Go的os.OpenFile标志参数中管道字符的目的是什么？

Question

When using the OpenFile function in Go's os package, what exactly is the purpose of the pipe character?

Example:

os.OpenFile("foo.txt", os.O_RDWR|os.O_APPEND, 0660)

Does it serve as a logical OR? If so, does Go choose the first one that is "truthy"?? Being that the constants those flags represent, at the heart of them are just integers written in hexadecimal, when compiled how does Go choose which flag to apply?

After all, if the function call were to go by the largest number, os.O_APPEND would take precedence over all other flags passed in as seen below:

os.O_RDWR == syscall.O_RDWR == 0x2 == 2
os.O_APPEND == syscall.O_APPEND == 0x400 == 1024
os.O_CREATE == syscall.O_CREAT == 0x40 == 64

UPDATE 1

To follow up on the comment below, if I have a bitwise operator calculation using os.O_APPEND|os.O_CREATE will that error if the file exists, or simply create/append as needed?

UPDATE 2

My question is two fold, one to understand the purpose of the bitwise operator, which I understand now is being used more as a bitmask operation; and two, how to use the os.OpenFile() function as a create or append operation. In my playing around I have found the following combination to work best:

file, _ := os.OpenFile("foo.txt", os.O_RDWR|os.O_CREATE|os.O_APPEND, 0660)
file.WriteString("Hello World
")
file.Sync()

Is this the correct way or is there a more succinct way to do this?

Answer 1

It is a bitwise, not a logical OR.

If you write out the numbers in binary, and assign each a truth value 0/1, and apply the logical OR to each of the bits in place i between the arguments, and then reassemble the result into an integer by binary expansion - that's the | operator.

It is often used in a way that is commonly described as a "bitmask" - you use a bitmask when you want a single int value to represent a (small) set of switches that could be turned on or off. One bit per switch.

You should see in this context, A | B means "all the switches in A that are on, as well as all the switches in B that are on". In your case, the switches define the exact behavior of the file open/creation function, as described by the Go manual. (And probably more in detail by the Unix manpage I linked above).

In a bitmask, constants are typically defined that represent each switch - that's how those O_* constants are determined. Each is an int with exactly one bit set and represents a particular switch. (though, be careful, because sometimes they represent combinations of switches!).

Also:

 ^A  // All of the "switches" not currently on in A
 A&^B // All of the "switches" on in A but not on in B
 A^B  // All of the "switches" on in exactly one of A or B

, etc.

The operator | itself is described in the Go manual here.