无法释放分配给一维和二维数组的内存

Question

I'm working on a numeric program to approximate solutions of an hyperbolic equation.

I have to allocate memory for two 1d-arrays and two 2d-arrays every iteration of a for loop. So I allocate memory and free memory in every iteration.

The code is

int main()
{
    int mesh, n_max, m_max;
    int i;
    double v, delta_t, delta_x, lambda;
    double *t, *x, **u, **u_exact, error[MAX_MESH]; 

    for (mesh = 0; mesh < MAX_MESH; mesh++) {
        m_max = pow(2, 5) * pow(2, mesh);
        n_max = 16 * m_max;

        t = vector_alloc((n_max + 1) * sizeof(double));
        x = vector_alloc((m_max + 2) * sizeof(double));
        u = matrix_alloc(m_max + 2, n_max);
        u_exact = matrix_alloc(m_max + 2, n_max);

        //functions manipulating t, x, u, u_exact

        free(t);
        free(x);
        for (i = 0; i < m_max + 2; i++) free(u[i]);
        free(u);
        for (i = 0; i < m_max + 2; i++) free(u_exact[i]);
        free(u_exact);
    }

    return (0);

}

double *vector_alloc(int n)
{
    double *result; /* ponteiro para o vetor */

    result = malloc(n * sizeof(double));
    if (result == NULL) printf("Error: no memmory available");
    return(result);
}

double **matrix_alloc(int m, int n)
{
    int i;
    double **result; 

    /* Allocates matrix raws */
    result = malloc(m * sizeof(double *));
    if (result == NULL) {
        printf ("Error: no memmory available");
        return (NULL);
    }
    /* Allocates matrix columns */
    for (i = 0; i < m; i++) {
        result[i] = malloc(n * sizeof(double));
        if (result[i] == NULL) {
            printf ("Error: no memmory available");
            return (NULL);
        }
    }
    return (result);
}

The free statements are not working. If I try to only free the 1d-arrays t and x , after the second iteration I get a segmentation fault error.

If try to only free the 2d-arrays u and u_exact I get the error

*** glibc detected *** ./a.out: double free or corruption (!prev): 0x0900a968 ***

Answer 1

I suspect that you are running out of memory. The memory allocation seems to be growing exponentially in the loop.

For MAX_MESH as 8, if we take the last iteration which will be the worst case :

I assume double size is 8 bytes.

m_max = 2^12
n_max = 2^16

Memory required in bytes by :
t = (2^16 + 1) * (2^3) = 2^19 + 2^3
x = (2^12 + 1) * (2^3) = 2^15 + 2^3
u = (2^12 + 2) * (2^16) * (2^3) = 2^31 + 2^20 (approx)
u_exact = 2^31 + 2^20 (approx)

For brevity, I left out the memory required to store each row pointer in u and u_exact calculation.

See the enormous memory required for u and u_exact!

Now, if you are running in a 32 bit system, then definitely your process will not have sufficient virtual memory to use. I would expect a "NULL" pointer returned by malloc in the matrix allocation function. Are you checking the return value and printing an error message similar to what you are doing in vector_alloc (like below)?

double **matrix_alloc(int m, int n)
{
    int i;
    double **p = malloc(sizeof(double **) * m);   
    if(!p) {printf("matrix_alloc:: Unable to allocate memory!!! \n");}
    for (i = 0; i < m; i++)
    {
        p[i] = malloc(sizeof(double) * n);
        if(!p[i]) {printf("matrix_alloc:: Unable to allocate memory!!! \n");}

    }
   return p;
}

malloc calls mmap if the allocation requested is large (the default is 128 KB) Refer http://man7.org/linux/man-pages/man3/malloc.3.html . mmap is restricted by RLIMIT_DATA resource limit. You can check the virtual memory limit for a process in your system by running the command "ulimit -v". If it returns "unlimited" then there is no restriction.

We can confirm whether memory is a problem by reducing the MAX_MESH value to 4 (if you want to check in a 32 bit machine). If you run the same program in a 64 bit machine, I would expect it to go through fine as the virtual memory available must be sufficient for your MAX_MESH value of 8.

Answer 2

Code runs out of memory, and prints an error, yet still continues on as if the allocation worked!

Code needs to change freeing of u an u_exact .

// for (i = 0; i < m_max + 2; i++) free(u[i]);
if (u != NULL) for (i = 0; i < m_max + 2; i++) free(u[i]);

---

The following leaks memory on failure as both the allocated matrix pointer and the partial columns allocated are lost. Could free allocated parts or error as below. Other approaches exist.

/* Allocates matrix columns */
for (i = 0; i < m; i++) {
    result[i] = malloc(n * sizeof(double));
    if (result[i] == NULL) {

        // add 2 lines
        do { free(result[i]); } while (i-- > 0);
        free(result);

        printf ("Error: no memmory available");
        return (NULL);
    }
}

---

Using size_t rather than int for array size is the C way as the size of int is not the definite type to use when sizing memory. Note that size_t is unsigned.

Allocating the memory all at once is a another approach. That may have alligmnet issues though, so follows is a 2 allocation approach.

void matrix_free(double **ptr) {
  if (ptr) {
    free(*ptr);
    free (ptr);
  }
}

double **matrix_alloc(size_t m, size_t n) {
  double **result = malloc(m * sizeof *result);
  double *data = malloc(m * n * sizeof *data);
  if (result == NULL || data == NULL) {
    free(result);
    free(data);
    printf ("Error: no memmory available");
    return (NULL);
  }
  for (size_t i=0; i<m; i++) {
    result[i] = &data[i*n];
  }
  return result;
}

void foo(void) {
  double **mat = matrix_alloc(3,4);
  ...
  matrix_free(mat);
  mat = NULL;
}