APUE reading notes-2

第二章

1. 2.1 We mentioned in Section 2.8 that some of the primitive system data types are defined in more than one header. For example, in FreeBSD 8.0, size_t is defined in 29 different headers. Because all 29 headers could be included in a program and because ISO C does not allow multiple typedefs for the same name, how must the headers be written?
   个人认为，遇到这种情况时，假设多重定义的类型为#size_t，那么可以采用include guard的策略，即在头文件中，声明size_t时，进行一次#ifndef，#endif的保护，从而可以避免size_t的多重定义。
2. 2.2 Examine your system’s headers and list the actual data types used to implement the primitive system data types.
   我的系统是macOS 10.12.6，也算是认证的Unix系统了，于是在对<sys/types.h>进行暗中观察😏之后，得出结论如下：

   copyc
   typedef	unsigned char		u_char;
   typedef	unsigned short		u_short;
   typedef	unsigned int		u_int;
   #ifndef _U_LONG
   typedef	unsigned long		u_long;
   #define _U_LONG
   #endif
   typedef	unsigned short		ushort;		/* Sys V compatibility */
   typedef	unsigned int		uint;		/* Sys V compatibility */
   #endif
   
   typedef	u_int64_t		u_quad_t;	/* quads */
   typedef	int64_t			quad_t;
   typedef	quad_t *		qaddr_t;
   
   typedef	char *			caddr_t;	/* core address */
   typedef	int32_t			daddr_t;	/* disk address */
   
   typedef	u_int32_t		fixpt_t;	/* fixed point number */
   
   typedef	int32_t			segsz_t;	/* segment size */
   typedef	int32_t			swblk_t;	/* swap offset */
   typedef __int32_t		fd_mask;

   这里仅仅列出了一些基本数据类型的实现，在sys/_types文件夹下的头文件中还有许多的其他的常见类型，如size_t、time_t等的实现，再此由于篇幅有限就不予详述了，总体而言，Unix下的不少以_t结尾的类型，都是随各个系统的实现方式而可能发生变化的，具体的实现只要观察<sys/types.h>即可得知。这个信息可以帮助我们解决一些潜在的bug。
   简单的介绍一些常见的unix定义的变量类型：

3. 2.3 Update the program in Figure 2.17 to avoid the needless processing that occurs when sysconf returns LONG_MAX as the limit for OPEN_MAX.
   Fig 2.17内容如下:

   copyC
   #include "apue.h"
   #include <errno.h>
   #include <limits.h>
   #ifdef  OPEN_MAX
   static long openmax = OPEN_MAX;
   #else
   static long openmax = 0;
   #endif
   /*
    * If OPEN_MAX is indeterminate, this might be inadequate.
    */
   #define OPEN_MAX_GUESS  256
   long
   open_max(void)
   {
       if (openmax == 0) {
           errno = 0;
           /* first time through */
           if ((openmax = sysconf(_SC_OPEN_MAX)) < 0) {
               if (errno == 0)
                   openmax = OPEN_MAX_GUESS;   /* it’s indeterminate */
               else
                   fprintf(stderr, "sysconf error for _SC_OPEN_MAX");
           }
       }
       return(openmax);
   }

   由于实在没读懂题意，我去Google了一下，获得了如下的答案：

   copyc
   #include "apue.h"
   #include <limits.h>
   #include <sys/resource.h>
   #define OPEN_MAX_GUESS 256
   long
   open_max(void)
   {
   	long openmax;
   	struct rlimit rl;
   	if ((openmax = sysconf(_SC_OPEN_MAX)) < 0 ||
   		 openmax == LONG_MAX) {
   		if (getrlimit(RLIMIT_NOFILE, &rl) < 0)
   			err_sys("can't get file limit");
   		if (rl.rlim_max == RLIM_INFINITY)
   			openmax = OPEN_MAX_GUESS;
   		else
   			openmax = rl.rlim_max;
   	}
   	return(openmax);
   }

   简单的来说，如果sysconf函数返回了LONG_MAX，那么这也表明OPEN_MAX是不确定的，同时，我们可以使用geTRlimit函数来获得每个进程最大的同时打开文件数量，从而使OPEN_MAX成为一个合理的上限值。