Han-Wen Nienhuysweird strncmp usage?
Hi,
the git source seems full of calls similar to
strncmp (x, "constant string", 15)
is there a reason not to use something like
int
strxmp (char const *x, char const *y)
{
return strncmp (x, y, strlen (y));
}everywhere?
-- Han-Wen Nienhuys - hanwen@xs4all.nl - http://www.xs4all.nl/~hanwen
Andy WhitcroftHan-Wen Nienhuys wrote:
>
> Hi,
>
> the git source seems full of calls similar to
>
> strncmp (x, "constant string", 15)
>
> is there a reason not to use something like
>
> int
> strxmp (char const *x, char const *y)
> {
> return strncmp (x, y, strlen (y));
> }
>
> everywhere?If you are doing these a _lot_ then there is a significant additional cost to using strlen on a constant string.
That said if you know its constant you can also use sizeof("foo") and that is done at compile time. Something like:
#define strxcmp(x, y) strncmp((x), (y), sizeof((y))
-apw
Jeff KingOn Thu, Nov 02, 2006 at 01:44:36AM +0000, Andy Whitcroft wrote:
> That said if you know its constant you can also use sizeof("foo") and
> that is done at compile time. Something like:
>
> #define strxcmp(x, y) strncmp((x), (y), sizeof((y))You would, of course, need to use sizeof(y)-1 to avoid comparing the NUL termination. :)
This is a slightly dangerous macro, because it _only_ works for string literals, but not pointers (which is fine in this case, but its limitations need to be documented).
Johannes SchindelinHi,
On Thu, 2 Nov 2006, Jeff King wrote:
> On Thu, Nov 02, 2006 at 01:44:36AM +0000, Andy Whitcroft wrote:
>
> > That said if you know its constant you can also use sizeof("foo") and
> > that is done at compile time. Something like:
> >
> > #define strxcmp(x, y) strncmp((x), (y), sizeof((y))
>
> You would, of course, need to use sizeof(y)-1 to avoid comparing the NUL
> termination. :)
>
> This is a slightly dangerous macro, because it _only_ works for string
> literals, but not pointers (which is fine in this case, but its
> limitations need to be documented).It would be even better to avoid these errors by doing something like
#define starts_with(x, y) !strncmp((x), #y, sizeof(#y) - 1)
which would be used like this:
if (starts_with(arg, --abbrev=))
However, in this case, you would need another macro, which automatically extracts the argument, and soon you will end up with yet another getopt package.
Ciao, Dscho
Han-Wen NienhuysJohannes Schindelin escreveu:
> However, in this case, you would need another macro, which automatically > extracts the argument, and soon you will end up with yet another getopt > package.
It puzzles me that git doesn't use getopt. Git is rare in not accepting
--option arg
but only
--option=arg
Also, -abc doesn't seem to work for combining options.
Petr BaudisDear diary, on Thu, Nov 02, 2006 at 02:44:36AM CET, I got a letter where Andy Whitcroft <apw@shadowen.org> said that...
> Han-Wen Nienhuys wrote:
> >
> > Hi,
> >
> > the git source seems full of calls similar to
> >
> > strncmp (x, "constant string", 15)
> >
> > is there a reason not to use something like
> >
> > int
> > strxmp (char const *x, char const *y)
> > {
> > return strncmp (x, y, strlen (y));
> > }
> >
> > everywhere?
>
> If you are doing these a _lot_ then there is a significant additional
> cost to using strlen on a constant string.
>
> That said if you know its constant you can also use sizeof("foo") and
> that is done at compile time. Something like:
>
> #define strxcmp(x, y) strncmp((x), (y), sizeof((y))At least in the #define, reasonable compilers should optimize out strlen("foo"). Hopefully as well if you would make strxmp() inlinable though I'm not so sure there.
--
Petr "Pasky" Baudis
Stuff: http://pasky.or.cz/
#!/bin/perl -sp0777i<X+d*lMLa^*lN%0]dsXx++lMlN/dsM0<j]dsj
$/=unpack('H*',$_);$_=`echo 16dio\U$k"SK$/SM$n\EsN0p[lN*1* Andy Whitcroft:
> If you are doing these a _lot_ then there is a significant additional > cost to using strlen on a constant string.
strlen has been a GCC builtin for quite some time. If the wrapper function is inlined, GCC will optimize it away. (It also turns the