I want to signal an error in the website documentation. In this URL :

https://www.mpich.org/static/docs/v3.2/www3/MPI_Igather.html

recvcount

number of elements in receive buffer (significant only at root) (non-negative integer)

should be  :

recvcount

number of elements for any single receive (integer, significant only at root

We can see this page 200 of the MPI standard, June 4, 2015.

Hugo T.