How can I make the system call write() print to the screen?

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

A system call is a service provided by Linux kernel. In C programming, functions are defined in libc which provide a wrapper for many system calls. The function call write() is one of these system calls.

The first argument passed to write() is the file descriptor to write to. The symbolic constants STDERR_FILENO, STDIN_FILENO, and STDOUT_FILENO are respectively defined to 2, 0, and 1 in unidtd.h. You want to write to either STDOUT_FILENO or STDERR_FILENO.

const char msg[] = "Hello World!";
write(STDOUT_FILENO, msg, sizeof(msg)-1);

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You can alternatively use the syscall() function to perform an indirrect system call by specifying the function number defined in syscall.h or unistd.h. Using this method, you can guarantee that you are only using system calls. You may find The Linux System Call Quick Refernence (PDF Link) to be helpful.

/* 4 is the system call number for write() */
const char msg[] = "Hello World!";
syscall(4, STDOUT_FILENO, msg, sizeof(msg)-1);

Answer 2

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;
}