Hello,

is inaccurate if I understood this function correctly.

in_values:  vector or pointer to storage that will contain the values to send to each process, indexed by the process rank.
This vector is not indexed by process rank. It contains all the values that would be scattered to each process (including the root process)

Ignoring displacements and focusing on this function

3 processes, root rank 0, and rank 1 and rank 2

Does "sizes" contain { s1, s2 } only or  { s0, s1, s2 }?

in_values = { 1, 2, 3, 4, 5, 6, 7 };
If we assume s1=2 and s2=3, therefore s0=2=7-2-3

In the root process:
std::vector<int> out_values( s0=2 );

scatterv(world, in_values, {s1,s2}, out_values.data(), rank=0);

and in rank 1:
std::vector<int> out_values( s1=2 );

scatterv(world, out_values.data(), outsize=2,  rank=0);

and rank 2:

Regards,