MPI - 在使用 MPI_Probe() 时发送我自己的结构

Question

我需要使用 MPI_Send() 发送我自己的结构。问题是，我不确定如何正确使用 MPI 中的结构。当然我也试过自己找，但是没有找到完全符合我需求的例子。

现在，当我运行 Makefile 时，我得到了这个错误：

probe_and_struct.c: In function ‘main’:
probe_and_struct.c:72:13: error: expected ‘;’ before ‘buf’
    myStruct buf;
             ^
probe_and_struct.c:73:4: error: ‘buf’ undeclared (first use in this function)
    buf = (myStruct *) malloc( sizeof(myStruct) * status_size );
    ^
probe_and_struct.c:73:4: note: each undeclared identifier is reported only once for each function it appears in
probe_and_struct.c:73:21: error: expected expression before ‘)’ token
    buf = (myStruct *) malloc( sizeof(myStruct) * status_size );
                     ^
make: *** [probe_and_struct] Error 1

那么，你能告诉我，我做错了什么，我应该如何正确使用这个结构？

编辑： 我已经重写了代码，但现在程序在 MPI_Send() 上因 Segmentation fault 而崩溃。

---

这是我的代码：

#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>

#define ARR_LEN 2 // length of test array and the size sent

int main() {
    //---------------------------------------------------------------------------------------------------------------------------------------
    // Create own structure
    //----------------------

    // Structure of structure :-)
    struct {
        int id;
        char c;
    } value;

    // Declare parts of structure
    MPI_Datatype myStruct;
    int blockLengths[2];
    MPI_Aint indices[2];
    MPI_Datatype types[2];

    // Initialize parts of structure
    blockLengths[0] = 1; // stucture's attributes' sizes
    blockLengths[1] = 1;
    types[0] = MPI_INT; // structure's attributes' data types
    types[1] = MPI_CHAR;
    MPI_Address( &value.id, &indices[0] );
    MPI_Address( &value.c, &indices[1] );

    // Create and commit new structure
    MPI_Type_struct( 2, blockLengths, indices, types, &myStruct );
    MPI_Type_commit( &myStruct );

    //---------------------------------------------------------------------------------------------------------------------------------------
    // Message passing
    //-----------------

    MPI_Init(NULL, NULL);

    value.id = 0;
    value.c = 'a';

    // Number of processes, ID of current process
    int world_size;
    MPI_Comm_size( MPI_COMM_WORLD, &world_size );

    int world_rank;
    MPI_Comm_rank( MPI_COMM_WORLD, &world_rank );

    // Test array to send
    int arr[ 2 ] = {10,20};

    MPI_Status status;

    switch( world_rank ) {
        case 0:
            printf("This is the process number %d.\n\t", world_rank);
            MPI_Send( &value, 2, myStruct, 1, 0, MPI_COMM_WORLD);
            printf("The array of INT was sent.\n");
            break;
        case 1:
            // Recognize the size of the message
            MPI_Probe( 0, 0, MPI_COMM_WORLD, &status );

            // Number of blocks sent
            int status_size;
            MPI_Get_count( &status, myStruct, &status_size );

            // Allocate buffer with the size needed
            myStruct buf;
            buf = (myStruct *) malloc( sizeof(myStruct) * status_size );

            // Receive and print message
            MPI_Recv( buf, status_size, myStruct, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE );
            printf("This is the process number %d.\n\t[ ", world_rank);
            for( int i = 0; i < status_size; i++ ) {
                printf( "%d ", buf[i] );
            }
            printf("]\n");
            break;
        default:
            printf("This is the process number %d.\n", world_rank);
            break;
    }

    MPI_Type_free( &myStruct );
    MPI_Finalize();
    return 0;
}

生成文件：

CC=mpicc

STD=-std=c11

all: probe_and_struct

probe_and_struct: probe_and_struct.c
    $(CC) -o probe_and_struct probe_and_struct.c $(STD)

clean:
    rm -f probe_and_struct

---

我的代码 - 第二版：

#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>

#define ARR_LEN 2 // length of test array and the size sent

int main() 
{
    MPI_Init(NULL, NULL);

    //---------------------------------------------------------------------------------------------------------------------------------------
    // Create own structure
    //----------------------

    // Structure of structure :-)
    typedef struct Values
    {
        int id;
        char c;
    } Values;

    Values value;

    // Declare parts of structure
    MPI_Datatype myStruct;
    int blockLengths[2];
    MPI_Aint indices[2];
    MPI_Datatype types[2];

    // Initialize parts of structure
    blockLengths[0] = 1; // stucture's attributes' sizes
    blockLengths[1] = 1;
    types[0] = MPI_INT; // structure's attributes' data types
    types[1] = MPI_CHAR;
    MPI_Address( &value.id, &indices[0] );
    MPI_Address( &value.c, &indices[1] );

    // Create and commit new structure
    MPI_Type_struct( 2, blockLengths, indices, types, &myStruct );
    MPI_Type_commit( &myStruct );

    //---------------------------------------------------------------------------------------------------------------------------------------
    // Message passing
    //-----------------

    value.id = 0;
    value.c = 'a';

    // Number of processes, ID of current process
    int world_size;
    MPI_Comm_size( MPI_COMM_WORLD, &world_size );

    int world_rank;
    MPI_Comm_rank( MPI_COMM_WORLD, &world_rank );

    // Test array to send
    //int arr[ 2 ] = {10,20};

    MPI_Status status;

    switch( world_rank ) {
        case 0:
            printf("Toto je proces cislo %d.\n\t", world_rank);
            MPI_Send( &value, sizeof(struct Values), myStruct, 1, 0, MPI_COMM_WORLD);
            printf("Odeslano pole INTu.\n");
            break;
        case 1:
            // Recognize the size of the message
            MPI_Probe( 0, 0, MPI_COMM_WORLD, &status );

            // Number of blocks sent
            int status_size;
            MPI_Get_count( &status, myStruct, &status_size );
puts("b");
            // Allocate buffer with the size needed
            Values * buf;
            buf = (Values *) malloc( sizeof(Values) * status_size );
puts("b");
            // Receive and print message
            //MPI_Recv( buf, status_size, myStruct, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE );
            printf("Toto je proces cislo %d.\n\t[ ", world_rank);
            for( int i = 0; i < status_size; i++ ) {
                printf( "%d %c", buf[i].id, buf[i].c );
            }
            printf("]\n");
            break;
        default:
            printf("Toto je proces cislo %d.\n", world_rank);
            break;
    }

    MPI_Type_free( &myStruct );
    MPI_Finalize();
    return 0;
}

Answer 1

您将myStruct 用作类型并尝试实例化buf，但myStruct 是MPI_Datatype 的实例（而不是类型）。

这部分代码（myStruct 被声明为MPI_Datatype）：

// Declare parts of structure
MPI_Datatype myStruct;

与这部分冲突（您尝试使用 myStruct 作为类型）：

        // Allocate buffer with the size needed
        myStruct buf;
        buf = (myStruct *) malloc( sizeof(myStruct) * status_size );

Answer 2

您将MPI_type 与变量的类型混淆了。例如，让我们使用一个简单的int。 int 的 MPI_type 是 MPI_INT，对吗？但这不是正确的代码：

MPI_INT a = 5; //wrong, MPI_INT is an MPI_Type
               //not the type we want to declare a variable
MPI_Send(&a, 1, MPI_INT, 1, 0, MPI_COMM_WORLD);

相反，你会使用：

int a = 5; //int is used to declare a variable
MPI_Send(&a, 1, MPI_INT, 1, 0, MPI_COMM_WORLD); //MPI_INT is the type to 
                                                //send the variable

同样，如果你有一个结构为：

struct value {
    int id;
    char c;
};

然后，在您使用MPI_Type_commit 声明一个新的 MPI 类型之后，您将使用：

struct value a = {4, 'b'}; //struct value is used to declare the variable
MPI_Send(&a, 1, myStruct, 1, 0, MPI_COMM_WORLD); //myStruct is the MPI_Type used to 
                                                 //send the variable

Answer 3

您没有正确构建 MPI 数据类型。或者更确切地说，您在发送结构时没有正确使用构造的数据类型。您应该执行以下任一操作：

1。修复数据类型内部的偏移量

这段代码：

// Initialize parts of structure
blockLengths[0] = 1; // stucture's attributes' sizes
blockLengths[1] = 1;
types[0] = MPI_INT; // structure's attributes' data types
types[1] = MPI_CHAR;
MPI_Address( &value.id, &indices[0] );
MPI_Address( &value.c, &indices[1] );

// Create and commit new structure
MPI_Type_struct( 2, blockLengths, indices, types, &myStruct );
MPI_Type_commit( &myStruct );

...

MPI_Send( &value, sizeof(struct Values), myStruct, 1, 0, MPI_COMM_WORLD);

应该变成：

// Initialize parts of structure
blockLengths[0] = 1; // stucture's attributes' sizes
blockLengths[1] = 1;
types[0] = MPI_INT; // structure's attributes' data types
types[1] = MPI_CHAR;
MPI_Address( &value.id, &indices[0] );
MPI_Address( &value.c, &indices[1] );

// Convert the absolute addresses into offsets
indices[1] -= indices[0];
indices[0] = 0;

// Create and commit new structure
MPI_Type_struct( 2, blockLengths, indices, types, &myStruct );
MPI_Type_commit( &myStruct );

...

MPI_Send( &value, 1, myStruct, 1, 0, MPI_COMM_WORLD);

此外，如果您愿意发送此类数据类型的数组，则应采取一些特殊措施来考虑编译器所做的类型填充。请参阅this question 如何。

2。发送数据时使用 MPI_BOTTOM

您可以保留绝对地址，而不是修复类型描述中的偏移量。缺点是使用绝对地址时，您只能发送构造数据类型时使用的特定变量 的内容（因为其他变量通常会在内存中的其他位置结束）。使用此类数据类型时，不指定缓冲区的地址，而是指定MPI_BOTTOM，即此代码：

MPI_Send( &value, sizeof(struct Values), myStruct, 1, 0, MPI_COMM_WORLD);

变成：

MPI_Send( MPI_BOTTOM, 1, myStruct, 1, 0, MPI_COMM_WORLD);

注意，在这种情况下，要发送的元素数量必须等于1，即不能发送数组（除非数据类型本身描述了数组的所有元素） .

---

还要注意，在这两种情况下sizeof(struct Values) 都是错误的，必须将其更正为1，因为MPI_Send 的第二个参数给出了要发送的元素数量，并且每个元素的实际数据大小已经编码在 MPI 数据类型中。

我强烈建议您使用选项 #1。