Line 12, mmap creates a new segment in virtual memory and assigns the start virtual address to buf.

The segment's content is the file specified by argv[1], but mmap doesn't load this file into physical memory.

The file is accessed through the buf variable and is loaded using virtual memory demand paging!

mmapEx.c

Line 9: Open the file for reading,

Line 10: fstat gets the file properties (size, permissions, last time modified, etc.) in a struct.

Lines 14-18: File is mapped to memory as if in an array pointed to by buf. Print the file backwards!

    1   
    2   int main(int argc, char* argv[])
    3   {
    4     char *buf;
    5     int i;
    6     struct stat s;
    7     int fd;
    8   
    9     fd = open(argv[1], O_RDONLY);
   10     fstat(fd, &s);
   11   
   12     buf = mmap(NULL, s.st_size, PROT_READ, MAP_PRIVATE, fd, 0);
   13   
   14     printf("\n\n*** File %s Backwards ***\n\n", argv[1]);
   15     for(i = s.st_size - 1; i >= 0; i--) {
   16       printf("%c", buf[i]);
   17     }
   18     printf("\n");
          ...
       }