int posix_mem_offset(const void *restrict addr, size_t
off_t *restrict off, size_t *restrict contig_len,
int *restrict fildes);
The posix_mem_offset() function shall return in the variable pointed to by off a value that identifies the offset (or location), within a memory object, of the memory block currently mapped at addr. The function shall return in the variable pointed to by fildes, the descriptor used (via mmap()) to establish the mapping which contains addr. If that descriptor was closed since the mapping was established, the returned value of fildes shall be -1. The len argument specifies the length of the block of the memory object the user wishes the offset for; upon return, the value pointed to by contig_len shall equal either len, or the length of the largest contiguous block of the memory object that is currently mapped to the calling process starting at addr, whichever is smaller.
If the memory object mapped at addr is a typed memory object, then if the off and contig_len values obtained by calling posix_mem_offset() are used in a call to mmap() with a file descriptor that refers to the same memory pool as fildes (either through the same port or through a different port), and that was opened with neither the POSIX_TYPED_MEM_ALLOCATE nor the POSIX_TYPED_MEM_ALLOCATE_CONTIG flag, the typed memory area that is mapped shall be exactly the same area that was mapped at addr in the address space of the process that called posix_mem_offset().
If the memory object specified by fildes is not a typed memory object, then the behavior of this function is implementation-defined.
Upon successful completion, the posix_mem_offset() function shall return zero; otherwise, the corresponding error status value shall be returned.
The posix_mem_offset() function shall fail if:
This function shall not return an error code of [EINTR].
The following sections are informative.
mmap() , posix_typed_mem_open() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/mman.h>