Implementing the ls -al command in C

Question

As a part of an assignment from one of my classes, I have to write a program in C to duplicate the results of the ls -al command. I have read up on the necessary materials but I

Answer 1

myfile->d_name is the file name not the path, so you need to append the file name to the directory "Downloads/file.txt" first, if it's is not the working directory:

char buf[512];    
while((myfile = readdir(mydir)) != NULL)
{
    sprintf(buf, "%s/%s", argv[1], myfile->d_name);
    stat(buf, &mystat);
....

As to why it prints 4096 that is the size of the links . and .. from the last call to stat().

Note: you should allocate a buffer large enough to hold the directory name, the file name the NULL byte and the separator, something like this

strlen(argv[1]) + NAME_MAX + 2;

Answer 2

This is the final code I got to work for anyone interested. It prints the correct file sizes. Credit goes to asker and mux for answering, just putting the code together. Input I got this to work for is "./main ." .

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>

int main(int argc, char* argv[])
{
    DIR *mydir;
    struct dirent *myfile;
    struct stat mystat;

    char buf[512];
    mydir = opendir(argv[1]);
    while((myfile = readdir(mydir)) != NULL)
    {
        sprintf(buf, "%s/%s", argv[1], myfile->d_name);
        stat(buf, &mystat);
        printf("%zu",mystat.st_size);
        printf(" %s\n", myfile->d_name);
    }
    closedir(mydir);
}

Answer 3

I believe you'll observe that if you ./a.out . you will get the behaviour you expect.

You have a slightly subtle bug, observable if you examine the return code of your call to stat(2).

The fundamental mistake: the dirents returned by readdir(2) (the myfile in your code) will have a d_name relative to mydir. Your code will stat .. first, succeed, and so mystat will contain valid data for .., then all subsequent calls to stat(2) will fail, returning -1, which you do not check for, so mystat will not be modified, and you will print the st_size for the old value, i.e. that of ...

Answer 4

or maybe just system("ls -al") will also work!

Answer 5

The trouble is that when you stat("ankur.txt", &mystat), you are not working on the file "Downloads/ankur.txt". Most likely, the stat() is failing; alternatively, it is reporting on a different file.

Consequently, you need to look at whether your system supports fstatat() — new in POSIX 2008 — or arrange to prefix the name of the file with name of the directory.