Setting file permissions when opening a file with ofstream

Question

Is there a way in C++'s standard libraries (or linux sys/stat.h, sys/types.h, sys/.... libraries) to set the file permissions of a file when creating it with ofstream (or using something else from those libraries)?

When I create a file it just gets created with some default set of file permissions (I assume whatever the current umask is), but I want to explicitly set the permissions to something other than the default (ex. 600), and I can't just set the umask before starting the program (b/c others will run it).

// Example of creating a file by writing to it
ofstream fp(filename.c_str())

/* write something to it */

Is there a way to do this in C++ or if not, a way to set the umask within the C++ program?

For example, in C's standard library you can just do:

open(filename, O_RDWR|O_CREAT, 0666)

but I don't want to resort to using the C function, as it'd be nice to be able to use the functions associated with fstream objects.

(Side Note: there was a question whose title was exactly what I was looking for, but it turned out to be unrelated.)

No, C++ is not aware of file permissions. (Create the file first, using system function(s) and than open the C++stream) — , Nov 05 '15 at 18:42
Do you mean something like opening the file using C's `open` function, close that file descriptor, then open it again using `ofstream`? — xgord, Nov 05 '15 at 18:57
There is the experimental [Filesystem library TS](http://en.cppreference.com/w/cpp/experimental/fs) — NathanOliver, Nov 05 '15 at 18:57
@NathanOliver, who would've guessed? In 2015 we finally realized we DO need a way to create or read directory in C++ :) — SergeyA, Nov 05 '15 at 18:58

score 8 · Accepted Answer · answered Nov 05 '15 at 18:46

8

You cannot. The standard library must be platform agnostic, and permissions like 0600 are meaningless on Windows for example. If you want to use platform-specific features, you'll need to use platform-specific APIs. Of course, you can always call umask() before you open the file, but that's not part of the C++ standard library, it's part of the platform API.

Note: open() isn't part of the C standard library either. It's a platform API. The C standard library function to open a file is fopen().

answered Nov 05 '15 at 18:46

Miles Budnek

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This is true right now but there is a TS out there for filesystems that will allow us do this portably. [Filesystem library TS](http://en.cppreference.com/w/cpp/experimental/fs) – NathanOliver Nov 05 '15 at 19:02
That TS is great for a lot of things, but permissions are not among the things it does well (on non-linux systems at least). – Miles Budnek Nov 05 '15 at 19:14
I haven't played with it yet so I'll take your word on it. – NathanOliver Nov 05 '15 at 19:17

Tom · Answer 2 · 2018-07-16T11:35:09.727

In C++17, std::filesystem::permissions was introduced. It will enable you to get and set permissions in a platform-agnostic manner.

Get permissions:

using fs = std::filesystem;
bool owner_can_read =
    fs::status("my_file.txt").permissions() & fs::perms::owner_read != fs::perms::none;

Set permissions (add all permissions for owner and group, that is, add modes 0x770 on unix):

using fs = std::filesystem;
fs::permissions("my_file.txt",
                fs::perms::owner_all | fs::perms::group_all,
                fs::perm_options::add);

Example based on an example from cppreference.

Setting file permissions when opening a file with ofstream

2 Answers2