APUE-第二章学习-UNIX标准化及实现-习题

2.1 在2.8节中，一些基本系统数据类型可以在多个头文件中定义，例如，在FreeBSD 5.2.1中，size_t在26个头文件中均有定义，由于一个程序可能包含这26个不同的头文件。并且ISO C不允许对同一个名字进行多次类型定义，因此如何编写这些头文件？

在UNIX系统时间中，定义了很多常数和常量，比如size_t，为了避免重复定义。

我们可以使用c语言中的条件编译

例如：

#ifdef size_t

#else
size_t 1
#endif

2.2 检查系统的头文件，列出实现基本系统数据类型所用到的实际数据类型。

在ubuntu10_04中书中的sys/types.h对应地址在/usr/include/sys/types.h

这里其实现方式为：

/* Copyright (C) 1991,1992,1994,1995,1996,1997,1998,1999,2000,2001,2002,2006
Free Software Foundation, Inc.
This file is part of the GNU C Library.

The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.

The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA.  */

/*
*  POSIX Standard: 2.6 Primitive System Data Types <sys/types.h>
*/

#ifndef _SYS_TYPES_H
#define _SYS_TYPES_H    1

#include <features.h>

__BEGIN_DECLS

#include <bits/types.h>

#ifdef  __USE_BSD
# ifndef __u_char_defined
typedef __u_char u_char;
typedef __u_short u_short;
typedef __u_int u_int;
typedef __u_long u_long;
typedef __quad_t quad_t;
typedef __u_quad_t u_quad_t;
typedef __fsid_t fsid_t;
#  define __u_char_defined
# endif
#endif

typedef __loff_t loff_t;

#ifndef __ino_t_defined
# ifndef __USE_FILE_OFFSET64
typedef __ino_t ino_t;
# else
typedef __ino64_t ino_t;
# endif
# define __ino_t_defined
#endif
#if defined __USE_LARGEFILE64 && !defined __ino64_t_defined
typedef __ino64_t ino64_t;
# define __ino64_t_defined
#endif

#ifndef __dev_t_defined
typedef __dev_t dev_t;
# define __dev_t_defined
#endif

#ifndef __gid_t_defined
typedef __gid_t gid_t;
# define __gid_t_defined
#endif

#ifndef __mode_t_defined
typedef __mode_t mode_t;
# define __mode_t_defined
#endif

#ifndef __nlink_t_defined
typedef __nlink_t nlink_t;
# define __nlink_t_defined
#endif

#ifndef __uid_t_defined
typedef __uid_t uid_t;
# define __uid_t_defined
#endif

#ifndef __off_t_defined
# ifndef __USE_FILE_OFFSET64
typedef __off_t off_t;
# else
typedef __off64_t off_t;
# endif
# define __off_t_defined
#endif
#if defined __USE_LARGEFILE64 && !defined __off64_t_defined
typedef __off64_t off64_t;
# define __off64_t_defined
#endif

#ifndef __pid_t_defined
typedef __pid_t pid_t;
# define __pid_t_defined
#endif

#if (defined __USE_SVID || defined __USE_XOPEN) && !defined __id_t_defined
typedef __id_t id_t;
# define __id_t_defined
#endif

#ifndef __ssize_t_defined
typedef __ssize_t ssize_t;
# define __ssize_t_defined
#endif

#ifdef  __USE_BSD
# ifndef __daddr_t_defined
typedef __daddr_t daddr_t;
typedef __caddr_t caddr_t;
#  define __daddr_t_defined
# endif
#endif

#if (defined __USE_SVID || defined __USE_XOPEN) && !defined __key_t_defined
typedef __key_t key_t;
# define __key_t_defined
#endif

#ifdef __USE_XOPEN
# define __need_clock_t
#endif
#define __need_time_t
#define __need_timer_t
#define __need_clockid_t
#include <time.h>

#ifdef __USE_XOPEN
# ifndef __useconds_t_defined
typedef __useconds_t useconds_t;
#  define __useconds_t_defined
# endif
# ifndef __suseconds_t_defined
typedef __suseconds_t suseconds_t;
#  define __suseconds_t_defined
# endif
#endif

#define __need_size_t
#include <stddef.h>

#ifdef __USE_MISC
/* Old compatibility names for C types.  */
typedef unsigned long int ulong;
typedef unsigned short int ushort;
typedef unsigned int uint;
#endif

/* These size-specific names are used by some of the inet code.  */

#if !__GNUC_PREREQ (2, 7)

/* These types are defined by the ISO C99 header <inttypes.h>. */
# ifndef __int8_t_defined
#  define __int8_t_defined
typedef char int8_t;
typedef short int int16_t;
typedef int int32_t;
#  if __WORDSIZE == 64
typedef long int int64_t;
#  elif __GLIBC_HAVE_LONG_LONG
__extension__ typedef long long int int64_t;
#  endif
# endif

/* But these were defined by ISO C without the first `_'.  */
typedef unsigned char u_int8_t;
typedef unsigned short int u_int16_t;
typedef unsigned int u_int32_t;
# if __WORDSIZE == 64
typedef unsigned long int u_int64_t;
# elif __GLIBC_HAVE_LONG_LONG
__extension__ typedef unsigned long long int u_int64_t;
# endif

typedef int register_t;

#else

/* For GCC 2.7 and later, we can use specific type-size attributes.  */
# define __intN_t(N, MODE) \
typedef int int##N##_t __attribute__ ((__mode__ (MODE)))
# define __u_intN_t(N, MODE) \
typedef unsigned int u_int##N##_t __attribute__ ((__mode__ (MODE)))

# ifndef __int8_t_defined
#  define __int8_t_defined
__intN_t (8, __QI__);
__intN_t (16, __HI__);
__intN_t (32, __SI__);
__intN_t (64, __DI__);
# endif

__u_intN_t (8, __QI__);
__u_intN_t (16, __HI__);
__u_intN_t (32, __SI__);
__u_intN_t (64, __DI__);

typedef int register_t __attribute__ ((__mode__ (__word__)));

/* Some code from BIND tests this macro to see if the types above are
defined.  */
#endif
#define __BIT_TYPES_DEFINED__   1

#ifdef  __USE_BSD
/* In BSD <sys/types.h> is expected to define BYTE_ORDER.  */
# include <endian.h>

/* It also defines `fd_set' and the FD_* macros for `select'.  */
# include <sys/select.h>

/* BSD defines these symbols, so we follow.  */
# include <sys/sysmacros.h>
#endif /* Use BSD.  */

#if defined __USE_UNIX98 && !defined __blksize_t_defined
typedef __blksize_t blksize_t;
# define __blksize_t_defined
#endif

/* Types from the Large File Support interface.  */
#ifndef __USE_FILE_OFFSET64
# ifndef __blkcnt_t_defined
typedef __blkcnt_t blkcnt_t;     /* Type to count number of disk blocks.  */
#  define __blkcnt_t_defined
# endif
# ifndef __fsblkcnt_t_defined
typedef __fsblkcnt_t fsblkcnt_t; /* Type to count file system blocks.  */
#  define __fsblkcnt_t_defined
# endif
# ifndef __fsfilcnt_t_defined
typedef __fsfilcnt_t fsfilcnt_t; /* Type to count file system inodes.  */
#  define __fsfilcnt_t_defined
# endif
#else
# ifndef __blkcnt_t_defined
typedef __blkcnt64_t blkcnt_t;     /* Type to count number of disk blocks.  */
#  define __blkcnt_t_defined
# endif
# ifndef __fsblkcnt_t_defined
typedef __fsblkcnt64_t fsblkcnt_t; /* Type to count file system blocks.  */
#  define __fsblkcnt_t_defined
# endif
# ifndef __fsfilcnt_t_defined
typedef __fsfilcnt64_t fsfilcnt_t; /* Type to count file system inodes.  */
#  define __fsfilcnt_t_defined
# endif
#endif

#ifdef __USE_LARGEFILE64
typedef __blkcnt64_t blkcnt64_t;     /* Type to count number of disk blocks. */
typedef __fsblkcnt64_t fsblkcnt64_t; /* Type to count file system blocks.  */
typedef __fsfilcnt64_t fsfilcnt64_t; /* Type to count file system inodes.  */
#endif

/* Now add the thread types.  */
#if defined __USE_POSIX199506 || defined __USE_UNIX98
# include <bits/pthreadtypes.h>
#endif

__END_DECLS

#endif /* sys/types.h */

2.3改进程序清单2-4程序，使其在sysconf为OPEN_MAX限制返回LONG_MAX 时，避免不必要的处理。

#include "apue.h"
#include <errno.h>
#include <limits.h>

#ifdef OPEN_MAX
static long openmax = OPEN_MAX;
#else
static long openmax = 0;
#endif

long open_max(void){
if(openmax == 0){
errno = 0;
if((openmax == sysconf(_SC_OPEN_MAX) ) < 0){
if(errno == 0)
openmax = LONG_MAX;   //it's determinate
else
err_sys("sysconf error for _SC_OPEN_MAX");
}
}
}