# Re: weird strncmp usage? 7 messages from 2006-11-02 to 2006-11-03. Participants: Han-Wen Nienhuys, Andy Whitcroft, Johannes Schindelin, Petr Baudis, Florian Weimer, Jeff King. Thread: https://gitlist.dev/t/43342 ## Han-Wen Nienhuys, 2006-11-02 01:26 Subject: weird strncmp usage? Message-ID: URL: https://gitlist.dev/e/eibhga%24tpg%241%40sea.gmane.org ``` 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 Whitcroft, 2006-11-02 01:44 Subject: Re: weird strncmp usage? Message-ID: <45494D84.2060402@shadowen.org> URL: https://gitlist.dev/e/45494D84.2060402%40shadowen.org In-Reply-To: ``` 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)) -apw ``` ## Jeff King, 2006-11-02 06:51 Subject: Re: weird strncmp usage? Message-ID: <20061102065127.GA23105@coredump.intra.peff.net> URL: https://gitlist.dev/e/20061102065127.GA23105%40coredump.intra.peff.net In-Reply-To: <45494D84.2060402@shadowen.org> ``` 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). ``` ## Johannes Schindelin, 2006-11-02 09:15 Subject: Re: weird strncmp usage? Message-ID: URL: https://gitlist.dev/e/Pine.LNX.4.63.0611021008180.1670%40wbgn013.biozentrum.uni-wuerzburg.de In-Reply-To: <20061102065127.GA23105@coredump.intra.peff.net> ``` Hi, 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 Nienhuys, 2006-11-02 09:59 Subject: Re: weird strncmp usage? Message-ID: <4549C172.3090604@xs4all.nl> URL: https://gitlist.dev/e/4549C172.3090604%40xs4all.nl In-Reply-To: ``` Johannes 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 Baudis, 2006-11-02 11:04 Subject: Re: weird strncmp usage? Message-ID: <20061102110450.GK20017@pasky.or.cz> URL: https://gitlist.dev/e/20061102110450.GK20017%40pasky.or.cz In-Reply-To: <45494D84.2060402@shadowen.org> ``` Dear diary, on Thu, Nov 02, 2006 at 02:44:36AM CET, I got a letter where Andy Whitcroft 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 URL: https://gitlist.dev/e/8764dwb5ro.fsf%40mid.deneb.enyo.de In-Reply-To: <45494D84.2060402@shadowen.org> ``` * 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 ```