来自系统调用创建的奇怪行为

Question

I am creating a file as follows

 int fd  = creat(file_path.c_str() ,S_IRWXU|S_IRWXG|S_IRWXO);

Though i am providing all permissions to all three entities, it creates the files with the below permission.

-rwxr-xr-x

The directory i am creating this in has permissions set as

drwxrwxrwx

Umask

0022

Can you guys please suggest what could be wrong?

Edit : I can chmod the file to give it the permissions i plan to. I am curious why the above is failing.

Answer 1

You said it yourself, you have a umask of 022, so the write permission is omitted; From the creat man page:

The effective permissions are modified by the process's umask in the usual way: The permissions of the created file are (mode & ~umask).

Answer 2

You are likely running under a umask restriction

man umask

should get you onto the right track

Answer 3

The umask is combined with the file permissions you specify in the creat call in this way file_perms & ~umask to arrive at the permission bits that are actually set for the file. The umask basically specifies which bits should not be set even if you request they be set.

The chmod call ignores the umask. And your directory permissions are irrelevant here (well, not totally irrelevant, it would fail completely if you didn't have write permission to the directory you're creating the file in).

Answer 4

Permissions in the umask are turned off from the mode argument to open and mkdir (and creat and friends, similarly). For example, if the mode argument is specified as:

S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH (0666) when creating a file with the typical umask of 0022, the result will be

S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH (0644)

So, you are correctly seeing write access not being given to the directory you're creating.