Using O_Rdwr vs O_Rdonly | O_Wronly
In My Simple Program: #Include #Include #Include #Include Using Namespace Std; Int Main(Int Argc, Char *Argv[]) { Stringstream Ss; Ss
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.
Sarah Jenkins is a veteran tech journalist with over 12 years of experience covering artificial intelligence, mobile innovations, and digital ethics. Her insights have appeared in leading technology publications worldwide.
