CODEWITHSUNDEEP
cmpi
MrRempton
MrRempton

Reputation: 23

Seg fault near MPI Bcast when using multiple processes

I am new to MPI, and I am currently working on a project that requires me to do array analysis on my local beowulf cluster. My code is written in C, and it compiles correctly. It runs correctly when only using a single process, but when I try to run it with multiple processes every process besides the root (rank 0) tends to die right around the point when I try to broadcast my data. My code looks something like this 

//1. Initialize global variables
//2. Initialize MPI, get  number of processes, get rank 
//3. All processes create two dimensional arrays 
    array1 = (char **) malloc(sizeArray1 * sizeof(char *));
    array1[0] = (char *) malloc(sizeArray1 * lineLength * sizeof(char)); 
    for(i = 1; i < sizeArray1; i++)
    {
            array1[i] = array1[i - 1] + lineLength;
    }
    //4. Only server will populate it's arrays, then broadcast to all processes
    if(rank == 0)
    {
            f = fopen("path..../testFile1.txt", "r");
            if(NULL == f) {
                    perror("FAILED: ");
                    return -1;
            }
            numWords = 0;
            while(err != EOF && numWords < sizeArray2)
            {
                    err = fscanf(f, "%[^\n]\n", array2[numWords]);
                    numWords ++;
            }
            fclose(f);

    }

//5. Broadcast each line from both arrays to all processes 
MPI_Bcast(array1, sizeArrray1 * lineLength, MPI_CHAR, 0, MPI_COMM_WORLD);
 //6. do further work on arrays

The root node will accomplish all of this perfectly fine, while the other nodes will usually try to broadcast once, print a line, and then die. The exact error that I am getting is 

Signal: Segmentation fault (11)
Signal code: Address not mapped (1)
Failing at address: 0x37
malloc.c:2392: sysmalloc: Assertion `(old_top == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)' failed.

If you need to look at any other portions of my code let me know

Note: I have edited my code to correspond with suggestions from other users, but the error still persists

Upvotes: 1

Views: 337

Answers (1)

Gilles Gouaillardet
Gilles Gouaillardet

Reputation: 8395

So your arrays are made of char and not int. so you should　MPI_Bcast() MPI_CHAR instead of MPI_INT. for example

MPI_Bcast(&(array1[i][0]), lineLength, MPI_CHAR, 0, MPI_COMM_WORLD);

as a matter of style, you can also write this as

MPI_Bcast(array1[i], lineLength, MPI_CHAR, 0, MPI_COMM_WORLD);

Also, you might want to allocate array1 in one chunk, so you can MPI_Bcast() it with one call (this is generally more efficient)

the allocation would look like

array1 = (char **)malloc(sizeArray1 * sizeof(char *);
array1[0] = (char *)malloc(sizeArray1 * lineLength * sizeof(char));
for (int i=1; i<sizeArray1; i++) array1[i] = array1[i-1] + lineLength;

and then

MPI_Bcast(array1, sizeArray1 * lineLength, MPI_CHAR, 0, MPI_COMM_WORLD);

Upvotes: 1

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