A Glimpse of GNU/Linux

All of us (well, at least some of us) like to test the limit of existing things.. That’s what I tried to do with the below given program. I decided to find the limit of the directories created in directories for an ext3 file system using mkdir(). Any one of you could find the reason behind it ? Well, it was for fun

# cat test_max_directories.c
————————————————
#include <stdio.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <errno.h>

int main()
{
char dirname[50];
int i;
/* The max number of subdirectories in one directory for ext3 is 32000 */
int limit = 32001;

for (i = 0; i < limit; i++) {
sprintf(dirname, “testdir%d”, i);
if ((mkdir(dirname, 00777)) == -1) {
perror(“Error in creating directory!”);
printf(“errno = %d\n”, errno);
exit(1);
}
}
return 0;

}

———————–

As expected, strace showed that it failed, however with an EMLINK error:

mkdir(“testdir31998″, 0777)             = -1 EMLINK (Too many links)

Here comes the next  thing…. EMLINK error is not present in the man page of mkdir(2) … Wo hoo… a big chance to submit a patch to Linux Kernel Man page project …. I have started the exciting process of preparing the patch, mail and follow up process.

First, I should download the latest kernel man pages which is now in a git repository:

#apt-get install git-core

#git clone git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git

After check out, I have made the necessary changes in mkdir.2 page and prepared the patch using the command:

# git diff

I got the below listed patch:
————

diff –git a/man2/mkdir.2 b/man2/mkdir.2
index e6cb28c..d01bafd 100644
— a/man2/mkdir.2
+++ b/man2/mkdir.2
@@ -73,6 +73,10 @@ is a symbolic link, dangling or not.
Too many symbolic links were encountered in resolving
.IR pathname .
.TP
+.B EMLINK
+The new directory cannot be created because the number of sub directories
+in a directory is limited to LINK_MAX defined by the file system.
+.TP
.B ENAMETOOLONG
.IR pathname ” was too long.”
.TP

—————————

Here comes the most exciting part… Create a proper mail and convince the man page maintainer to include this to the mkdir man page.

First, I should choose the right mail id’s to send the patch. From the mailing list it should be mailed with:

To: mtk.manpages@googlemail.com

CC: linux-man@vger.kernel.org

(make sure that you are already subscribed to linux-man@vger.kernel.org)

Then it is important to give the correct subject line. It gives the required importance to the patch. A wrong subject line could ruin to chances to mainline your patch . As I have previous experience with this mailing list, I chose the subject line:

[PATCH] mkdir.2: Add EMLINK error

Next important thing is what we should write in the mail. We should support our patch and show it’s importance. It should have the “Signed-off-by : Name <mail id> and properly placed patch along with the mail. Some groups will reject the patch right away if we attach the patch with the mail as a separate attachment.  So, observe the mailing list and then decide how to send the patch.

——————————————-

Dear Michael,

While executing the below given program in ext3 file system, I came
across the EMLINK error in mkdir(2) syscall.

# cat test_max_directories.c

#include <stdio.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <errno.h>

int main()
{
char dirname[50];
int i;
/* The max number of subdirectories in one directory for ext3 is 32000 */
int limit = 32001;

for (i = 0; i < limit; i++) {
sprintf(dirname, “testdir%d”, i);
if ((mkdir(dirname, 00777)) == -1) {
perror(“Error in creating directory!”);
printf(“errno = %d\n”, errno);
exit(1);
}
}
return 0;
}

# ./a.out
Error in creating directory!: Too many links
errno = 29

An strace of this execution shows that the mkdir(2) call generates an
EMLINK error when it reaches the limit of  maximum number of sub
directories in one directory

#strace ./a.out
——————————

Bingo.. they accepted the patch, with some modifications though…

Come, be a man … Submit some kernel man page patches