Daniel Spångber's C code, when reading whole file, and using table to count. See http://saml.rilspace.org/node/248

gc_ds_wholefile_tbl.c

#include <stdio.h>
#include <stdlib.h>

#define MAXFLEN 70000000 /* Larger than the file. */

int main()
{
  char *m=malloc(MAXFLEN);
  char tablegc[256];
  char tableat[256];
  int gc=0;
  int at=0;
  FILE *f=fopen("Homo_sapiens.GRCh37.67.dna_rm.chromosome.Y.fa","r");
  int items=fread(m,1,MAXFLEN,f); /* Read the whole file into memory. */
  char *ptr=m;
  int nums, i;
  while (*ptr++!='\n'); /* Find end of first line. */
  for (i=0; i<256; i++)
    tablegc[i]=0;
  for (i=0; i<256; i++)
    tableat[i]=0;
  tableat['A']=1;
  tableat['T']=1;
  tablegc['C']=1;
  tablegc['G']=1;
  nums=items-(ptr-m);
  for (i=0; i<nums; i++) {
    char c=*ptr++;
    at+=tableat[(int)c];
    gc+=tablegc[(int)c];
  }
  fclose(f);
  free(m);
  int total = at + gc;
  printf("%.10f\n",(100.*gc)/total);
  return 0;
}