c++ MPI broadcasting vector

Question

I have problem with MPI library. I have to read the text from file and send it to another processes for example as a vector.

I've written the following code:

#include "mpi.h"
#include <stdio.h>
#include<string.h>
#include<stdlib.h>
#include<string>
#include <fstream>
#include <cstring>
#include <vector>
class PatternAndText
{
public:
    static std::string textPreparaation()
    {
        std::ifstream t("file.txt");
        std::string str((std::istreambuf_iterator<char>(t)), std::istreambuf_iterator<char>());
        std::string text = str;
        return text;
    }

};

int main(int argc, char* argv[])
{
   int size, rank ;
   std::string text;
   std::vector<char> cstr;

    MPI_Init(&argc, &argv);
    MPI_Status status;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    if (rank == 0)
    {
      text = PatternAndText::textPreparaation();
      std::vector<char> cstr(text.c_str(), text.c_str() + text.size() + 1);   
   }

   MPI_Bcast(cstr.data(), cstr.size(), MPI_CHAR,0,MPI_COMM_WORLD);

    if (rank  != 0 )
   {
      std::cout<<"\n";
      std::cout<<cstr[1]<<" "<<rank;
      std::cout<<"\n";
   }

        MPI_Finalize();
        return 0; 
}

I want to read the text from file by main process and broadcast i to the others.

When I try to run, it gives me:


[alek:26408] *** Process received signal ***
[alek:26408] Signal: Segmentation fault (11)
[alek:26408] Signal code: Address not mapped (1)
[alek:26408] Failing at address: 0x1
[alek:26408] [ 0] /lib/x86_64-linux-gnu/libc.so.6(+0x3ef20)[0x7fc7c1a8af20]
[alek:26408] [ 1] spli(+0xc63d)[0x55b0104bb63d]
[alek:26408] [ 2] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x7fc7c1a6db97]
[alek:26408] [ 3] spli(+0xc3ba)[0x55b0104bb3ba]
[alek:26408] *** End of error message ***

[alek:26406] *** Process received signal ***
[alek:26406] Signal: Segmentation fault (11)
[alek:26406] Signal code: Address not mapped (1)
[alek:26406] Failing at address: 0x1
[alek:26406] [ 0] /lib/x86_64-linux-gnu/libc.so.6(+0x3ef20)[0x7f01ef5f5f20]
[alek:26406] [ 1] spli(+0xc63d)[0x5579714df63d]
[alek:26406] [ 2] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x7f01ef5d8b97]
[alek:26406] [ 3] spli(+0xc3ba)[0x5579714df3ba]
[alek:26406] *** End of error message ***

[alek:26414] *** Process received signal ***
[alek:26414] Signal: Segmentation fault (11)
[alek:26414] Signal code: Address not mapped (1)
[alek:26414] Failing at address: 0x1



[alek:26413] *** Process received signal ***

[alek:26422] *** Process received signal ***
[alek:26417] *** Process received signal ***
[alek:26417] Signal: Segmentation fault (11)
[alek:26417] Signal code: Address not mapped (1)
[alek:26417] Failing at address: 0x1
[alek:26413] Signal: Segmentation fault (11)
[alek:26413] Signal code: Address not mapped (1)
[alek:26413] Failing at address: 0x1
[alek:26422] Signal: Segmentation fault (11)
[alek:26422] Signal code: Address not mapped (1)
[alek:26422] Failing at address: 0x1
[alek:26413] [alek:26425] *** Process received signal ***
[alek:26425] Signal: Segmentation fault (11)
[alek:26425] Signal code: Address not mapped (1)
[alek:26425] Failing at address: 0x1
[alek:26414] [ 0] [alek:26422] [ 0] /lib/x86_64-linux-gnu/libc.so.6(+0x3ef20)[0x7ff9c3740f20]
[alek:26414] [ 1] spli(+0xc63d)[0x563e8a58563d]
[alek:26417] [ 0] [alek:26425] [ 0] [ 0] [alek:26414] [ 2] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7/lib/x86_64-linux-gnu/libc.so.6/lib/x86_64-linux-gnu/libc.so.6(+0x3ef20)[0x7f5a4dd75f20]
[alek:26417] /lib/x86_64-linux-gnu/libc.so.6(/lib/x86_64-linux-gnu/libc.so.6(+0x3ef20)[0x7f90009f0f20]
[alek:26425] [ 1] spli(+0xc63d))[0x7ff9c3723b97]
[alek:26414] [ 3] spli+0x3ef20)[0x7f2a6faf6f20]
[alek:26413] [ 1] spli(+0xc63d)[0x5557de07763d]
[0x557dee98063d]
[alek:26425] [ 2] (+0xc3ba)[0x563e8a5853ba]
[alek:26414] *** End of error message ***
(+0x3ef20)[0x7f8c41861f20]
[alek:26422] [ 1] spli(+0xc63d)[ 1] spli(+0xc63d)[0x5650e93dc63d]
[alek:26417] [alek:26413] [ 2] [0x561eb2de463d]
[alek:26422] [ 2] [ 2] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x7f2a6fad9b97]
[alek:26413] [ 3] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x7f5a4dd58b97]
[alek:26417] [ 3] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x7f8c41844b97]
[alek:26422] [ 3] spli(+0xc3ba)[0x5557de0773ba]
[alek:26413] *** End of error message ***
spli(+0xc3ba)[0x5650e93dc3ba]
[alek:26417] *** End of error message ***
spli(+0xc3ba)[0x561eb2de43ba]
[alek:26422] *** End of error message ***
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x7f90009d3b97]
[alek:26425] [ 3] spli(+0xc3ba)[0x557dee9803ba]
[alek:26425] *** End of error message ***

[alek:26411] *** Process received signal ***
[alek:26411] Signal: Segmentation fault (11)
[alek:26411] Signal code: Address not mapped (1)
[alek:26411] Failing at address: 0x1
[alek:26411] [ 0] /lib/x86_64-linux-gnu/libc.so.6(+0x3ef20)[0x7f1a339adf20]
[alek:26411] [ 1] spli(+0xc63d)[0x555737c1263d]
[alek:26411] [ 2] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xe7)[0x7f1a33990b97]
[alek:26411] [ 3] spli(+0xc3ba)[0x555737c123ba]
[alek:26411] *** End of error message ***
[warn] Epoll ADD(4) on fd 88 failed.  Old events were 0; read change was 0 (none); write change was 1 (add): Bad file descriptor
--------------------------------------------------------------------------
mpirun noticed that process rank 5 with PID 0 on node alek exited on signal 11 (Segmentation fault).
--------------------------------------------------------------------------

When I check the size instead of [0] processes print 0 What should I change to make it works?

Community · Accepted Answer · 2020-06-20 09:12:55Z

5

The local variable std::vector<char> cstr inside if (rank == 0) {...} shadows that inside main. The variable cstr inside main is not affected.

To assign data to cstr, use cstr.assign(...):
```
if (rank == 0) {
    const std::string text = PatternAndText::textPreparaation();
    cstr.assign(text.c_str(), text.c_str() + text.size() + 1);
}
```
Other processes should first allocate storage in cstr by calling cstr.resize(...). To do that they should know its size. You can first broadcast size and then resize cstr:
```
unsigned long long size = cstr.size();
MPI_Bcast(&size, 1, MPI_UNSIGNED_LONG_LONG, 0, MPI_COMM_WORLD);
if (rank != 0)
    cstr.resize(size);
```
before broadcasting the vector itself:
```
MPI_Bcast(cstr.data(), size, MPI_CHAR, 0, MPI_COMM_WORLD);
```

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered Dec 7, 2019 at 19:40

Evg

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1 Comment

alekq Over a year ago

It explains me far more the I expected , thanks, it works now

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