CODEWITHSUNDEEP
cfilefile-permissions
Ranveer
Ranveer

Reputation: 6863

File creation permissions in C

I'm creating a file in C using the following code:

int outfd = open(arg,O_CREAT|O_TRUNC|O_WRONLY, f_per);

f_per being the file permission numbers.

Setting f_per to 0644, executing the code and doing an ls -l gives me the (output) file permissions set as -rw-r--r-- which is expected. However, setting things to 0777 gives permissions as -rwxrwxr-x instead of -rwxrwxrwx. Any idea why this happens?

Upvotes: 2

Views: 279

Answers (1)

paxdiablo
paxdiablo

Reputation: 881333

As per the POSIX page for the open call, under O_CREAT:

... the access permission bits of the file mode shall be set to the value of the argument following the oflag argument taken as type mode_t modified as follows: a bitwise AND is performed on the file-mode bits and the corresponding bits in the complement of the process' file mode creation mask.

The mode creation mask (or umask) can be considered a subtractive one. For example, if you attempt to create a file with permissions rwxrwxrwx/0777 when your file mode creation mask is -------w-/0002, you'll actually end up with:

  rwxrwxrwx
& rwxrwxr-x (complement of -------w-)
  =========
  rwxrwxr-x

This appears to be the situation you are encountering.

If you want to actually create a file of the specific permissions, you can temporarily disable the umask by setting it to zero (and restoring it afterwards), something like:

mode_t oldmask = umask(0); // needs sys/stat.h
int outfd = open(arg, O_CREAT|O_TRUNC|O_WRONLY, 0777);
umask(oldmask);

Upvotes: 1

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