default_mode

This is the default mode given to POSIX operating systems for creating files. 0o666 is "-rw-rw-rw-" which is counter-intuitive at first, since most people would expect "-rw-r--r--", for example, when using the touch command, which would correspond to 0o644. However, POSIX libc implementations use 0o666 inside fopen and then rely on the process-scoped "umask" setting to adjust this number for file creation.

Source

Implementation

pub const default_mode = switch (builtin.os.tag) {
    .windows => 0,
    .wasi => 0,
    else => 0o666,
}