Why are the open system call mode values different for equivalent code?

Question

Below code and the quoted statement are from Linux System Programming by Robert Love. I don't understand how are the modes in the first and the second code examples the same.

From the first example, I calculated the mode values as

USR -> 2+1 = 3
GRP -> 2+1 = 3
OTH -> 1 =   1

So my expectation was 0331, but it the book says it's 0664.

What's wrong in my logic?

Copied from the book below:

int fd;
fd = open (file, O_WRONLY | O_CREAT | O_TRUNC, S_IWUSR | S_IRUSR | S_IWGRP | S_IRGRP | S_IROTH);
if(fd == -1)
 /* error */

Trading portability (in theory at least) for readability, we could have written the following, to identical effect:

int fd;
fd = open(file, O_WRONLY | O_CREAT | O_TRUNC, 0664);
if(fd == -1)
 /*error*/

Answer 1

For whichever reason, you have chosen to use the wrong values for the constants.

• Instead of S_IRUSR being 0400, you decided it was 0200.
• Instead of S_IWUSR being 0200, you decided it was 0100.
• etc

If you use the correct values for the constants, you will get the same result.