What does mode_t 0644 mean?

Question

#define COPYMODE 0644
creat(argV[2],COPYMODE);

I have these two lines of code in a copy.c file. I don't know what it means. Please give some example about it

Answer 1

There are 3x3 bit flags for a mode:

• (owning) User
  • read
  • write
  • execute
• Group
  • read
  • write
  • execute
• Other
  • read
  • write
  • execute

So each triple encodes nicely as an octal digit.

rwx oct    meaning
--- ---    -------
001 01   = execute
010 02   = write
011 03   = write & execute
100 04   = read
101 05   = read & execute
110 06   = read & write
111 07   = read & write & execute

So 0644 is:

* (owning) User: read & write
* Group: read
* Other: read

Note that in C, an initial 0 indicates octal notation, just like 0x indicates hexadecimal notation. So every time you write plain zero in C, it's actually an octal zero (fun fact).

This might also be written:

-rw-r--r--

Whereas full permissions, 0777 can also be written:

-rwxrwxrwx

So the octal number passed to creat corresponds directly (via octal encoding of the bit-pattern) to the file permissions as displayed by ls -l.