in this unix book that i have, it says:
the statement:

filedes = open(pathname, O_CREAT, mode);

is actually

filedes = open(pathname, O_CREAT, (~mask)&mode); /* ~ is the negation symbol */

like it's doing some type of masking.
for example,

fd = open("newfile", O_CREAT, 0644);

will actually given the mode of 0640.

how did it come up with that? what value does it mask to my mode set to come up with 0640?
why is it doing that?
thanks

It would be more accurate to say something like the thing that the first thing that the open() system call does is:
mode=(~mask)&mode;

The "mask" is called umask. It is set with the umask() system call. And there is a umask command for users. Umask will also affect creat(), mkdir(), and mknod().

If you use bind() on a unix_domain socket you will create a socket in the filesystem. bind() is immune to umask which is actually something I find to be a little odd.

As for why, well I don't really know. However, here is my guess... I think that after people wrote a bunch of programs that did:
fd = open("newfile", O_CREAT, 0644);
or even
fd = open("newfile", O_CREAT, 0666);
they one day woke up and realized that was not great for security. So rather than rewrite all the programs, they just invented umask() to give users some control over what the mode is.