SEEK_* Problem with MPICH2

This is a frequently encountered error when writing MPI programs in C++.

/usr/include/mpicxx.h:26:2: error: #error "SEEK_SET is #defined but must not be for the C++ binding of MPI"
/usr/include/mpicxx.h:30:2: error: #error "SEEK_CUR is #defined but must not be for the C++ binding of MPI"
/usr/include/mpicxx.h:35:2: error: #error "SEEK_END is #defined but must not be for the C++ binding of MPI"

I remember (not very sure) it was MPICH2 that introduced this problem. It was not present in MPICH1.

This happens if <mpi.h> is inlcuded AFTER some system headers like <stdio.h> and <iostream>. The workaround is simple: always include <mpi.h> first in any C/C++ source that uses MPI APIs. (Another workaround is to pass -DMPICH_IGNORE_CXX_SEEK to mpicxx, but it seems this has other drawbacks. Do this only if you are unable to modify the sources.)

This certainly should be considered a bug of MPICH2: SEEK_SET, SEEK_CUR, and SEEK_END are constant symbols required by the ANSI C standard, but MPICH2 has global variables with exactly the same names. MPICH2 has not changed this, though they apparently have realized this problem for long. (Otherwise they could not have introduced MPICH_IGNORE_CXX_SEEK. Probably they have binary compatibility concerns?)