Apue Chapter 5 习题答案

Q5.1 Implement setbuf using setvbuf. 

Q5.2 Type in the program that copies a file using line-at-a-time I/O (fgets and fputs) from Figure 5.5, but use a MAXLINE of 4. What happens if you copy lines that exceed this length?

Explain what is happening.

正常状况下，在stdin 输入字符数小于4096时，我只显示一行 hello this is a test. 

但当我改成4自后，fgets每到4个字符就从新执行一次fgets.

正常状况下

$ ./fgets

hello

hello thi sis a test

zhangbo

hello thi sis a test

改过自后 MAXLINE1 = 4 能够看到输入两个字符时没有关系，fgets 运行一次。 

当我输入6个字符时它运行3次。 

$ ./fgets

ldsakfjlsdk

hello thi sis a test

hello thi sis a test

hello thi sis a test

hello thi sis a test

sfdfds

hello thi sis a test

hello thi sis a test

hello thi sis a test

sd

hello thi sis a test

sdf

hello thi sis a test

hello thi sis a test

sd

hello thi sis a test

源代码

$ cat fgets.c

#include "apue.h"

#define MAXLINE1 4

int

main (void)

{

  char buf[MAXLINE1];

  while ( fgets(buf, MAXLINE1, stdin) != NULL)    

    printf("hello thi sis a test\n");

    if( fputs(buf,  stdout) == EOF )

      err_sys("output error");

     if ( ferror(stdin))

        err_sys("input error");

    exit(0);

}

Q5.3 What does a return value of 0 from printf mean?

打印空的 printf(" ");

Q5.4 The following code works correctly on some machines, but not on others, What could be the problem?

#include <stdio.h>

int

main(void)

{

 char c;

 while (( c = getchar()) !== EOF)

          putchar(c);

}

Q5.5 How would you use the fsync function(Section 3.13) with a standard I/O stream?

对于标准I/O流如何使用fsync函数？

 The function fsync refers only to a single file, specified by the file descriptor fd, and waits for the disk writes to complete before returning, This function is used when an application, such as a database, needs to be sure that the modified blocks have been written to the disk. 

with fsync, the file's attributes are also updated synchronously. 

使用方法为：先调用fflush，而后在调用fsync,fsync所使用的参数有fileno函数得到。 

若是不调用fflush,全部的数据仍然在内存缓冲区中，此时调用fsync将没有任何效果。 

Q5.6 In the programs in Figures 1.7 and 1.10, the prompt that is printed does not contain a newline, and we don't call fflush. What causes the prompt to be output?

在交互式设备中，标准输入和标准输出都是行缓冲方式，每次调用fgets时标准输出自动冲洗。 

Q5.7 BSD-based systems provide a function called funopen that allows us to intercept read, write, seek, and close calls on a stream. Use this function to implement fmemopen for FreeBSD and Mac OS X.