How to initialize all members of an array to the same value?

Question

I have a large array in C (not C++ if that makes a difference). I want to initialize all members of the same value.

I could swear I

Answer 1

There is a fast way to initialize array of any type with given value. It works very well with large arrays. Algorithm is as follows:

• initialize first element of the array (usual way)
• copy part which has been set into part which has not been set, doubling the size with each next copy operation

---

For 1 000 000 elements int array it is 4 times faster than regular loop initialization (i5, 2 cores, 2.3 GHz, 4GiB memory, 64 bits):

loop runtime 0.004248 [seconds]

memfill() runtime 0.001085 [seconds]

---

#include 
#include 
#include 
#define ARR_SIZE 1000000

void memfill(void *dest, size_t destsize, size_t elemsize) {
   char   *nextdest = (char *) dest + elemsize;
   size_t movesize, donesize = elemsize;

   destsize -= elemsize;
   while (destsize) {
      movesize = (donesize < destsize) ? donesize : destsize;
      memcpy(nextdest, dest, movesize);
      nextdest += movesize; destsize -= movesize; donesize += movesize;
   }
}    
int main() {
    clock_t timeStart;
    double  runTime;
    int     i, a[ARR_SIZE];

    timeStart = clock();
    for (i = 0; i < ARR_SIZE; i++)
        a[i] = 9;    
    runTime = (double)(clock() - timeStart) / (double)CLOCKS_PER_SEC;
    printf("loop runtime %f [seconds]\n",runTime);

    timeStart = clock();
    a[0] = 10;
    memfill(a, sizeof(a), sizeof(a[0]));
    runTime = (double)(clock() - timeStart) / (double)CLOCKS_PER_SEC;
    printf("memfill() runtime %f [seconds]\n",runTime);
    return 0;
}