Re: CRC in Ada?

[ohk@edeber.tfdt-o.nta.no (Ole-Hjalmar Kristensen FOU.TD/DELAB)]

In article <dewar.857693222@merv> dewar@merv.cs.nyu.edu (Robert Dewar) writes:

   Fergus asks

   <<>In my experience it is generally C programmers who make this mistake.

   Why is it a mistake?>>

   Tell you what Fergus, do a whole lot of timing tests comparing reading
   character by character and reading large blocks, and come back and tell
   us if it did not tell you the answer to that questin :-)

OK, I'll bite:

The following two simple programs have been tested on a 24443068 byte
file, both compiled wit -O7 on the Centerline C compiler:

#include <stdio.h>

main() {
   setvbuf(stdin,0,_IOFBF,4096);
   int c,total=0;
   while ((c = getc(stdin)) != EOF) total++;
   printf("total %d\n",total);
}

Average results obtained with time:
2.28u 0.28s 0:02.66 96.2%

char buf[4096];

main() {
   int c,n,total=0;
   while ((n = read(0,buf,sizeof(buf))) > 0) {
      int i;
      for (i = 0; i < n; i++) {
         c = buf[i];
         total++;
      }
   }
   printf("total %d\n",total);
}

Average results obtained with time:
0.12u 0.29s 0:00.41 100.0%


As you can see, the time used by the OS is pretty much the same in
both cases. 

It is somewhat surprising that the difference between the user CPU
times is so large, but I'm more surprised by the speed of the second
version than by the slowness of the first.

Depending on what your program is really going to do with the 24443068
bytes, the 2.5 seconds spent in the C standard IO library may or may
not be significant....