primes in C/C++

primes.c

#include <stdlib.h>
#include <alloca.h>
#include <stdio.h>
#include <sys/time.h>

int* get_primes(int n, int *l) {
  char* compo=(char*)alloca(n);
  int* primes=(int*)malloc(sizeof(int)*n);
  int i,j,c=0;
  for(i=0;i<n;++i) compo[i]=0;
  for(i=2;i<n;++i) {
     if (compo[i]) continue;
     primes[c++]=i;
     for(j=i*2;j<n;j+=i) compo[j]=1;
  }
  *l=c;
  return primes;
}

int main() {
  struct timeval timer[2];
  int i, l;
  double dt;
  int *primes=get_primes(100, &l);
  /*
  printf("we have %d primes: ", l);
  for(i=0;i<l;++i) printf("%d ", primes[i]);
  printf("\n");
  */
  gettimeofday(&timer[0], NULL);
  for(i=0;i<500;++i) get_primes(10000, &l);
  gettimeofday(&timer[1], NULL);
  printf("t0: %d %d\n", timer[0].tv_sec, timer[0].tv_usec);
  printf("t1: %d %d\n", timer[1].tv_sec, timer[1].tv_usec);
  dt=timer[1].tv_sec + timer[1].tv_usec/1000000.0;
  dt-=timer[0].tv_sec + timer[0].tv_usec/1000000.0;
  printf("dt=%g\n", dt);
}