c++ MPI broadcasting vector - c++

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?

Answer 1

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);