Does readdir() guarantee an order?

K

7

65

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 don't see anything in the man pages about this order being guaranteed.

Can anyone tell me whether or not readdir guarrantees an order?

Khachaturian answered 23/1, 2012 at 19:30 Comment(5)

As a rule of thumb, odds are if it's not in the documentation, the answer is no. Even if there happened to be a consistent order on all implementations, if it's not documented, it's probably not guaranteed. – Hardpressed 23/1, 2012 at 19:34

If they're coming out alphabetical, that's almost surely just that they were originally created in alphabetical order, e.g. by unzip or tar extracting them as such... readdir provides no order. – Frymire 23/1, 2012 at 19:39

By the way, scandir may be useful if you want to order the results or have random access to them. It's standardized in POSIX 2008 and was a common extension before then. – Frymire 23/1, 2012 at 19:42

The ordering you gets depends almost surely from the filesystem. On FAT32 I get the files in order of creation, on NTFS usually in alphabetical order but with some exceptions, on ext4 in no apparent order. There goes your guaranteed order. :) – Suppressive 23/1, 2012 at 19:42

I've even seen a case where "." and ".." weren't the first entries (in a network mounted file system). And I wouldn't count much on the order of creation when creations and deletions are intermixed. – Stringhalt 24/1, 2012 at 10:51

T

55

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, its history and how its implementation is typically traversal order.

Tisbe answered 23/1, 2012 at 19:34 Comment(0)

M

14

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

Maturate answered 23/1, 2012 at 19:37 Comment(0)

G

13

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.

Gomes answered 24/1, 2012 at 9:56 Comment(0)

C

9

It's explicitly not guaranteed. The ordering often follows some rules, but the rules are complicated enough that you should not rely on them. The ordering may, for example, be affected by other operations happening in the same directory, and you can't control those. Treat the ordering as random, and sort things yourself if you need to.

Cavin answered 23/1, 2012 at 19:34 Comment(0)

S

8

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.)

Sawhorse answered 23/1, 2012 at 19:36 Comment(0)

N

3

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

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.

source: readdir() beginning with dots instead of files

#include <stdio.h>
#include <stdlib.h>
#include <dirent.h>
#include <string.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;
}

Newsletter answered 11/2, 2015 at 13:3 Comment(1)

just for fun: while(1==1) can be written as while(1) or simply for(;;) – Observer 30/8, 2018 at 22:16

S

1

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.

Spectral answered 27/11, 2017 at 14:39 Comment(0)

Recommended topics

Hot tags