How to get metadata from a file in c - c

I need to create a program in c and then build some code in the way that allows me to see what the metadata is in a file, not creating a header, just from any file. I've been searching this all around but still didn't find anything. I need to create a program in c able to read file system information about that file, size, type of file (normal, diredctory, link, etc.), i-node, owner, creation time and last modification. I know how to open the file and such, but I have no idea how to read that metadata. I appreciate de help. Also it's in linux

Did you try fstat()?
Man page link: https://linux.die.net/man/2/fstat

Related

How would I know that file is opened and it is saved after some writing operation using C code?

I have a set of configuration files (10 or more), and if user opens any of these file using any editor (e.g vim,vi,geany,qt,leafpad..). How would I come to know that which file is opened and if some writing process is done, then it is saved or not (using C code).
For the 1st part of your question, please refer e.g. to How to check if a file has been opened by another application in C++?
One way described there is to use a system tool like lsof and call this via a system() call.
For the 2nd part, about knowing whether a file has been modified, you will have to create a backup file to check against. Most editors already do that, but their naming scheme is different, so you might want to take care of that yourself. How to do that? Just automatically create a (hidden) file .mylogfile.txt if it does not exist by simply copying mylogfile.txt. If .mylogfile.txt exists, is having an older timestamp than mylogfile.txt, and differs in size and/or hash-value (using e.g. md5sum) your file was modified.
But before re-implementing this, take a look at How do I make my program watch for file modification in C++?

permission denied for rename function in C on windows

I have an application developed in C. This application is supported across multiple platforms. There is one functionality where we are transferring files via file transfer protocol to different machine or to any other directory on local machine. I want to include a functionality where I can transfer the file with some temporary name and once the transfer is complete, I want to rename the file with the correct name (the actual file name).
I tried using simple rename() function. It works fine in Unix and Linux machines. But it does not work on Windows. It is giving me an error code of 13(Permission denied error).
First thing, I checked in msdn to know the functionality of rename if I have to grant some permissions to the file etc.
I granted full permissions to the file (lets say it is 777).
I read in few other posts that I should close the file descriptor before renaming the file. I did that too. It still gives the same error.
Few other posts mentioned about the owner of the file and that of the application. The application will run as a SYSTEM user. (But this should not affect the behavior, because I tried the same rename function in my application as follows:
This works fine from my application:
rename("C:/abc/aaa.txt","C:/abc/zzz.txt");
but
rename(My_path,"C:/abc/zzz.txt");
doesn't work, where My_path when printed displays C:/abc/test.txt.
How can I rename a file? I need it to work on multiple platforms.
Are there any other things I should be trying to make it work.?
I had this same problem, but the issue was slightly different. If I did the following sequence of function calls, I got "Permission Denied" when calling the rename function.
fopen
fwrite
rename
fclose
The solution was to close the file first, before doing the rename.
fopen
fwrite
fclose
rename
If
rename("C:/abc/aaa.txt","C:/abc/zzz.txt");
works but
rename(My_path,"C:/abc/zzz.txt");
does not, in the exact same spot in the program (i.e. replacing one line with another and making no changes), then there might be something wrong with the variable My_path. What is the type of this variable? If it is a char array (since this is C), is it terminated appropriately? And is it exactly equal to "C:/abc/aaa.txt"?
(I wish I could post this as a comment/clarification rather than as an answer but my rep isn't good enough :( )

Check if another program has a file open

After doing tons of research and nor being able to find a solution to my problem i decided to post here on stackoverflow.
Well my problem is kind of unusual so I guess that's why I wasn't able to find any answer:
I have a program that is recording stuff to a file. Then I have another one that is responsible for transferring that file. Finally I have a third one that gets the file and processes it.
My problem is:
The file transfer program needs to send the file while it's still being recorded. The problem is that when the file transfer program reaches end of file on the file doesn't mean that the file actually is complete as it is still being recorded.
It would be nice to have something to check if the recorder has that file still open or if it already closed it to be able to judge if the end of file actually is a real end of file or if there simply aren't further data to be read yet.
Hope you can help me out with this one. Maybe you have another idea on how to solve this problem.
Thank you in advance.
GeKod
Simply put - you can't without using filesystem notification mechanisms, windows, linux and osx all have flavors of this. I forget how Windows does it off the top of my head, but linux has 'inotify' and osx has 'knotify'.
The easy way to handle this is, record to a tmp file, when the recording is done then move the file into the 'ready-to-transfer-the-file' directory, if you do this so that the files are on the same filesystem when you do the move it will be atomic and instant ensuring that any time your transfer utility 'sees' a new file, it'll be wholly formed and ready to go.
Or, just have your tmp files have no extension, then when it's done rename the file to an extension that the transfer agent is polling for.
Have you considered using stream interface between the recorder program and the one that grabs the recorded data/file? If you have access to a stream interface (say an OS/stack service) which also provides a reliable end of stream signal/primitive you could consider that to replace the file interface.
There is no functions/libraries available in C to do this. But a simple alternative is to rename the file once an activity is over. For example, recorder can open the file with name - file.record and once done with recording, it can rename the file.record to file.transfer and the transfer program should look for file.transfer to transfer and once the transfer is done, it can rename the file to file.read and the reader can read that and finally rename it to file.done!
you can check if file is open or not as following
FILE_NAME="filename"
FILE_OPEN=`lsof | grep $FILE_NAME`
// if [ -z $FILE_NAME ] ;then
// "File NOT open"
// else
// "File Open"
refer http://linux.about.com/library/cmd/blcmdl8_lsof.htm
I think an advisory lock will help. Since if one using the file which another program is working on it, the one will get blocked or get an error. but if you access it in force,the action is Okey, but the result is unpredictable, In order to maintain the consistency, all of the processes who want to access the file should obey the advisory lock rule. I think that will work.
When the file is closed then the lock is freed too.Other processes can try to hold the file.

How do i use vfs_readdir in linux kernel?

I'm trying to use a system call to display the contents of a directory. I've been pointed in the direction of vfs_readdir, but I have no clue of how to use it or what to pass to it to get the contents of a directory. All I want to do is be able to list files in a directory similar to how ls works. (I eventually intend to store this in some sort of buffer, but for now just being able to print the contents of a dir would be enough).
I think you probably have to open the directory using filp_open.
For the "flags" argument, you proably need to put some combination of the LOOKUP_ flags listed in include/linux/namei.h
You can see what build_open_flags does here: http://lxr.linux.no/#linux+v3.1.5/fs/open.c#L876 to provide flags to filp_open.
As far as I know, filp_open IS the correct way of opening a file in kernel-space. However, doing so is discouraged.
Provided you do so from the context of a "normal" thread belonging to a user process, I think you will be ok.

C - Reading multiple files

just had a general question about how to approach a certain problem I'm facing. I'm fairly new to C so bear with me here. Say I have a folder with 1000+ text files, the files are not named in any kind of numbered order, but they are alphabetical. For my problem I have files of stock data, each file is named after the company's respective ticker. I want to write a program that will open each file, read the data find the historical low and compare it to the current price and calculate the percent change, and then print it. Searching and calculating are not a problem, the problem is getting the program to go through and open each file. The only way I can see to attack this is to create a text file containing all of the ticker symbols, having the program read that into an array and then run a loop that first opens the first filename in the array, perform the calculations, print the output, close the file, then loop back around moving to the second element (the next ticker symbol) in the array. This would be fairly simple to set up (I think) but I'd really like to avoid typing out over a thousand file names into a text file. Is there a better way to approach this? Not really asking for code ( unless there is some amazing function in c that will do this for me ;) ), just some advice from more experienced C programmers.
Thanks :)
Edit: This is on Linux, sorry I forgot to metion that!
Under Linux/Unix (BSD, OS X, POSIX, etc.) you can use opendir / readdir to go through the directory structure. No need to generate static files that need to be updated, when the file system has the information you want. If you only want a sub-set of stocks at a given time, then using glob would be quicker, there is also scandir.
I don't know what Win32 (Windows / Platform SDK) functions are called, if you are developing using Visual C++ as your C compiler. Searching MSDN Library should help you.
Assuming you're running on linux...
ls /path/to/text/files > names.txt
is exactly what you want.
opendir(); on linux.
http://linux.die.net/man/3/opendir
Exemple :
http://snippets.dzone.com/posts/show/5734
In pseudo code it would look like this, I cannot define the code as I'm not 100% sure if this is the correct approach...
for each directory entry
scan the filename
extract the ticker name from the filename
open the file
read the data
create a record consisting of the filename, data.....
close the file
add the record to a list/array...
> sort the list/array into alphabetical order based on
the ticker name in the filename...
You could vary it slightly if you wish, scan the filenames in the directory entries and sort them first by building a record with the filenames first, then go back to the start of the list/array and open each one individually reading the data and putting it into the record then....
Hope this helps,
best regards,
Tom.
There are no functions in standard C that have any notion of a "directory". You will need to use some kind of platform-specific function to do this. For some examples, take a look at this post from Cprogrammnig.com.
Personally, I prefer using the opendir()/readdir() approach as shown in the second example. It works natively under Linux and also on Windows if you are using Cygwin.
Approach 1) I would just have a specific directory in which I have ONLY these files containing the ticker data and nothing else. I would then use the C readdir API to list all files in the directory and iterate over each one performing the data processing that you require. Which ticker the file applies to is determined only by the filename.
Pros: Easy to code
Cons: It really depends where the files are stored and where they come from.
Approach 2) Change the file format so the ticker files start with a magic code identifying that this is a ticker file, and a string containing the name. As before use readdir to iterate through all files in the folder and open each file, ensure that the magic number is set and read the ticker name from the file, and process the data as before
Pros: More flexible than before. Filename needn't reflect name of ticker
Cons: Harder to code, file format may be fixed.
but I'd really like to avoid typing out over a thousand file names into a text file. Is there a better way to approach this?
I have solved the exact same problem a while back, albeit for personal uses :)
What I did was to use the OS shell commands to generate a list of those files and redirected the output to a text file and had my program run through them.
On UNIX, there's the handy glob function:
glob_t results;
memset(&results, 0, sizeof(results));
glob("*.txt", 0, NULL, &results);
for (i = 0; i < results.gl_pathc; i++)
printf("%s\n", results.gl_pathv[i]);
globfree(&results);
On Linux or a related system, you could use the fts library. It's designed for traversing file hierarchies: man fts,
or even something as simple as readdir
If on Windows, you can use their Directory Management API's. More specifically, the FindFirstFile function, used with wildcards, in conjunction with FindNextFile

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