MPI_Issend is the synchronous non-blocking send (the capital 'I' standing for immediate return). Unlike its blocking counterpart MPI_Ssend, MPI_Issend will not block until the recipient has received the message. In other words, when MPI_Issend returns, the buffer passed may not have been sent yet, and it must be considered unsafe to reuse the buffer passed. The user must therefore check for completion with MPI_Wait or MPI_Test before safely reusing the buffer passed. Note that MPI_Issend may be implicitly invoked by the standard non-blocking send (MPI_Isend). Other non-blocking sends are MPI_Isend, MPI_Ibsend and MPI_Irsend. Refer to MPI_Ssend to see the blocking counterpart of MPI_Issend.