Question

For my OS class I’m supposed to implement Linux’s cat using only system calls (no printf)

Reading this reference I found it being used to print to a file. I guess I should manipulate ofstream.

In the example appears: ofstream outfile ("new.txt",ofstream::binary);

How can I make it write to the screen?

EDIT: I realized this write() is part of iostream library, is this the same as the int write (int fd, char *buf , int size) system call?

Answer 1

No, std::ostream::write is not the same as the write system call. It does (almost certainly) use the write system call, at least on a system like Linux that has such a thing, and it normally does pretty similar things, but it’s still a separate thing of its own.

Linux will, however, pre-open standard input, standard output and standard error streams for your process. To write to the screen, you’d normally use write (i.e., the one that is a system call) to write to stream number 1 or stream number 2 (which are standard output and standard error respectively).

If you need to write to the screen even if those are re-directed, you’d normally open a stream to /dev/tty and (again) use write to write to it:

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main() { 
    char msg[] = "hello\n";

    int fd = open("/dev/tty", O_WRONLY);
    write(fd, msg, sizeof(msg));
    return 0;
}