[C - open()] How to calculate the bit flags/enum flags for oflag in open()?

Question

I'm working on a networking assignment and we are tasked with creating a remote file access server with a protocol we were given. My difficulties come in the lack of information I can find that explains the process of calculating the bits for the oflag argument in open().

I receive a message from a client to open a file and in the message I parse characters for the flags to use in oflag. Specifically they are:

• R - O_RDONLY
• W - O_WRONLY
• RW - O_RDWR
• A - O_APPEND
• C - O_CREAT
• T - O_TRUNC
• E - O_EXCL

I went around Google and searched bitwise operations, enumeration flags, bit flags, calculating bit flags, etc. and couldn't find something that was useful in figuring out how to create the bits for oflag. Maybe I just didn't know what I was looking for and overlooked useful information?

Could someone please:

• Point me in the direction/provide links to documentation/example of how to calculate the bits/# I ought to put into oflags given my parsed characters or
• Show me the enumeration types for the flags and the ordering they ought to go in

Thanks a bunch for you help and if I wasn't clear on my problem or what I am trying to do, just let me know and I will clarify ASAP.

Answer 1

The usual expression would be something like O_RDWR | O_CREAT. And note that O_RDWR is exactly O_RDONLY | O_WRONLY

You can do something like this:

char *flags = "r";
int oflag = 0;

if (strchr(flags,'r')) oflag |= O_RDONLY;

and so on for the rest of the flags.

Answer 2

The O_... flags are numbers each with a different single bit set. For example on my system they are defined in fcntl.h as

#define O_RDONLY             00
#define O_WRONLY             01
#define O_RDWR               02
#define O_CREAT            0100 /* not fcntl */
#define O_EXCL             0200 /* not fcntl */
#define O_NOCTTY           0400 /* not fcntl */
#define O_TRUNC           01000 /* not fcntl */
#define O_APPEND          02000

You use | (logical OR) to combine the flags and pass in a single number to open with all the bits set for each option you want. So e.g. open("file", O_RDWR | O_CREAT).

You can compute an int and pass that into open too if you want.

int flags = 0;
if (...)
    flags |= O_RDWR;
...
open('file', flags);