open() and permissions not quite as documented?

mpersico has asked for the wisdom of the Perl Monks concerning the following question:

Is it a known behavior that if you 'open' a file for writing that does not already exist, umask settings are properly applied, yet if a file that exists is opened for (over)writing, it keeps the existing permissions, ignoring the umask settings? This drove me nuts for an hour this morning on a Linux box with Perl 5.16.

Comment on open() and permissions not quite as documented?

Replies are listed 'Best First'.
Re: open() and permissions not quite as documented? by Marshall (Canon) on May 27, 2016 at 18:37 UTC
That is exactly what is supposed to happen. umask applies only to creating new files. Opening an existing file for "re-write" is not the same as creating a new file. You would need to delete the existing file, then create a new one for the umask to apply. On Linux and other Unix-like operating systems, new files are created with a default set of permissions. Specifically, a new file's permissions may be restricted in a specific way by applying a permissions "mask" called the umask. The umask command is used to set this mask, or to show you its current value.	[reply]
Re: open() and permissions not quite as documented? by Discipulus (Canon) on May 28, 2016 at 13:16 UTC
as side note, using perl you can also trap the occurence of this behaviour, if it is important in your application. sysopen `MODE` and combination of them, can be used in order to track the overwrite: `O_WRONLY\|O_CREAT # possible rewrite of an existing file O_WRONLY\|O_CREAT\|O_EXCL # only create new file (fails if file exists +)` [download] With sysopen you can also specify `PERM` in octal to be applied to the new created file. If not specified `0666` is the default. Then `umask` will be applied. L* There are no rules, there are no thumbs.. Reinvent the wheel, then learn The Wheel; may be one day you reinvent one of THE WHEELS.	[reply] [d/l] [select]