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

Write to the file descriptor for standard output or standard error (1 and 2 respectively).

Answer 2

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 3

#include <unistd.h>
/* ... */
const char msg[] = "Hello world";
write( STDOUT_FILENO, msg, sizeof( msg ) - 1 );

First argument is the file descriptor for STDOUT (usually 1), the second is the buffer to write from, third is the size of the text in the buffer (-1 is to not print zero terminator).

Answer 4

#define _GNU_SOURCE         /* See feature_test_macros(7) */    
#include <unistd.h> // For open, close, read, write, fsync
#include <sys/syscall.h>  //For SYSCALL id __NR_xxx

//Method 1 : API    
write(1,"Writing via API\n",\
        strlen("Writing via API\n") ); 
fsync(1);
//Method 2  : Via syscall id
const char msg[] = "Hello World! via Syscall\n";
syscall(__NR_write, STDOUT_FILENO, msg, sizeof(msg)-1);     
syscall(__NR_fsync, STDOUT_FILENO );    // fsync(STDOUT_FILENO);

Answer 5

Your reference is incorrect. It's part of C++ and has nothing to do with your assignment. The correct reference is http://www.opengroup.org/onlinepubs/9699919799/functions/write.html