Using O_RDWR vs O_RDONLY | O_WRONLY

In my simple program:

#include <iostream> #include <unistd.h> #include <fcntl.h> #include <sstream> using namespace std; int main(int argc, char *argv[]) { stringstream ss; ss << "What does the quick brown fox say?" << endl; int file_descriptor = open("/dev/tty", O_RDONLY | O_WRONLY); write(file_descriptor, ss.str().c_str(), ss.str().size()); } 

I open the terminal stream using the combination O_RDONLY | O_WRONLY, and this seems to work fine. I get that you should use O_RDWR because it makes clearer semantic sense, but my question is why bother creating a whole other flag if joining two existing flags already works? Is there some historical reason for this, or am I just overlooking something, and this really doesn't actually work?

2 Answers

O_RDONLY | O_WRONLY (at least on my Linux machine) is not the same thing as O_RDWR.

#define O_RDONLY 00 #define O_WRONLY 01 #define O_RDWR 02 

The fact that it works seems like a bug/feature/coincidence rather than "it works because it should work that way".

From Linux manual page for open(2):

Unlike the other values that can be specified in flags, the access mode values O_RDONLY, O_WRONLY, and O_RDWR do not specify individual bits. Rather, they define the low order two bits of flags, and are defined respectively as 0, 1, and 2. In other words, the combination O_RDONLY | O_WRONLY is a logical error, and certainly does not have the same meaning as O_RDWR.