1，一些基本系统数据类型可以在多个头文件中定义。例如，在FreeBSD8.0中，size_t在29个不同的头文件中都有定义，由于一个程序可能包含这29个不同的头文件，但是ISO C却不允许对同一个名字进行多次typedef，那么如何编写这些头文件呢？
答：采取下面的方式
1 #ifndef __XXX_t_defined 2 typedef __XXX_t XXX; 3 #dedine __XXX_t_defined 4 #endif
这样如果有头文件已经定义了这个名字，那就不会冲突了。
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。
答：ubuntu18系统。如下图，还有查看/usr/include/sys/types.h和/usr/include/bits/types.h文件可知
dog@dog:/usr/include/sys$ cat ../bits/typesizes.h #ifndef _BITS_TYPES_H # error "Never include <bits/typesizes.h> directly; use <sys/types.h> instead." #endif #ifndef _BITS_TYPESIZES_H #define _BITS_TYPESIZES_H 1 /* See <bits/types.h> for the meaning of these macros. This file exists so that <bits/types.h> need not vary across different GNU platforms. */ /* X32 kernel interface is 64-bit. */ #if defined __x86_64__ && defined __ILP32__ # define __SYSCALL_SLONG_TYPE __SQUAD_TYPE # define __SYSCALL_ULONG_TYPE __UQUAD_TYPE #else # define __SYSCALL_SLONG_TYPE __SLONGWORD_TYPE # define __SYSCALL_ULONG_TYPE __ULONGWORD_TYPE #endif #define __DEV_T_TYPE __UQUAD_TYPE #define __UID_T_TYPE __U32_TYPE #define __GID_T_TYPE __U32_TYPE #define __INO_T_TYPE __SYSCALL_ULONG_TYPE #define __INO64_T_TYPE __UQUAD_TYPE #define __MODE_T_TYPE __U32_TYPE #ifdef __x86_64__ # define __NLINK_T_TYPE __SYSCALL_ULONG_TYPE # define __FSWORD_T_TYPE __SYSCALL_SLONG_TYPE #else # define __NLINK_T_TYPE __UWORD_TYPE # define __FSWORD_T_TYPE __SWORD_TYPE #endif #define __OFF_T_TYPE __SYSCALL_SLONG_TYPE #define __OFF64_T_TYPE __SQUAD_TYPE #define __PID_T_TYPE __S32_TYPE #define __RLIM_T_TYPE __SYSCALL_ULONG_TYPE #define __RLIM64_T_TYPE __UQUAD_TYPE #define __BLKCNT_T_TYPE __SYSCALL_SLONG_TYPE #define __BLKCNT64_T_TYPE __SQUAD_TYPE #define __FSBLKCNT_T_TYPE __SYSCALL_ULONG_TYPE #define __FSBLKCNT64_T_TYPE __UQUAD_TYPE #define __FSFILCNT_T_TYPE __SYSCALL_ULONG_TYPE #define __FSFILCNT64_T_TYPE __UQUAD_TYPE #define __ID_T_TYPE __U32_TYPE #define __CLOCK_T_TYPE __SYSCALL_SLONG_TYPE #define __TIME_T_TYPE __SYSCALL_SLONG_TYPE #define __USECONDS_T_TYPE __U32_TYPE #define __SUSECONDS_T_TYPE __SYSCALL_SLONG_TYPE #define __DADDR_T_TYPE __S32_TYPE #define __KEY_T_TYPE __S32_TYPE #define __CLOCKID_T_TYPE __S32_TYPE #define __TIMER_T_TYPE void * #define __BLKSIZE_T_TYPE __SYSCALL_SLONG_TYPE #define __FSID_T_TYPE struct { int __val[2]; } #define __SSIZE_T_TYPE __SWORD_TYPE #define __CPU_MASK_TYPE __SYSCALL_ULONG_TYPE #ifdef __x86_64__ /* Tell the libc code that off_t and off64_t are actually the same type for all ABI purposes, even if possibly expressed as different base types for C type-checking purposes. */ # define __OFF_T_MATCHES_OFF64_T 1 /* Same for ino_t and ino64_t. */ # define __INO_T_MATCHES_INO64_T 1 /* And for __rlim_t and __rlim64_t. */ # define __RLIM_T_MATCHES_RLIM64_T 1 #else # define __RLIM_T_MATCHES_RLIM64_T 0 #endif /* Number of descriptors that can fit in an `fd_set'. */ #define __FD_SETSIZE 1024 #endif /* bits/typesizes.h */
可知pid_t的实际数据类型是有符号32位整数，同理可知其他。
3，改写一下程序，使其在sysyconf为OPEN_MAX限制返回LONG_MAX时，避免进行不必要的处理。
#include "apue.h" #include <errno.h> #include <limits.h> #ifdef OPEN_MAX static long openmax = OPEN_MAX; #else static long openmax = 0; #endif /* * If OPEN_MAX is indeterminate, this might be inadequate. */ #define OPEN_MAX_GUESS 256 long open_max(void) { if (openmax == 0) { /* first time through */ errno = 0; if ((openmax = sysconf(_SC_OPEN_MAX)) < 0) { if (errno == 0) openmax = OPEN_MAX_GUESS; /* it's indeterminate */ else err_sys("sysconf error for _SC_OPEN_MAX"); } } return(openmax); }
答：如果OPEN_MAX是未确定的或大得出奇（即等于LONG_MAX，效果等同于无限制），那个可以使用getrlimit得到每个进程的最大打开文件描述符数。因为可以修改对每个进程的限制，所以我们不能将前一个调用得到的值高速缓存起来，所以要把openmax改成局部变量。
#include <stdio.h> #include <errno.h> #include <limits.h> #include <unistd.h> #include <sys/resource.h> /* * If OPEN_MAX is indeterminate, this might be inadequate. */ #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) perror("can't get file limit"); if (rl.rlim_max == RLIM_INFINITY) openmax = OPEN_MAX_GUESS; else openmax = rl.rlim_max; } return(openmax); } int main(int argc, char *argv[]) { printf("openmax is %ld ", open_max()); }