如何在GCC 4.6.3中更改堆和堆栈内存大小？

Question

使用malloc时我注意到指定的指针为null。我想在执行代码期间将大约30 MB的内存分配给堆内存。如何在Ubuntu 12.04 LTS上使用GCC 4.6.3作为编译器？我应该分配更多的堆栈内存吗？它会更好吗？

由于

编辑代码中有问题的部分，代码在if语句之后终止。

#include <stdio.h>
#include <math.h>
#include <limits.h>
#include <stdlib.h>
#define a0 0.53 
#define N LONG_MAX
// This value of N is the highest possible number in long double
// data format over 100. Higher is not possible due to the size of 
// the default heap memory. Change its value  to adjust the precision of 
// integration and computation time. Since exponential decays really fast
// a modest value of N is enough for this integration.
// The discrete integral may be defined as follows:
long double trapezoid(long double x[],long double f[]) {
  int i;
  long double dx = x[1]-x[0];
  long double sum = 0.5*(f[0]+f[N]);

    for (i = 1; i <  N; i++) 
        sum+=f[i];
  return sum*dx;  
}
main() {
    printf("LONG_MAX constant has the following value: %ld \n", LONG_MAX);
    long double * P = malloc(N * sizeof(long double));
    long double * r = malloc(N * sizeof(long double));
    long double * PP = malloc(N * sizeof(long double));
    // The if statement is to check if the memory allocation has 
    // been done and terminate the program otherwise.
    if (P == 0 || r == 0)
        {
            printf("ERROR: Out of memory\n");
            return 1;
        }
    // Declare and initialize the loop variable
}