[TOC]

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## MPI程序基本框架与常用函数

### 任务描述

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。

Open MPI 文档：https://docs.open-mpi.org/en/main/

### 相关知识

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。

#### MPI常用函数

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##### MPI_Init

`MPI_Init` — Initializes the MPI execution environment.

```c
int MPI_Init(int *argc, char ***argv)
```

INPUT PARAMETERS

- `argc`: C only: Pointer to the number of arguments.
- `argv`: C only: Argument vector.

OUTPUT PARAMETER

- `ierror`: Fortran only: Error status (integer).

---

##### MPI_Comm_size

`MPI_Comm_size` — Returns the size of the group associated with a communicator.

```c
int MPI_Comm_size(MPI_Comm comm, int *size)
```

INPUT PARAMETER

- `comm`: Communicator (handle).

OUTPUT PARAMETERS

- `size`: Number of processes in the group of comm (integer).

---

##### MPI_Comm_rank

`MPI_Comm_rank` — Determines the rank of the calling process in the communicator.

```c
int MPI_Comm_rank(MPI_Comm comm, int *rank)
```

INPUT PARAMETERS

- `comm`: Communicator (handle).

OUTPUT PARAMETERS

- `rank`: Rank of the calling process in group of comm (integer).
- `ierror`: Fortran only: Error status (integer).

---

##### MPI_Send

`MPI_Send` — Performs a standard-mode blocking send.

```c
int MPI_Send(const void *buf, int count, MPI_Datatype datatype, int dest,
     int tag, MPI_Comm comm)
```

INPUT PARAMETERS

- `buf`: Initial address of send buffer (choice).
- `count`: Number of elements in send buffer (nonnegative integer).
- `datatype`: Datatype of each send buffer element (handle).
- `dest`: Rank of destination (integer).
- `tag`: Message tag (integer).
- `comm`: Communicator (handle).

OUTPUT PARAMETER

- `ierror`: Fortran only: Error status (integer).

---

##### MPI_Recv

`MPI_Recv` — Performs a standard-mode blocking receive.

```c
int MPI_Recv(void *buf, int count, MPI_Datatype datatype,
     int source, int tag, MPI_Comm comm, MPI_Status *status)
```

INPUT PARAMETERS

- `count`: Maximum number of elements to receive (integer).
- `datatype`: Datatype of each receive buffer entry (handle).
- `source`: Rank of source (integer).
- `tag`: Message tag (integer).
- `comm`: Communicator (handle).

OUTPUT PARAMETERS

- `buf`: Initial address of receive buffer (choice).
- `status`: Status object (status).
- `ierror`: Fortran only: Error status (integer).

---

##### MPI_Finalize

`MPI_Finalize` — Terminates MPI execution environment.

```c
int MPI_Finalize()
```

OUTPUT PARAMETER

- `ierror`: Fortran only: Error status (integer).

#### MPI程序编译运行

```bash
$ mpicc -g -Wall -o mpi_hello mpi_hello.c
$ mpiexec -n 4 ./mpi_hello
```

### 编程要求

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。

### 测试说明

平台会对你编写的代码进行测试：

测试输入：无；
预期输出：
```bash
Greetings from process 0 of 4!
Greetings from process 1 of 4!
Greetings from process 2 of 4!
Greetings from process 3 of 4!
```

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开始你的任务吧，祝你成功！