Does readdir() guarantee an order?

Question

I\'m getting a list of files on a linux-like system using opendir/readdir. It appears that the directory entries are returned in alphabetical order of file name. However, I

Answer 1

In short, no, readdir() does not guarantee any particular order.

from a readdir example in the glibc manual

The order in which files appear in a directory tends to be fairly random. A more useful program would sort the entries (perhaps by alphabetizing them) before printing them

Answer 2

The readdir method doesn't guarantee any ordering. If you want to ensure they are sorted alphabetically you'll need to do so yourself.

Note: I searched for a bit for definitive documentation saying this is the case. The closest I came is the following link

• http://utcc.utoronto.ca/~cks/space/blog/unix/ReaddirOrder

It's by no means definitive but it does give a nice overview of the command, it's history and how it's implementation is typically traversal order.

Answer 3

No, readdir does not guarantee any order.

(Some file systems might store directory entries in a certain order, in such cases readdir might return them to you in the same order, but that's not a feature of readdir itself.)

Answer 4

readdir() does not guarantee any other order above that which is OS disk read order.

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According to test which i made on few platforms - Solaris - sun4sol, x86 sol, linux, Windows with the sample code all results were displated in a random manner.

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source: readdir() beginning with dots instead of files

#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>

int main() {

    DIR *dir;
    struct dirent *dp;
    char * file_name;
    char dirpath [100] ;


    while(1==1){
        printf("Choose dir:");
        scanf("%s",dirpath);
        dir = opendir(dirpath);
        while ((dp=readdir(dir)) != NULL) {
            if ( !strcmp(dp->d_name, ".") || !strcmp(dp->d_name, "..") )
            {
                // do nothing (straight logic)
            } else {
                file_name = dp->d_name; // use it
                printf("file_name: \"%s\"\n",file_name);
            }
        }
        closedir(dir);
    }

    return 0;
}

Answer 5

From "The linux programming interface":

The filenames returned by readdir() are not in sorted order, but rather in the order in which they happen to occur in the directory (this depends on the order in which the file system adds files to the directory and how it fills gaps in the directory list after files are removed). (The command ls –f lists files in the same unsorted order that they would be retrieved by readdir().)

We can use the function scandir(3) to retrieve a sorted list of files matching programmer-defined criteria; see the manual page for details. Although not specified in SUSv3, scandir() is provided on most UNIX implementations.

Note: scandir is a part of POSIX.1-2008. A permissibly-copyrighted version defined around readdir is available in FreeBSD libc.

Answer 6

In addition to the other answers, the readdir man page is pretty clear about files ordering.

The order in which filenames are read by successive calls to readdir() depends on the filesystem implementation; it is unlikely that the names will be sorted in any fashion.

Some file systems, like ReiserFS do list the files in lexical order.

In your case, you have to store the names in an array, then sort the array.

For instance, use qsort() to sort the array.