Let's say I have a file called hello.txt in the folder called coding, and I want to open that in python. I know that if I don't use os, I would have to write open("coding/hello.txt") but if I would write os.open would I still have to specify the folder like ("coding/hello.txt") or can I just write os.open("hello.txt") because I am using os?
"File" and "operating system" can mean a lot of different things, but typically operating systems have the concept of a "current" or "working" directory. Each process has its own current directory, and if you don't specify a directory for a file it uses the current directory.
Do not rely on this. Too many things can change the current directory unexpectedly, and your program will suddenly start using a different file.
Instead always specify the full file path like open("/usr/tmp/coding/hello.txt") or whatever is appropriate for your operating system; it will probably provide environment variables or something for the user's home or temporary directories.
Note that your examples "coding/hello.txt" and "hello.txt" both use the current directory, and are different files.
Related
If you have a program that writes to an output file in C, how do you access/see that output file? For instance, I'm trying to write a program that writes the values from a .ppm image file to another .ppm image file, but I don't know how to access the output file after I've done so. I know that's a pretty general question, but I don't have a block of code I can share just yet.
When creating a file with fopen by only specifying a file name, without specifying a path, then the file will be put in the current working directory of your program.
If you are using an integrated development environment (IDE) to launch your program, then you can probably see and set your program's initial working directory in your IDE. If you are running your program directly from a command-line shell, then the file will be placed in the current working directory of the shell.
On most operating systems, you can also determine your program's current working directory by calling a certain function provided by the operating system. For example, on POSIX-compliant operating systems, such as Linux, you can call getcwd. On Microsoft Windows, you can call _getcwd or GetCurrentDirectory. That way, you should easily be able to find out in which directory your file is being created.
At some point in my C program I have to deal with something like this.
FILE * fptr = fopen("/Parent/child/.../file.dat");
Which means in order to access any file I need to know it's location. That's all understandable.
But, how can I make this generic? In my computer "/Parent/child/.../file.dat" will work because that's where the file is stored, but I'm making a software to distribute to other users so the path obviously differs. My question is, how can I install a specific file into the user's computer such that I can know and get the location of that file. I a but confused about this concept so any resources that could help me understand it better would be greatly appreciated.
In Linux the default path to application files should be hardcoded. There is a standard which applications should follow. For example, architecture-independent files should go to /usr/share/ and then either your application name or, if you expect the data to be shared between applications, a generic category such as images. User-specific configuration files should go $HOME/.config/<app-name>. Older applications place their default configuration in $HOME/.<app-name> instead.
You should also provide an ability to override the default path to the data with a command line switch and/or an environment variable and/or a user configuration file (the location of the latter should also be overridable with a command line switch and/or an environment variable).
Some applications search for their data directory relatively to the executable position. An executable can know its own absolute path by reading /proc/self/exe symbolic link. For example, if an executable finds itself in /usr/local/bin/somename, it can look for /usr/local/share/<app-name> (two levels up from the executable name and down to share/<app-name>.).
Finally, if you distribute source code for the users to build, the file locations should be configuration parameters.
Many commands access files, and they usually take a path relative to ~/ which I gather is the "home" directory. In my case, C:\Users\Grant\AppData\Roaming\ which is where .emacs.d\ and my init files live.
This is great for Emacs, but not as useful for me. I typically would be over in C:\Users\Grant\Documents\ or my Python script files directory.
Also, I'm using Customize and typing in absolute file names for things like Org capture templates (currently anything I capture goes into files in one particular directory). Not that it's likely that these locations will change, but I was wondering if there are variables I can set for these places, which I can use in Customize or when invoking Dired - maybe some sort of environment variable? And if so, how can I refer to them when typing in a file name?
I am working at an OS independent file manager, and I divide files in groups, usually based on the extension. On Linux, I check if a file has the executable permissions or not, and if it does, I add it to the executables group.
This works great for Windows or Linux, but if you combine them it doesn't work so well. For example, while using it on Linux and exploring a windows mounted drive, all the files appear to be executable. I am trying to find a way to ignore those files and not add them to the executables group.
My code (on Linux) uses stat:
#ifndef WINDOWS
stat(ep->d_name, &buf);
....
if(!files_list[i].is_dir && buf.st_mode & 0111)
files_list[i].is_exe=1;
#endif
The first part of the answer is to find what filesystem the file is mounted on. To do that you need to find the filesystem using the st_dev field of the stat information for the file. (You can also do this by checking the file path, but you then have to check every path element for symbolic links).
You can then cross-reference the st_dev field with the mount table in /proc/mounts using getmntent_r(). There's an example of that in a previous answer. The mnt_type field will give you the text of the filesystem type, and you'll need to compare the string with a list of Windows filesystems.
Once you've found the filesystem, the only way to identify an executable is by heuristics. As other people have suggested, you can look at the file extension for Windows executables, and look at the initial bytes of the file for Linux executables. Don't forget executable scripts with the #! prefix, and you may need to read into a Jar file to find out if it contains an executable static main() method.
If you are browsing Windows files then you need to apply Windows rules for whether or not a file is executable. If the file extension is .EXE, .COM, .BAT, or .CMD then it is executable. If you want a more complete list then you should check MSDN. Note that it is possible to add registry entries on a machine that makes any extension you want to be considered executable, but it is best to ignore that kind of thing when you are browsing a drive from the network.
The fact is that you are fighting an uphill battle. The reason all the files have executable permissions is that the windows filesystem driver on Linux allows you to specify that as an option. This masks whether or not any files are Linux exceutables, for instance.
However, you could look into the file header for EVERY file and see if it is a Linux ELF executable (just like the Linux file command does).
It might be helpful to start by checking all the information about mounted filesystems so that you know what you are dealing with. For instance, do you have a CIFS filesystem mounted that is actually a Linux filesystem served up by SAMBA? If you enumerate every bit of information available about the mounted filesystem plus the complete set of stat info, you can probably identify combinations that act as fingerprints of the different scenarios.
Another option I could imagine, is to call the file util, and depend on its output (maybe its enough to grep for the words executable / script). This util exist/is compileable for windows (basically it just checks for some magic bytes in the files), too.
Hi all I was wondering how (if possible) to change to an external HDD in C. I am writing a program that works with an external HDD.
Thanks much,
Mr. Man
Be wary of changing directory within a program - all that's usually needed is to open the files on the external device without actually changing directory to it.
However, on both Windows and Unix, the basic answer is via a 'change directory' operation. On Unix-like platforms, that is the 'chdir(2)' system call; your program should specify the name of the directory where the external hard disk is mounted, and then relative pathnames will write to appropriate locations on the disk (subject to the path name not containing too many "../" sequences).
On Windows, you would need to ensure you specify the drive letter as well as the path on the hard drive.