1:  /*

2:  author:justinzhang

3:  email:uestczhangchao@gmail.com

4:  time：2012-8-22 16:23:08

5:  desc: mmap()practice from man mmap

6:  */

7:  #include 
     

8:  #include 
     

9:  #include 
     

10:  #include 
     

11:  #include 
     

12:  #include 
     

13:   

14:  #define handle_error(msg)\

15:  do { perror(msg); exit(EXIT_FAILURE); } while(0)

16:  int

17:  main(int argc, char *argv[])

18:  {

19:      char *addr;

20:      int fd;

21:      struct stat sb;

22:      off_t offset, pa_offset;

23:      size_t length;

24:      ssize_t s;

25:      if(argc <3 || argc > 4)

26:      {

27:          fprintf(stderr, "%s file offset [length]\n",argv[0]);

28:      exit(EXIT_FAILURE);

29:      }

30:      fd = open(argv[1],O_RDONLY);

31:      if(fd == -1)

32:      {

33:          handle_error("open");

34:      }

35:      if(fstat(fd, &sb) == -1)/*to obtain file size*/

36:      {

37:          handle_error("fstat");

38:      }

39:      offset = atoi(argv[2]);

40:      pa_offset = offset & ~(sysconf(_SC_PAGE_SIZE)-1);

41:         /*offset for mmap() must be page aligned*/

42:      if( offset >= sb.st_size)

43:      {

44:          fprintf(stderr, "offset is past end of file\n");

45:      exit(EXIT_FAILURE);

46:      }

47:      if(argc == 4)

48:      {

49:          length = atoi(argv[3]);

50:      if(offset + length > sb.st_size)

51:          length = sb.st_size - offset;

52:          /*can't display bytes past end of file*/

53:      }else {
   /*No length arg ==> display to end of file*/

54:          length = sb.st_size - offset;

55:      }

56:      addr = mmap(NULL, length + offset - pa_offset, PROT_READ,MAP_PRIVATE, fd

57:                  ,pa_offset);

58:      if(addr == MAP_FAILED)

59:          handle_error("mmap");

60:      s = write(STDOUT_FILENO, addr + offset - pa_offset, length);

61:      if (s != length)

62:      {

63:          if (s == -1)

64:          handle_error("write");

65:      fprintf(stderr, "partial write");

66:      exit(EXIT_FAILURE);

67:      }

68:      exit(EXIT_SUCCESS);

69:  }