simple_copy example on pmem.io - c

Question

I have created the emulated device given at http://pmem.io/2016/02/22/pm-emulation.html, successfully.
It shows the device correctly:
:~/Prakash/nvml/src/examples/libpmem$ mount | grep pmem
/dev/pmem0 on /mnt/pmemd type ext4 (rw,relatime,dax,errors=continue,data=ordered)
However, when I execute the simple_copy sample given with pmem nvml, it gives this error:
amd#amd:~/Prakash/nvml/src/examples/libpmem$ ./simple_copy logs
/dev/pmem0 pmem_map_file: File exists
amd#amd:~/Prakash/nvml/src/examples/libpmem$ ./simple_copy logs
/dev/pmem0/logs pmem_map_file: Not a directory
Am I not using the program correctly?
Also, I have mounted the device as dax and I clearly see the performance advantage with
:~/Prakash/nvml/src/examples/libpmem$ sudo dd if=/dev/zero of=/dev/pmem0 bs=2G count=1
0+1 records in
0+1 records out
2147479552 bytes (2.1 GB, 2.0 GiB) copied, 0.910729 s, 2.4 GB/s
:~/Prakash/nvml/src/examples/libpmem$ sudo dd if=/dev/zero of=/mnt/pmem0/test bs=2G count=1
0+1 records in
0+1 records out
2147479552 bytes (2.1 GB, 2.0 GiB) copied, 6.39032 s, 336 MB/s

Answer 1

from the errors posted, is seems reasonable to believe:
without the appropriate option, it will not create a directory
without the appropriate option, it will not replace a file

Answer 2

If you open the example you are referring to, you will see the following:
if ((pmemaddr = pmem_map_file(argv[2], BUF_LEN,
PMEM_FILE_CREATE|PMEM_FILE_EXCL,
0666, &mapped_len, &is_pmem)) == NULL) {
perror("pmem_map_file");
exit(1);
}
This is the part that is giving you trouble. To understand why, let's look at the man 7 libpmem. You can find the relevant part here.
This is the paragraph we are interested in:
The pmem_map_file() function creates a new read/write mapping for a
file. If PMEM_FILE_CREATE is not specified in flags, the entire
existing file path is mapped, len must be zero, and mode is ignored.
Otherwise, path is opened or created as specified by flags and mode,
and len must be non-zero. pmem_map_file() maps the file using mmap(2),
but it also takes extra steps to make large page mappings more likely.
So, the pmem_map_file function effectively calls open(2) and then mmap(2). In the simple_copy.c example we can observe that the flags which were used are: PMEM_FILE_CREATE and PMEM_FILE_EXCL, and as we can learn from the manpage, they roughly translate to O_CREAT and O_EXCL respectively.
This means that the error messages are correct and you've received them because in your first attempt you've provided an existing file, whilst on the second attempt you tried a directory.
There's an in-depth explanation of libpmem here.