mmap brokes after strdup - mmap

Question

I tried the following configuration with mmap:
open file (file is over 2 kB)
request statistics from file *f_file*
map file (file is smaller than a page, offset page 0, size is expected size)
verify values of *f_footer* in the map *f_fpage*
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?