C faster way to check if a directory exists

Question

I'm using opendir function to check if a directory exists. The problem is that I'm using it on a massive loop and it's inflating the ram used by my app.

What is the best (fastest) way to check if a directory exists in C? What is the best (fastest) way to create it if doesn't exists?

And the faster way might be to do it asynchronously, but you should consider the capabilities of your media. If it is regular HDD then it might fail into seek-storm and this will work slow. But if you use RAID/SSD or even working in the cloud, this will work. — LiMar, Feb 16 '12 at 16:09

score 50 · Answer 1 · answered Feb 16 '12 at 16:08

50

Consider using stat. S_ISDIR(s.st_mode) will tell you if it's a directory.

Sample:

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

...
struct stat s;
int err = stat("/path/to/possible_dir", &s);
if(-1 == err) {
    if(ENOENT == errno) {
        /* does not exist */
    } else {
        perror("stat");
        exit(1);
    }
} else {
    if(S_ISDIR(s.st_mode)) {
        /* it's a dir */
    } else {
        /* exists but is no dir */
    }
}
...

answered Feb 16 '12 at 16:08

Johannes Weiss

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1

how faster does it is comparing with opendir? – Frederico Schardong Feb 16 '12 at 16:22
3

That depends on the operating system and the number of entries there are in the parent directory. It will be faster in any way and that's what `stat` is made for. – Johannes Weiss Feb 16 '12 at 16:24
Would lstat also be an option? – cwd Jun 08 '13 at 22:09
@cwd , yes, I think `lstat` would possibly be the even better option because it does not follow symlinks. Depends on what is wanted though... – Johannes Weiss Jun 08 '13 at 22:32
I am using this to test for "D:\path\subpath" and I've created the directory. It is telling me the dir does **not** exist. :/ *(using Windows/MinGW)* – IAbstract Mar 30 '15 at 13:22

score 25 · Accepted Answer · edited May 22 '13 at 08:32

25

You could call mkdir(). If the directory does not exist then it will be created and 0 will be returned. If the directory exists then -1 will be returned and errno will be set to EEXIST.

edited May 22 '13 at 08:32

Kevin Brydon

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answered Feb 16 '12 at 16:04

ckruse

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With the caveat that `ENOENT` may occur if something along the path doesn't exist as well. – user7116 Feb 16 '12 at 16:07
True. Should have mentioned that. – ckruse Feb 16 '12 at 16:08
Since you want to create the directory if it doesn't exist: yes. You save a ``stat()`` call. – ckruse Feb 16 '12 at 20:53
1

Note that you need to include (for `mkdir`) and (to handle `errno` and `EEXIST`). – Skippy le Grand Gourou Jul 21 '13 at 18:09
@ckruse : If the directory exists in 99 % of cases, and if `stat` is faster than `mkdir` (which I don't have a clue about), your statement may not be true. – Skippy le Grand Gourou Jul 21 '13 at 18:18
1

Note however that if you DON'T WANT to create the directory (but only check for existence, per first half of the question), then then `mkdir` costs you I/O (might be synchronized, costing an even greater hit in I/O performance for other actors on the system) and might fail for reasons unrelated to existence such as current account's permissions, mount options (R/O FS), or available filesystem space or quota. – Jim Klimov Jul 21 '20 at 09:15

score 21 · Answer 3 · answered Mar 07 '13 at 15:12

21

I prefer using access()

if (0 != access("/path/to/possible_dir/", F_OK)) {
  if (ENOENT == errno) {
     // does not exist
  }
  if (ENOTDIR == errno) {
     // not a directory
  }
}

If you ensure a trailing / in the directory name, this works perfectly.

answered Mar 07 '13 at 15:12

To1ne

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Could you point me to where you found the bit about the trailing slash? The `access()` man page does not seem to say anything about it. – domsson Nov 23 '18 at 08:21
This is the best reference I could find: https://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap04.html#tag_04_11 – To1ne Nov 26 '18 at 09:16

score 3 · Answer 4 · answered Feb 16 '12 at 16:06

3

I would use stat(), if available.

answered Feb 16 '12 at 16:06

unwind

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score 0 · Answer 5 · answered Feb 16 '12 at 16:09

0

It sounds like you have a memory leak. Calling opendir should not inflate the RAM of your app as long as you remember to always call closedir after successfully opening a directory. Also, make sure you are freeing any buffers you allocated to compute the directory name.

answered Feb 16 '12 at 16:09

David Grayson

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Yeap I'm closing it, I'll just use stat, thanks for your time! – Frederico Schardong Feb 16 '12 at 16:28

C faster way to check if a directory exists

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