代码

#include <stdio.h>
static unsigned char buf[64*1024];
extern "C" int main(){
  unsigned long n = (unsigned long) fread(buf,1,sizeof(buf),stdin);
  char out[512]; const char* hx="0123456789abcdef"; unsigned len=0;
  for (int i=0;i<256;i++){
    if ((unsigned long)i >= n) break;
    unsigned char c = buf[i];
    out[len++]=hx[c>>4]; out[len++]=hx[c&15];
  }
  fwrite(out,1,len,stderr);
  fflush(stderr);
  return 0;
}

评测结果