APUE_1.5InputAndOutput Figure1.4CopyStandardInputToStandardOutput

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// 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.

 1 // apuesunyj.h
 2 #ifndef APUE_SUNYJ
 3 #define APUE_SUNYJ
 4 
 5 #include <errno.h>
 6 #include <stdio.h>
 7 #include <string.h>
 8 #include <stdarg.h>
 9 #include <stdlib.h>
10 #include <stdint.h>
11 #include <unistd.h>
12 
13 void err_quit(const char *fmt, ...);
14 void err_sys(const char *fmt, ...);
15 
16 
17 #endif

 1 // error.cpp
 2 #include "apuesunyj.h"
 3 
 4 int64_t const MAXLINE = 4096; // max line length
 5 /*
 6  *  * Print a message and return to caller.
 7  *   * Caller specifies "errnoflag".
 8  *    */
 9 static void err_doit(int errnoflag, int error, const char *fmt, va_list ap)
10 {
11     char buf[MAXLINE];
12 
13     vsnprintf(buf, MAXLINE, fmt, ap);
14     if (errnoflag)
15         snprintf(buf + strlen(buf), MAXLINE - strlen(buf), ": %s", strerror(
16                     error));
17     strcat(buf, "\n");
18     fflush(stdout);/* in case stdout and stderr are the same */
19     fputs(buf, stderr);
20     fflush(NULL);/* flushes all stdio output streams */
21 }
22 
23 /*
24  *  * Fatal error unrelated to a system call.
25  *   * Print a message and terminate.
26  *    */
27 void err_quit(const char *fmt, ...)
28 {
29     va_list ap;
30 
31     va_start(ap, fmt);
32     err_doit(0, 0, fmt, ap);
33     va_end(ap);
34     exit(1) ; // process terminate, not just like return, totally different
35 }
36 
37 /*
38  *  * Fatal error related to a system call.
39  *   * Print a message and terminate.
40  *    */
41 void err_sys(const char *fmt, ...)
42 {
43     va_list ap;
44 
45     va_start(ap, fmt);
46     err_doit(1, errno, fmt, ap);
47     va_end(ap);
48     exit(1) ;
49 }