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[TOC]
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---
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## MPI程序基本框架与常用函数
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### 任务描述
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本关任务：练习`MPI`程序的基本框架与常用函数，包括`MPI_Init`、`MPI_Comm_size`、`MPI_Comm_rank`、`MPI_Send`、`MPI_Recv`以及`MPI_Finalize`的使用。
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Open MPI 文档：https://docs.open-mpi.org/en/main/
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### 相关知识
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为了完成本关任务，你需要掌握：1.MPI程序的基本框架，2.MPI常用函数。
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#### MPI常用函数
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---
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##### MPI_Init
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`MPI_Init` — Initializes the MPI execution environment.
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```c
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int MPI_Init(int *argc, char ***argv)
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```
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INPUT PARAMETERS
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- `argc`: C only: Pointer to the number of arguments.
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- `argv`: C only: Argument vector.
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OUTPUT PARAMETER
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- `ierror`: Fortran only: Error status (integer).
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---
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##### MPI_Comm_size
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`MPI_Comm_size` — Returns the size of the group associated with a communicator.
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```c
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int MPI_Comm_size(MPI_Comm comm, int *size)
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```
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INPUT PARAMETER
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- `comm`: Communicator (handle).
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OUTPUT PARAMETERS
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- `size`: Number of processes in the group of comm (integer).
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---
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##### MPI_Comm_rank
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`MPI_Comm_rank` — Determines the rank of the calling process in the communicator.
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```c
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int MPI_Comm_rank(MPI_Comm comm, int *rank)
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```
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INPUT PARAMETERS
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- `comm`: Communicator (handle).
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OUTPUT PARAMETERS
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- `rank`: Rank of the calling process in group of comm (integer).
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- `ierror`: Fortran only: Error status (integer).
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---
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##### MPI_Send
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`MPI_Send` — Performs a standard-mode blocking send.
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```c
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int MPI_Send(const void *buf, int count, MPI_Datatype datatype, int dest,
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int tag, MPI_Comm comm)
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```
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INPUT PARAMETERS
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- `buf`: Initial address of send buffer (choice).
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- `count`: Number of elements in send buffer (nonnegative integer).
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- `datatype`: Datatype of each send buffer element (handle).
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- `dest`: Rank of destination (integer).
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- `tag`: Message tag (integer).
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- `comm`: Communicator (handle).
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OUTPUT PARAMETER
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- `ierror`: Fortran only: Error status (integer).
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---
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##### MPI_Recv
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`MPI_Recv` — Performs a standard-mode blocking receive.
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```c
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int MPI_Recv(void *buf, int count, MPI_Datatype datatype,
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int source, int tag, MPI_Comm comm, MPI_Status *status)
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```
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INPUT PARAMETERS
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- `count`: Maximum number of elements to receive (integer).
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- `datatype`: Datatype of each receive buffer entry (handle).
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- `source`: Rank of source (integer).
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- `tag`: Message tag (integer).
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- `comm`: Communicator (handle).
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OUTPUT PARAMETERS
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- `buf`: Initial address of receive buffer (choice).
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- `status`: Status object (status).
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- `ierror`: Fortran only: Error status (integer).
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---
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##### MPI_Finalize
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`MPI_Finalize` — Terminates MPI execution environment.
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```c
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int MPI_Finalize()
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```
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OUTPUT PARAMETER
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- `ierror`: Fortran only: Error status (integer).
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#### MPI程序编译运行
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```bash
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$ mpicc -g -Wall -o mpi_hello mpi_hello.c
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$ mpiexec -n 4 ./mpi_hello
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```
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### 编程要求
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根据提示，在右侧编辑器补充六处代码。启动四个进程分别将问候语发送给进程`0`，由进程`0`将接收到的问候语打印输出出来。
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### 测试说明
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平台会对你编写的代码进行测试：
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测试输入：无；
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预期输出：
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```bash
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Greetings from process 0 of 4!
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Greetings from process 1 of 4!
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Greetings from process 2 of 4!
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Greetings from process 3 of 4!
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```
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---
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开始你的任务吧，祝你成功！
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