c,arrays,double,mmap , Using mmap to allocate double 2D array


Using mmap to allocate double 2D array

Question:

Tag: c,arrays,double,mmap

I am attempting to use mmap to create/access twi different (N+2)*(N+2) 2D array of doubles, so that multiple threads can look at their own portion of it and change it, applying the changes so that all others can see. Here is what I have:

int main(int argc, char *argv[]) {
    int N = atoi(argv[1]);
    int numProcs = atoi(argv[2]);

    int ARRAY_SIZE = (N+2)*(N+2)*sizeof(double);

    double **grid = (double **) mmap(NULL, ARRAY_SIZE, PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, -1, 0);
    if (grid == MAP_FAILED) {
        printf("Error mmapping grid\n");
    }
    double **newGrid = (double **) mmap(NULL, ARRAY_SIZE, PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, -1, 0);
    if (newGrid == MAP_FAILED) {
        printf("Error mmapping grid\n");
    }

When I run it, or try to access anything in it, I get a segmentation fault. I tried to allocate memory as well, with:

for(i = 0; i < N+2; i++) {
    for(j = 0; j < N+2; j++) {
        grid[i][j] = malloc(sizeof(double));
    }
}

for(i = 0; i < N+2; i++) {
    for(j = 0; j < N+2; j++) {
        newGrid[i][j] = malloc(sizeof(double));
    }
}

But I am met with: error: assigning to 'double' from incompatible type 'void *' newGrid[i][j] = malloc(sizeof(double));

I believe I am missing something here with how mmap works, could anyone point me in the right direction?


Answer:

If N is a compile-time constant or if your compiler supports variable length arrays (VLAs), then you could simply do:

double (*grid)[N+2] = (double (*)[N+2]) mmap(NULL, ARRAY_SIZE, ...

grid[4][5] = 2.0;  // setting an element

If N is not constant and your compiler doesn't support VLAs, then you need to manually offset:

double *grid = (double *) mmap(NULL, ARRAY_SIZE, ...

grid[4 * (N + 2) + 5] = 2.0;  // setting the same element using manual offsets

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