Open Flags in UNIX

Open flags are specified as macros provided to pass to the system call open(), to define what permissions and types we want to open a file with. These come in the form of binary numbers, and are usually defined as constants.

#define O_RDONLY        0b00000000 
#define O_CREAT         0b00000100 
#define O_TRUNC         0b00001000

We can define a combination of these flags using the bitwise OR operator.

O_RDONLY | O_CREAT | O_TRUC

The binary representation of these flags 0b00001100. How does open() use this to intepret what flags we have set? It uses the bitwise AND operator.

Bitmasks

A bitmask is data used in bitwise operations. The & bitwise operator will set the bit only if both bits are 'on' (i.e 1 and 1).

Using our example above if we & O_CREATE with our flags, we obtain

00000100 &
00001100
---
00000100

The idea is, if we & any of the flags with the oflag above and the result is non-zero then we know that flag is set.

This is how we open() checks for which flags have been set

if (flags & O_CREAT) {
	// do something
}

In practice, we can use binary numbers like this to set states and check for states in any system, similar to boolean flags.