mmap brokes after strdup

Question

I tried the following configuration with mmap:

1. open file (file is over 2 kB)
2. request statistics from file *f_file*
3. map file (file is smaller than a page, offset page 0, size is expected size)
4. verify values of *f_footer* in the map *f_fpage*
5. usage strdup

Code:

union{
  ...
  struct {
    char    *f_fname;
    struct clog_footer *f_footer;
    char    *f_fpage;
    size_t  f_size;
  } f_ring;                   /* circular log file */
  char *f_fname;              /* Name use for Files|Pipes|TTYs.  */
} f_un;
...
struct clog_footer {
        uint32_t cf_magic;
};
...
1995            f->f_file = open(p+1, O_RDWR, 0 );
1996          if (f->f_file == -1) {
2000          }
2001          if (fstat(f->f_file,&sb)<0) {
2006          }
2014          f->f_un.f_ring.f_fpage = mmap(NULL,sb.st_size,PROT_READ|PROT_WRITE,MAP_SHARED,f->f_file,0);
2015          if (f->f_un.f_ring.f_fpage == MAP_FAILED) {
2020          }
2021          f->f_un.f_ring.f_footer = (struct clog_footer*)(f->f_un.f_ring.f_fpage + sb.st_size-sizeof(struct clog_footer));
2022          if (memcmp(&(f->f_un.f_ring.f_footer->cf_magic),MAGIC_CONST,4)!=0) {
2029          }
2031          f->f_un.f_fname = strdup (p+1);
...

I used read/write, file is filled with zero's up to 2 Kb. I parametrized mmap with file size and zero page size, but mmap fails to map the file.

Should the file to map have additional properties?

Is "0" a acceptable parameter as offset in mmap, once i want to map the file from beginning?