CODEWITHSUNDEEP
cfilefile-permissions
Marine Biologist Kujo
Marine Biologist Kujo

Reputation: 143

c Open() function

I have some trouble understanding the arguments in the open function, specifically used in the context of creating an output file. I do not quite understand the roles of flags and file permissions (the 2nd and 3rd arguments in the function). For instance, if I have the file permission 00200 (user has write permission), and the flag O_RDONLY (Read only), then can I read the file or write the file?

Upvotes: 1

Views: 1295

Answers (1)

nanofarad
nanofarad

Reputation: 41271

The signature of open is as follows:

 int open(const char *pathname, int flags, mode_t mode);

There are three sets of "permissions" at play: The permissions of the file itself, the flags, and the mode.

The permissions of the file itself (e.g. 00200 meaning only user can write) specify what the operating system allows a program to do.

When you specify the flags, you indicate what you want to do with the file. For example, if the file is readonly to you (e.g. rwxr-xr-x and you're not the owner), you will be allowed to open the file with O_RDONLY. If you attempt to open the file with O_RDWR or O_WRONLY, you will receive an EPERM (operation not permitted) error in errno.

The mode parameter is only relevant when you create a new file, such as when you open a file that doesn't exist1 and the flag O_CREAT is specified. The file is created on the filesystem and its permissions are given by mode & ~umask (see man 2 umask for more details).

1 Of course, the containing directory must exist and you must have write+exec permissions on that directory.

Upvotes: 3

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