Random C issue, was (Re: [vorbis-dev] getting playback length from ogg vorbis file)

[Jack Angel]

On Fri, 11 Jul 2003 23:33:22 -0400
"Kenneth C. Arnold" <kcarnold at arnoldnet.net> wrote:

> Jack Angel wrote:
> 
> >This doesn't work because <math.h> isn't included, and floor definition is picked up from somewhere else. I don't know from where, but check this:
> >
> >~# cat test.c
> >//#include <math.h>
> >
> >int main()
> >{
> >  double time = 800.510;
> >  printf("%f %f\n", time, floor(time));
> >}
> >~# ./test
> >800.510000 80298907385280950682755444182387957006805762791233118433533549849731864380320828326878948200747786192971450917286372883160777116601003408797083834721392632613345848270168843448871733524979042229249230391219772018492598287395874874133804539689308263302039462290421043577395770761936896.000000
> >
> 
> Compile with -Wall and you'll see why:
> 
> test.c:6: warning: implicit declaration of function `printf'
> test.c:6: warning: implicit declaration of function `floor'
> test.c:6: warning: double format, different type arg (arg 3)
> test.c:7: warning: control reaches end of non-void function
> 
> gcc sees no prototype for "floor" and assumes that it is:
> 
> int floor(int)
> 
> (which is the "implicit declaration") because some C standard said that 
> "int" is default. Not only are "int" and "double" different sizes (so 
> you just corrupted your stack; "main" pushed 8 byes and libc popped 4), 

<p>why doesn't it coerce this double to int as it would if i declared floor as
"int floor(int)" ?

<p>> but the first 4 byes of a "double" are absolutely meaningless when 
> interpreted as an "int".
> 
> This is, of couse, why compiler warnings are actually a good thing. 
> CFLAGS += "-Wall".
> 
> Ken
> 
> 
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<p>
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