Linux- C program - c

Question

I have recently(yesterday) started trying to learn linux and to program in this os. Now, one interesting and probably easy problem I came across while surfing the net was something like this:
Consider a C program that takes a directory as an argument in the command line and calculates the sum of all the files' dimensions that are in the directory's tree.
Now, due to the fact that I've been doing a lot of reading and researching in a short matter of time, all my knowledge is piled up in my brian creating a cloud of confusion. If anyone could help me with the code, I'd be really thankful.

Answer 1

what you are asking is a basic task. It can be done in linux but can also be done in microsoft windows with minor code tweaks if you are writing a program in C or C++. you would be writing code, which is sort of at a lower level compared to other ways of doing it, to accomplish what you want.
However you don't need to write a program C, which then requires you to compile it into an executable. Because what you are asking is a basic task, you might be able to do it with a bash shell script which would be linux specific. And if you wanted to do this in Windows then you would write a .bat file which is either the DOS scripting language, or Windows Powershell. I am not that familiar with Windows, i only mention it to help give you a general understanding for "all the knowledge piled up in your brain creating a cloud of confusion".
There is the windirstat program which runs under Microsoft Windows, can get it free from sourceforge and I think it does mostly what you are asking. I am not sure if you can get source code for it.
For linux there is kdirstat and that you can get the source code for from
http://kdirstat.cvs.sourceforge.net/viewvc/kdirstat/
you can download it as GNU tarball.
Look at how that program is written, which is C++ as you'll see a bunch of .cpp files. That would be a good template to work off of, and you can see what libraries they are using to accomplish file system functions. There are 21 .cpp files, look at the file kdirstatmain.cpp first.
For C/C++ code the start of execution is with the function int main(int argc, char *argv[]).
Regarding accomplishing this task with a bash shell script in linux, the best i can tell you is web search on bash shell scripting for linux.
And in linux to calculate the sum of all the files' dimensions that are in the directory's tree we can quickly do that at the linux prompt with the du -sh . command. In linux at the prompt do man du so read about the disk usage command. And then consider looking for the source code for du to use it as a template, and work off how they implemented du to learn and then modify their ways to meet your needs.
linux du command source code

Answer 2

Use opendir(3) to "open" the directory. Since you are interested in learning how to program in GNU/Linux, start by typing man opendir in the terminal to read how this function works. The (3) in opendir(3) means that the help for this function can be found in the section 3 of the manpages. Notice, at the top of the page, that the manpage tells you which #includes you'll need.
If everything goes right, opendir(3) will return a DIR* object. To know which files or subdirectories it contains, you use this object with readdir(3). This should return a pointer of type struct dirent*. You can heck the manual pages for details on the fields of this structure, but the most important for you will probably be d_type and d_name. A second call to this function will return the next entry. When it returns NULL, that either means you have read all files or an error occurred. To know which happened, you should check errno.
Here's a short example that list all entries in /tmp:
#include <stdio.h>
#include <dirent.h>
#include <sys/types.h>
int main(void)
{
DIR *dir;
struct dirent *entry;
dir = opendir("/tmp");
/* should check if dir != NULL */
while ((entry = readdir(dir)) != NULL) {
printf("Found %s\n", entry->d_name);
}
/* You may want to check errno here to see if readdir returned
* NULL because all files were read or because of some error;
* but this is beyond the purposes of my example.
*/
closedir(dir);
return 0;
}
Now you have to process each entry. If it is a directory, you have to descend into it an read its contents. A recursive function will probably help you here. If it is a file, then you have at least two options:
Open it with fopen(3), then use fseek(3) to seek the end of file. Use the return value of fseek(3) to calculate the size of the file in bytes;
Use stat(2) to get a structure with information on the file. Do not confuse it with stat(1). If you simply type man stat, you'll get information about the latter. To force man to read from section 2, type man 2 stat in the command line.
The first approach is certainly simpler. The second will require you to do a bit of reading on how stat(2) works. My advice: you should do it. Not only because it's more in the lines of Linux, but also because it gives you information that fseek(3) doesn't give. For instance, you can use stat(2) to see not only how many bytes the file contains, but how many bytes it occupies in the disk (like du does).
While reading the directory, you may stumble on other types of entries other than files and directories. stat(2) will probably help you figure the sizes of them as well. But you may want to simply ignore them for now.