APUE_1.5InputAndOutput Figure1.4CopyStandardInputToStandardOutput

// 1.4CopyStandardInputToStandardOutput.cpp
#include "../apuesunyj.h"

#define BUFFSIZE 4096

int main()
{
	int n;
	char buf[BUFFSIZE];

    // By convention, all shells open three descriptors whenever a new program is run:
    // standard input, standard output, and standard error.
    // STDIN_FILENO specify the file descriptor for standard input
    // Unbuffered I/O is provided by the functions open, read, write, lseek, and close.
    // These functions all work with file descriptors.
    // The read function returns the number of bytes that are read, and this value is used
    // as the number of bytes to write. When the end of the input file is encountered, read
    // returns 0 and the program stops. If a read error occurs, read returns -1.
    while ((n = read(STDIN_FILENO, buf, BUFFSIZE)) > 0)
    {
        if (write(STDOUT_FILENO, buf, n) != n)
        {
            err_sys("write error");
        }
    }

    // usually n is bigger than 0, if input ctrl + d
    // then n is equal to 0
    if (n < 0)
    {
        err_sys("read error");
    }

    return 0;
}
// The standard I/O functions provide a buffered interface to the unbuffered I/O
// functions. Using standard I/O relieves us from having to choose optimal buffer sizes,
// such as the BUFFSIZE constant in Figure 1.4. The standard I/O functions also simplify
// dealing with lines of input (a common occurrence in UNIX applications). The fgets
// function, for example, reads an entire line. The read function, in contrast, reads a
// specified number of bytes. As we shall see in Section 5.4, the standard I/O library
// provides functions that let us control the style of buffering used by the library.
// The most common standard I/O function is printf. In programs that call
// printf, we'll always include <stdio.h> normally by including apue.h as this
// header contains the function prototypes for all the standard I/O functions.

// apuesunyj.h
#ifndef APUE_SUNYJ
#define APUE_SUNYJ

#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <stdarg.h>
#include <stdlib.h>
#include <stdint.h>
#include <unistd.h>

void err_quit(const char *fmt, ...);
void err_sys(const char *fmt, ...);


#endif

// error.cpp
#include "apuesunyj.h"

int64_t const MAXLINE = 4096; // max line length
/*
 *  * Print a message and return to caller.
 *   * Caller specifies "errnoflag".
 *    */
static void err_doit(int errnoflag, int error, const char *fmt, va_list ap)
{
    char buf[MAXLINE];

    vsnprintf(buf, MAXLINE, fmt, ap);
    if (errnoflag)
        snprintf(buf + strlen(buf), MAXLINE - strlen(buf), ": %s", strerror(
                    error));
    strcat(buf, "
");
    fflush(stdout);/* in case stdout and stderr are the same */
    fputs(buf, stderr);
    fflush(NULL);/* flushes all stdio output streams */
}

/*
 *  * Fatal error unrelated to a system call.
 *   * Print a message and terminate.
 *    */
void err_quit(const char *fmt, ...)
{
    va_list ap;

    va_start(ap, fmt);
    err_doit(0, 0, fmt, ap);
    va_end(ap);
    exit(1) ; // process terminate, not just like return, totally different
}

/*
 *  * Fatal error related to a system call.
 *   * Print a message and terminate.
 *    */
void err_sys(const char *fmt, ...)
{
    va_list ap;

    va_start(ap, fmt);
    err_doit(1, errno, fmt, ap);
    va_end(ap);
    exit(1) ;
}