How can I deny access to a file I open with fstream? I want to unable access to the file while I'm reading/writing to it with fstream?
You cannot do it with the standard fstream, you'll have to use platform specific functions.
On Windows, you can use CreateFile() or LockFileEx(). On Linux, there is flock(), lockf(), and fcntl() (as the previous commenter said).
If you are using MSVC, you can pass a third parameter to fstream's constructor. See the documentation for Visual Studio 6 or newer versions. Of course it won't work with other compilers and platforms.
Why do you want to lock others out anyway? There might be a better solution...
Expanding on the comment by Casebash:
To open a file in windows so that other processes cannot write to it use
file.rdbuf()->open(path, std::ios_base::app, _SH_DENYWR);
_SH_DENYRW will deny both read and write access
There is no way to do this in native C++, as it would be highly platform dependent. On Linux/UNIX, you can do this with flock or fcntl. I'm not really sure how to do it on Windows.
On windows, it looks like you have to pass some flags to CreatFile or use LockFileEx (which allows byte range locking).
Note that, all of these methods work on the underlying OS file descriptors/handles, not with fstreams. You will either need to use the Posix or Windows API to read/write from the file, or wrap the file descriptor/handle in an fstream. This is platform dependent again. I'm sure there is a way to do it, but I don't remember it off the top of my head.
Related
A question came up at work where I am not sure of the answer. Suppose there is a handle, within a process, to a directory. The handle has RW set and lets say it points to the desktop. Is there a way to use nothing but the handle in order to read or write files to the desktop?
Thanks for any ideas!
Edit: To clarify, this is on Windows and is a File handle pointing at the desktop. Lets assume you already know the value of the handle and are looking to use it within the same process in order to read/write files to the desktop that it points at. Any code is allowed, including the Windows API.
You can use GetFinalPathNameByHandle() on the directory handle to obtain the pathname, append the file you want to write onto the end of it, then use CreateFile() as normal.
Standard C and C++ have no notion of a directory, and thus the strict answer is no.
To say anything more, we need more information. What do you mean by a handle? What are the platforms/libraries you are allowed work with?
Also, it's not clear what you mean by "nothing but the handle." Functions are not the handle; are you allowed to call functions?
I have a requirement for reading, updating and deleting a file. I want to write a class for this.
For example
class FileManagement {
private:
fstream myFile;
public:
void read();
void update();
void delete();
};
My question is while updating is it possible to delete only one line in a file in C++ and should be portable, if it is possible how we can achieve this.
Other question is if above option is not possible how we can achieve the above.
In C++ how we can delete a file in portable way.
Thanks!
Use standard C/C++ functions fopen(), fread(), fwrite(), rename() and remove() for that. http://www.cplusplus.com/reference/clibrary/cstdio/
I recommend Boost Filesystem.
Its description reads:
"The Boost Filesystem Library provides portable facilities to query and manipulate paths, files, and directories."
You appear to be asking two different questions at once, in a confusing way.
To delete a file, use the remove function, found in stdio.h.
To erase one line of a file, you have to read the entire file and write it back out with the line removed. There is no library routine for this. The standard "safe" technique is to read the entire file, write it back out (with the line you don't want removed) to a new file in the same directory, fsync the new file, close it, then rename the new file to the old name. If you don't care about concurrent readers or the computer crashing in the middle of the operation, you can instead open the old file read/write, read its contents into memory, rewind the file handle, and rewrite it directly.
You should look at the posix standard, and find the file operations (like fopen()). Where platforms do not support posix, or diverge from the standard, you'll likely need to
#ifdef NONPOSIXOS1 // really, this should be a good identifier of hte OS
// write code to handle the special case
#else
// write code to handle the posix compliant case
#endif
Most systems will accept posix compliant statements. You could always just define abstract base class and create different concrete implementations that use whatever platform specific instructions you need. You could have one if def that instantiates the correct concrete class.
If you are looking for a higher-level C++ library that is object-oriented and can handle both filename manipulation and file I/O, POCO is a decent choice:
http://pocoproject.org
ACE is an older, battle-tested framework that includes lots of I/O support. It's commonly used for it's excellent CORBA support, but there's a lot in there:
http://www.cs.wustl.edu/~schmidt/ACE-overview.html
And, finally, there's QT. Normally known for its cross-platform UI library, QT actually includes several other useful pieces (including file management and I/O), and you don't even have to link in the UI stuff if you don't need it.
http://qt.nokia.com/
If you'd rather not bring in another framework, I would recommend rolling your own File I/O classes using boost::filesystem and either the standard iostream or stdio functions. You can use the interfaces in the above frameworks as a reference, but you will also want to familiarize yourself with modern C++ design, as demonstrated by Boost and explained in Modern C++ Design.
I'd like to simulate a file without writing it on disk. I have a file at the end of my executable and I would like to give its path to a dll. Of course since it doesn't have a real path, I have to fake it.
I first tried using named pipes under Windows to do it. That would allow for a path like \\.\pipe\mymemoryfile but I can't make it works, and I'm not sure the dll would support a path like this.
Second, I found CreateFileMapping and GetMappedFileName. Can they be used to simulate a file in a fragment of another ? I'm not sure this is what this API does.
What I'm trying to do seems similar to boxedapp. Any ideas about how they do it ? I suppose it's something like API interception (Like Detour ), but that would be a lot of work. Is there another way to do it ?
Why ? I'm interested in this specific solution because I'd like to hide the data and for the benefit of distributing only one file but also for geeky reasons of making it works that way ;)
I agree that copying data to a temporary file would work and be a much easier solution.
Use BoxedApp and do not worry.
You can store the data in an NTFS stream. That way you can get a real path pointing to your data that you can give to your dll in the form of
x:\myfile.exe:mystreamname
This works precisely like a normal file, however it only works if the file system used is NTFS. This is standard under Windows nowadays, but is of course not an option if you want to support older systems or would like to be able to run this from a usb-stick or similar. Note that any streams present in a file will be lost if the file is sent as an attachment in mail or simply copied from a NTFS partition to a FAT32 partition.
I'd say that the most compatible way would be to write your data to an actual file, but you can of course do it one way on NTFS systems and another on FAT systems. I do recommend against it because of the added complexity. The appropriate way would be to distribute your files separately of course, but since you've indicated that you don't want this, you should in that case write it to a temporary file and give the dll the path to that file. Make sure you write the temporary file to the users' temp directory (you can find the path using GetTempPath in C/C++).
Your other option would be to write a filesystem filter driver, but that is a road that I strongly advise against. That sort of defeats the purpose of using a single file as well...
Also, in case you want only a single file for distribution, how about using a zip file or an installer?
Pipes are for communication between processes running concurrently. They don't store data for later access, and they don't have the same semantics as files (you can't seek or rewind a pipe, for instance).
If you're after file-like behaviour, your best bet will always be to use a file. Under Windows, you can pass FILE_ATTRIBUTE_TEMPORARY to CreateFile as a hint to the system to avoid flushing data to disk if there's sufficient memory.
If you're worried about the performance hit of writing to disk, the above should be sufficient to avoid the performance impact in most cases. (If the system is low enough on memory to force the file data out to disk, it's probably also swapping heavily anyway -- you've already got a performance problem.)
If you're trying to avoid writing to disk for some other reason, can you explain why? In general, it's quite hard to stop data from ever hitting the disk -- the user can always hibernate the machine, for instance.
Since you don't have control over the DLL you have to assume that the DLL expects an actual file. It probably at some point makes that assumption which is why named pipes are failing on you.
The simplest solution is to create a temporary file in the temp directory, write the data from your EXE to the temp file and then delete the temporary file.
Is there a reason you are embedding this "pseudo-file" at the end of your EXE instead of just distributing it with our application? You are obviously already distributing this third party DLL with your application so one more file doesn't seem like it is going to hurt you?
Another question, will this data be changing? That is are you expecting to write back data this "pseudo-file" in your EXE? I don't think that will work well. Standard users may not have write access to the EXE and that would probably drive anti-virus nuts.
And no CreateFileMapping and GetMappedFileName definitely won't work since they don't give you a file name that can be passed to CreateFile. If you could somehow get this DLL to accept a HANDLE then that would work.
And I wouldn't even bother with API interception. Just hand the DLL a path to an acutal file.
Reading your question made me think: if you can pretend an area of memory is a file and have kind of "virtual path" to it, then this would allow loading a DLL directly from memory which is what LoadLibrary forbids by design by asking for a path name. And this is why people write their own PE loader when they want to achieve that.
I would say you can't achieve what you want with file mapping: the purpose of file mapping is to treat a portion of a file as if it was physical memory, and you're wanting the reciprocal.
Using Detours implies that you would have to replicate everything the intercepted DLL function does except from obtaining data from a real file; hence it's not generic. Or, even more intricate, let's pretend the DLL uses fopen; then you provide your own fopen that detects a special pattern in the path and you mimmic the C runtime internals... Hmm is it really worth all the pain? :D
Please explain why you can't extract the data from your EXE and write it to a temporary file. Many applications do this -- it's the classic solution to this problem.
If you really must provide a "virtual file", the cleanest solution is probably a filesystem filter driver. "clean" doesn't mean "good" -- a filter is a fully documented and supported solution, so it's cleaner than API hooking, injection, etc. However, filesystem filters are not easy.
OSR Online is the best place to find Windows filesystem information. The NTFSD mailing list is where filesystem developers hang out.
How about using a some sort of RamDisk and writing the file to this disk? I have tried some ramdisks myself, though never found a good one, tell me if you are successful.
Well, if you need to have the virtual file allocated in your exe, you will need to create a vector, stream or char array big enough to hold all of the virtual data you want to write.
that is the only solution I can think of without doing any I/O to disk (even if you don't write to file).
If you need to keep a file like path syntax, just write a class that mimics that behaviour and instead of writing to a file write to your memory buffer. It's as simple as it gets. Remember KISS.
Cheers
Open the file called "NUL:" for writing. It's writable, but the data are silently discarded. Kinda like /dev/null of *nix fame.
You cannot memory-map it though. Memory-mapping implies read/write access, and NUL is write-only.
I'm guessing that this dll cant take a stream? Its almost to simple to ask BUT if it can you could just use that.
Have you tried using the \?\ prefix when using named pipes? Many APIs support using \?\ to pass the remainder of the path directly through without any parsing/modification.
http://msdn.microsoft.com/en-us/library/aa365247(VS.85,lightweight).aspx
Why not just add it as a resource - http://msdn.microsoft.com/en-us/library/7k989cfy(VS.80).aspx - the same way you would add an icon.
I've been looking over the Doom 3 SDK code, specifically their File System implementation.
The system works (the code I have access to at least) by passing around an 'idFile' object and I've noticed that this class provides read and write methods as well as maintaining a FILE* member.
This suggests to me that either the FILE* is 'opened' with read and write access or the file is closed and reopened (with the appropriate access) between calls to Read() and Write().
Does this sound correct or am I over simplifying it?
If this isn't the case (which part of me suspects it isn't - due to speed etc.) does anyone have any suggestions as to how they would achieve this elegant interface?
Please bare in mind that I am fairly new to both C++ and stdio (which I'm pretty sure iD favours).
You can open a FILE* in read-write mode.
If you do that, you should flush and seek to a known location when changing between reading and writing, but you don't have to reopen the file.
Without ever having looked at the Doom code (I'm guessing you can specify a mode when you create the object), you can use freopen() to re-open a file (in a different mode, if you want) without closing it first.
I want to create a file that only resides in memory... In looking through some documentation I saw a recommendation to use a shell extension as a virtual file. Im not sure that is a workable solution but I would like to know
Is it a good approach (or should I be using a ramdisk instead)
Where is a good place to start to read up on it
Note: This is on the Windows platform
As I understand, you want your program to create a "file", which resides only in memory and that you can pass on to another external program (say, Microsoft Word).
AFAIK this is not possible, short of a ramdrive. I'd suggest using a temporary folder. You will however have to come up with a strategy for deleting the file when it's not needed anymore.
Added: On second though, you might want to check out Reparse points. I'm not familiar with them myself, and they will only work for NTFS formatted disks, but perhaps they can provide you with what you want. It will be a lot of coding though.
You don't say on which plateform you are but I'm guessing Windows. Is mmap() available? If not, I think BerkeleyDB has been ported to Windows so you should be able to use that. Win32 API may have something akin to mmap() but I don't know it.
If you want a file that resides only in memory, use a named pipe or something, though I question your scenario - can you go up a level and describe what you want to do?