How to make sense of O_RDONLY = 0?

Question

I am dealing with file status flags. Among test I performed, I found

#include <stdio.h>
#include "fcntl.h"

int main() {
    const int flag = O_RDONLY;
    printf( "*** Flag O_RDONLY = %5d\n", flag);
    return 0;
}

produces this output

*** Flag O_RDONLY =     0

which is fully consistent with

#define O_RDONLY         00

from fcntl-linux.h.

How can the value zero be used as a flag?

I expect an "atomic" flag to be 2^n (n>=1), and "composite" flags (like O_ACCMODE) to be simply the sum of several atomic flags (which is the same as bitwise-or'ing those atomic flags).
As far as I understand, I cannot "detect" anything, and such flag cannot be ever set. A bitwise-and'ed expression like (stat & O_RDONLY) will always be false.

The file status flags are not a bit mask. They're just 3 alternative options: O_RDONLY = 0, O_WRONLY = 1, O_RDWR = 2 — Barmar, May 20 '20 at 22:11
values for o_flags are constructed using bitwise-inclusive OR — Andreas DM, May 20 '20 at 22:13
True: *`_O_RDONLY` Opens a file for reading only. Cannot be specified with `_O_RDWR` or `_O_WRONLY`*. — Weather Vane, May 20 '20 at 22:15
@Barmar: to my pain when I need a handle to something I can't read or write (for `fstat()` or `fchdir()`). — Joshua, May 20 '20 at 22:22

score 7 · Accepted Answer · edited May 21 '20 at 10:58

7

Although these are called flags in the documentation, these three are not actually atomic flags that can be combined like the rest. They're mutually exclusive alternative values for the O_ACCMODE bits. You don't use stat & RDONLY to test for it, you use (stat & O_ACCMODE) == O_RDONLY.

edited May 21 '20 at 10:58

sancho.s ReinstateMonicaCellio

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answered May 20 '20 at 22:20

Barmar

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Great! Actually, the man page http://man7.org/linux/man-pages/man2/open.2.html first describes the file access modes, and then adds "In addition, zero or more file creation flags and file status flags can be bitwise-or'd in flags...." But it does not mention that file access mode can be bitwise operated on. For me, the word "flag" is an absolute misnomer, and misleading. – sancho.s ReinstateMonicaCellio May 20 '20 at 22:39
Related, can you please check https://stackoverflow.com/questions/61909358/how-to-get-the-mode-of-a-file-descriptor/61909773 and give your insight? – Tony Tannous May 21 '20 at 12:01

How to make sense of O_RDONLY = 0?

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