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

c linux system-calls
andandandand picture andandandand · Oct 5, 2010 · Viewed 50k times · Source

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

jschmier picture jschmier · Oct 5, 2010

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);

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);

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