Exercise 1 (easy) - Size of files

The first step for falling into the darkness of c

What it will be shown below isn't just an exercise to illustrate some functions from lab 2, but rather showcase some optional syscalls, learn how to search about them, and recap some c functions (good to know).

The following information is also useful for the project part (after lab 8).

Description

Given a folder "tests01" with 3 .txt files in it, find the size of each file and write in "output.txt" the following : [ filename ]: [ size in bytes] Bytes.

Download the test folder

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Example

By running the code the file "output.txt" will look like this:

file1 copy.txt: 817 Bytes
file1.txt: 812 Bytes
test3.txt: 4 Bytes

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Idea

Let's take the task and split in more parts. The first thing we need to do is finding a way to open each .txt file from a folder. So the first step is to open the folder and print each file name.

💡Hint 1

Read about opendir(), readdir() and dirent (a structure);

🤖Code sample for step 1

#include <stdio.h>
#include <dirent.h>
int main() {
    struct dirent *entry;
    DIR *dir = opendir("tests01");
    while((entry = readdir(dir)) != NULL) {
        printf("%s\n", entry->d_name);
    }
    closedir(dir);
}

Did we actually make 'ls' ?

Now, how do we get access to the size of a file? That is the step 2

🤖Code sample for step 2

       struct stat st;
       char path[265];
       snprintf(path, sizeof(path), "tests01/%s", entry->d_name);
       stat(path, &st); //stat will need a path and it will start from the location of the .c file.

       int size = st.st_size; //tadaa

All right, now is the final step, merge them together. For this part you'll need the information from lab 2 for open, write in order to open the output.txt and write there.

🤖The final code

#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <dirent.h>
int main() {
    int destination_fd = open("output.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);

    struct dirent *entry; //information about a file from a director
    DIR *dir = opendir("tests01");

    while((entry = readdir(dir)) != NULL) {
        if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0) {
            continue;
        }

        struct stat st;
        char path[265];
        snprintf(path, sizeof(path), "tests01/%s", entry->d_name);
        stat(path, &st);

        write(destination_fd, entry->d_name, strlen(entry->d_name));
        write(destination_fd, ": ", 2);

        char size_str[32];
        snprintf(size_str, sizeof(size_str), "%ld Bytes\n", st.st_size);
        write(destination_fd, size_str, strlen(size_str));
    }
    closedir(dir);
    close(destination_fd);
    return 0;
}

Hope this problem helped you in some way 😊 and get ready for exercise 2, which will take some of the information presented here and add something spicy.