that only the last 500 events are saved in the array.
After that, new events overwrite the oldest events.
Currently I just keep revolving through the array until the program terminates, then I write the array to a file.
Going forward I would like to view the log in real time, any time I wish, without disturbing the event processing and logging process.
I considered opening the file for "appending" but here are my concerns:
(1) The program is running on a Raspberry Pi which has a flash memory as a "disk drive". I believe flash memories have a limited number of write cycles before problems can occur. This program runs 24/7 "forever" so I am afraid the "disk drive" will "wear out".
(2) I am using pretty much all the CPU capacity of the RPi so I don't want to add a lot of overhead/CPU cycles.
How would experienced programmers attack this problem?
Please go easy on me, this is my first C program.
[EDIT]
I began reviewing all the information and I became intrigued by Mark A's suggestion for tmpfs. I looked into it more and I am sure this answers my question. It allows the creation of files in RAM not the SD card. They are lost on power down but I don't care.
In order to keep the files from growing to large I created a double buffer approach. First I write 500 events to file A then switch to file B. When 500 events have been written to file B I close and reopen file A (to delete the contents and start at 0 events) and switch to writing to file A. I found I needed to fflush(file...) after each write or else the file was empty until fclose.
Normally that would be OK but right now I am fighting a nasty segmentation fault so I want as much insight into what is going on. When I hit the fault, I never get to my fclose statements.
Welcome to Stack Overflow and to C programming! A wonderful world of possibilities awaits you.
I have several thoughts in response to your situation.
The very short summary is to use stdout and delegate the output-file management to the shell.
The longer, rambling answer full of my personal musing is as follows:
1 : A very typical thing for C programs to do is not be in charge of how outputs are kept. You might have heard of the "built in" file handles, stdin, stdout, and stderr. These file handles are (under normal circumstances) always available to your program for input (from stdin) and output (stdout and stderr). As you might guess from their names stdout is customarily used for regular output and stderr is customarily used for error / exception output. It is exceedingly typical for a C program to simply read from stdin and output to stdout and stderr, and let something else (e.g., the shell) take care of what those actually are.
For example, reading from stdin means that your program can be used for keyboard entry and for file reading, without needing to change your program's code. The same goes for stdout and stderr; simply output to those file handles, and let the user decide whether those should go to the screen or be redirected to a file. And, because stdout and stderr are separate file handles, the user can have them go to separate 'destinations'.
In your case, to implement this, drop the array entirely, and simply
fprintf(stdout, "event notice : %s\n", eventdetailstring);
(or similar) every time your program has something to say. Take a look at fflush(), too, because of potential output buffering.
2a : This gets you continuous output. This itself can help with your concern about memory wear on the Pi's flash disk. If you do something like:
eventmonitor > logfile
then logfile will be being appended to during the lifetime of your program, which will tend to be writing to new parts of the flash disk. Of course, if you only ever append, you will eventually run out of space on the disk, so you might set up a cron job to kill the currently running eventmonitor and restart it every day at midnight. Done with the above command, that would cause it to overwrite logfile once per day. This prevents endless growth, and it might even use a new physical area of the flash drive for the new file (even though it's the same name; underneath, it's a different file, with a different inode, etc.) But even if it reuses the exact same area of the flash drive, now you are down to worrying if this will last more than 10,000 days, instead of 10,000 writes. I'm betting that within 10,000 days, new options will be available -- worst case, you buy a new Pi every 27 years or so!
There are other possible variations on this theme, as well. e.g., you could have a sophisticated script kicked off by cron every day at midnight that kills any currently running eventmonitor, deletes output files older than a week, and starts a new eventmonitor outputting to a file whose filename is based partly on the date so that past days' file aren't overwritten. But all of this is in the realm of using your program. You can make your program easier to use by writing it to use stdin, stdout, and stderr.
2b : Or, you can just have stdout go to the screen, which is typically how it already is when a program is started from an interactive shell / terminal window. I imagine you could have the Pi running headless most of the time, and when you want to see what your program is outputting, hook up a monitor. Generally, things will stay running between disconnecting and reconnecting your monitor. This avoids affecting the flash drive at all.
3 : Another approach is to have your event monitoring program send its output somewhere off-system. This is getting into more advanced programming territory, so you might want to save this for a later enhancement, after you've mastered more of the basics. But, your program could establish a network connection to, say, a JSON API and send event information there. This would let you separate the functions of event monitoring from event reporting.
You will discover as you learn more programming that this idea of separation of concerns is an important concept, and applies at various levels of a program or a system of interoperating programs. In this case, the Pi is a good fit for the data monitoring aspect because it is a lightweight solution, and some other system with more capacity and more stable storage can cover the data collection aspect.
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.
I have a program that is used to exercise several disk units in a raid configuration. 1 process synchronously (O_SYNC) writes random data to a file using write(). It then puts the name of the directory into a shared-memory queue, where a 2nd process is waiting for the queue to have entries to read the data back into memory using read().
The problem that I can't seem to overcome is that when the 2nd process attempts to read the data back into memory, none of the disk units show read accesses. The program has code to check whether or not the data read back in is equal to the code that is written to disk, and the data always matches.
My question is, how can I make the OS (IBM i) not buffer the data when it is written to disk so that the read() system call accesses the data on the disk rather than in cache? I am doing simple throughput calculations and the read() operations are always 10+ times faster than the write operations.
I have tried using the O_DIRECT flag, but cannot seem to get the data to write to the file. It could have to do with setting up the correct aligned buffers. I have also tried the posix_fadvise(fd, offset,len, POSIX_FADV_DONTNEED) system call.
I have read through this similar question but haven't found a solution. I can provide code if it would be helpful.
My though is that if you write ENOUGH data, then there simply won't be enough memory to cache it, and thus SOME data must be written to disk.
You can also, if you want to make sure that small writes to your file works, try writing ANOTHER large file (either from the same process or a different one - for example, you could start a process like dd if=/dev/zero of=myfile.dat bs=4k count=some_large_number) to force other data to fill the cache.
Another "trick" may be to "chew up" some (more like most) of the RAM in the system - just allocate a large lump of memory, then write to some small part of it at a time - for example, an array of integers, where you write to every 256th entry of the array in a loop, moving to one step forward each time - that way, you walk through ALL of the memory quickly, and since you are writing continuously to all of it, the memory will have to be resident. [I used this technique to simulate a "busy" virtual machine when running VM tests].
The other option is of course to nobble the caching system itself in OS/filesystem driver, but I would be very worried about doing that - it will almost certainly slow the system down to a slow crawl, and unless there is an existing option to disable it, you may find it hard to do accurately/correctly/reliably.
...exercise several disk units in a raid configuration... How? IBM i doesn't allow a program access to the hardware. How are you directing I/O to any specific physical disks?
ANSWER: The write/read operations are done in parallel against IFS so the stream file manager is selecting which disks to target. By having enough threads reading/writing, the busyness of SYSBASE or an IASP can be driven up.
...none of the disk units show read accesses. None of them? Unless you are running the sole job on a system in restricted state, there is going to be read activity on the disks from other tasks. Is the system divided into multiple LPARs? Multiple ASPs? I'm suggesting that you may be monitoring disks that this program isn't writing to, because IBM i handles physical I/O, not programs.
ANSWER I guess none of them is a slight exaggeration - I know which disks belong to SYSBASE and those disks are not being targeted with many read requests. I was just trying to generalize for an audience not familiar w/IBM i. In the picture below, you will see that the write reqs are driving the % busyness up, but the read reqs are not even though they are targeting the same files.
...how can I make the OS (IBM i) not buffer the data when it is written to disk... Use a memory starved main storage pool to maximise paging, write immense blocks of data so as to guarantee that the system and disk controller caches overflow and use a busy machine so that other tasks are demanding disk I/O as well.
One line of background: I'm the developer of Redis, a NoSQL database. One of the new features I'm implementing is Virtual Memory, because Redis takes all the data in memory. Thanks to VM Redis is able to transfer rarely used objects from memory to disk, there are a number of reasons why this works much better than letting the OS do the work for us swapping (redis objects are built of many small objects allocated in non contiguous places, when serialized to disk by Redis they take 10 times less space compared to the memory pages where they live, and so forth).
Now I've an alpha implementation that's working perfectly on Linux, but not so well on Mac OS X Snow Leopard. From time to time, while Redis tries to move a page from memory to disk, the redis process enters the uninterruptible wait state for minutes. I was unable to debug this, but this happens either in a call to fseeko() or fwrite(). After minutes the call finally returns and redis continues working without problems at all: no crash.
The amount of data transfered is very small, something like 256 bytes. So it should not be a matter of a very big amount of I/O performed.
But there is an interesting detail about the swap file that's target of the write operation. It's a big file (26 Gigabytes) created opening a file with fopen() and then enlarged using ftruncate(). Finally the file is unlink()ed so that Redis continues to take a reference to it, but we are sure that when the Redis process will exit the OS will really free the swap file.
Ok that's all but I'm here for any further detail. And BTW you can even find the actual code in the Redis git, but it's not trivial to understand in five minutes given that's a fairly complex system.
Thank you very much for any help.
As I understand it, HFS+ has very poor support for sparse files. So it may be that your write is triggering a file expansion that is initializing/materializing a large fraction of the file.
For example, I know mmap'ing a new large empty file and then writing at a few random locations produces a very large file on disk with HFS+. It's quite annoying since mmap and sparse files are an extremely convenient way of working with data, and virtually every other platform/filesystem out there handles this gracefully.
Is the swap file written to linearly? Meaning we either replace an existing block or write a new block at the end and increment a free space pointer? If so, perhaps doing more frequent smaller ftruncate calls to expand the file would result in shorter pauses.
As an aside, I'm curious why redis VM doesn't use mmap and then just move blocks around in an attempt to concentrate hot blocks into hot pages.
antirez, I'm not sure I'll be much help since my Apple experience is limited to the Apple ][, but I'll give it a shot.
First thing is a question. I would have thought that, for virtual memory, speed of operation would be a more important measure than disk space (especially for a NoSQL DB where speed is the whole point, otherwise you'd be using SQL, no?). But, if your swap file is 26G, maybe not :-)
Some things to try (if possible).
Try to actually isolate the problem to the seek or write. I have a hard time believing a seek could take that long since, at worst, it should be a buffer pointer change. Still, I didn't write OSX so I can't be sure.
Try adjusting the size of the swap file to see if that's what is causing the problem.
Do you ever dynamically expand the swap file (as opposed to pre-allocation)? If you do, that may be what is causing the problem.
Do you always write as low in the file as you can? It may be that creating a 26G file may not actually fill it with data but, if you create it then write to the last byte, the OS may have to zero out the bytes before then (deferring the initialization, if any).
What happens if you just pre-allocate the entire file (write to every byte) and not unlink it? In other words, leave the file there between runs of your program (creating it if it doesn't already exist of course). Then in your startup code for Redis, just initialize the file (pointers and such). This may get rid of any problems like those in point 4 above.
Ask on the various BSD sites as well. I'm not sure how much Apple changed under the covers but OSX is just BSD at the lowest level (Pax ducks for cover).
Also consider asking on the Apple sites (if you haven't already done so).
Well, that's my small contribution, hopefully it'll help. Good luck with your project.
Have you turned off file caching for your file? i.e. fcntl(fd, F_GLOBAL_NOCACHE, 1)
Have you tried debugging with DTrace and or Instruments (Apple's experimental dtrace front-end)?
Exploring Leopard with DTrace
Debugging Chrome on OS X
As Linus said once on the Git mailing list:
"I realize that OS X people have a hard time accepting it, but OS X
filesystems are generally total and utter crap - even more so than
Windows."
In holy wars about whether garbage collection is a good thing, people often point out that it doesn't handle things like freeing file handles. Putting this logic in a finalizer is considered a bad thing because the resource then gets freed non-deterministically. However, it seems like an easy solution would be for the OS to just make sure lots and lots of file handles are available so that they are a cheap and plentiful resource and you can afford to waste a few at any given time. Why is this not done in practice?
In practice, it cannot be done because the OS would have to allocate a lot more memory overhead for keeping track of which handles are in use by different processes. In an example C code as shown below I will demonstrate a simple OS process structure stored in a circular queue for an example...
struct ProcessRecord{
int ProcessId;
CPURegs cpuRegs;
TaskPointer **children;
int *baseMemAddress;
int sizeOfStack;
int sizeOfHeap;
int *baseHeapAddress;
int granularity;
int time;
enum State{ Running, Runnable, Zombie ... };
/* ...few more fields here... */
long *fileHandles;
long fileHandlesCount;
}proc;
Imagine that fileHandles is an pointer to an array of integers which each integer contains the location (perhaps in encoded format) for the offset into the OS's table of where the files are stored on disk.
Now imagine how much memory that would eat up and could slow down the whole kernel, maybe bring about instability as the 'multi-tasking' concept of the system would fall over as a result of having to keep track of how much filehandles are in use and to provide a mechanism to dynamically increase/decrease the pointer to integers which could have a knock on effect in slowing down the user program if the OS was dishing out file handles on a user program's demand basis.
I hope this helps you to understand why it is not implemented nor practical.
Hope this makes sense,
Best regards,
Tom.
Closing a file also flushes the writes to disk -- well, from the point of view of your application anyway. After closing a file, the application can crash, as long as the system itself doesn't crash the changes will not be lost. So it's not a good idea to let the GC close files at its leisure. Even if it may be technically possible nowadays.
Also, to tell the truth, old habits die hard. File handles used to be expensive and are still probably considered as such for historical reasons.
It's not just the amount of file handles, it's that sometimes when they're used in some modes, they can prevent other callers from being able to access the same file.
I'm sure more comprehensive answers will ensue, but based on my limited experience and understanding of the underlying operation of Windows, file handles (the structures used to represent them to the OS) are kernel objects and as such they require a certain type of memory to be available - not to mention processing on the kernel part to maintain consistency and coherence with multiple processes requiring access to the same resources (i.e. files)
I don't think they're necessarily expensive - if your application only holds a few unnessary ones open it won't kill the system. Just like if you leak only a few strings in C++ no one will notice, unless they're looking pretty carefully. Where it becomes a problem is:
if you leak hundreds or thousands
if having the file open prevents other operations from occurring on that file (other applications might not be able to open or delete the file)
it's a sign of sloppiness - if your program can't keep track of what it owns and is using or has stopped using, what other problems will the program have? Sometimes a small leak turns into a big leak when something small changes or a user does something a little differently than before.
In the Linux paradigm sockets are file descriptors. There are definite advantages to freeing up TCP ports as soon as possible.