Faster I/O in C

Question

I have a problem which will take 1000000 lines of inputs like below from console.

0 1 23 4 5
1 3 5 2 56
12 2 3 33 5
...
...

I have used scanf,

Answer 1

If your example is representative, that you indeed have a fixed format of five decimal numbers per line, I'd probably use a combination of fgets() to read the lines, then a loop calling strtol() to convert from string to integer.

That should be faster than scanf(), while still clearer and more high-level than doing the string to integer conversion on your own.

Something like this:

typedef struct {
  int number[5];
} LineOfNumbers;

int getNumbers(FILE *in, LineOfNumbers *line)
{
  char buf[128];  /* Should be large enough. */
  if(fgets(buf, sizeof buf, in) != NULL)
  {
    int i;
    char *ptr, *eptr;

    ptr = buf;
    for(i = 0; i < sizeof line->number / sizeof *line->number; i++)
    {
      line->number[i] = (int) strtol(ptr, &eptr, 10);
      if(eptr == ptr)
        return 0;
      ptr = eptr;
    }
    return 1;
  }
  return 0;
}

Note: this is untested (even uncompiled!) browser-written code. But perhaps useful as a concrete example.