MPI 3.0标准：同步发送MPI_Ssend完成时机的技术问询

Great question—this is a common point of confusion when working with MPI's synchronous send operations! Let's break this down clearly.

First, let's restate the key requirement from the MPI 3.0 standard (Section 3.4):

同步发送完成需满足两个条件：1. 发送缓冲区可被复用；2. 接收方已开始接收数据。标准中表述为"已开始接收"而非"已完成接收"

The critical thing to understand here is what "send operation completes" actually means for MPI_Ssend: when the call returns, the MPI implementation has already taken ownership of the data in your send buffer—either copying it to an internal intermediate buffer, or directly transferring it to the receiver's buffer via zero-copy mechanisms if supported.

This means your original send buffer is no longer tied to the ongoing receive operation. Even if the receiver hasn't finished its MPI_Recv call yet, the data that will eventually end up in the receiver's buffer is already safely stored in MPI's internal infrastructure—not your send buffer.

To put it in practical terms: if you call MPI_Ssend(buf, count, type, dest, tag, comm) and it returns successfully, you can immediately overwrite buf with new data, free it, or repurpose it however you want. The receiver will still get the original data you sent, because MPI already pulled that data out of buf before allowing the send call to return.

There's no risk of data inconsistency or corruption here. The MPI standard explicitly guarantees that the send buffer is reusable once the synchronous send completes, and compliant MPI implementations are required to enforce this by ensuring the data is no longer dependent on the sender's buffer at that point. The receiver's "in-progress" receive is working with MPI's internal copy of the data, not your original buffer.

In short: trust the standard's wording. When MPI_Ssend returns, your buffer is yours again—no strings attached to the receiver's ongoing operation.

内容的提问来源于stack exchange，提问作者F.L.