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| From | Shao Miller <sha0.miller@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: a = b or memset/cpy? |
| Date | 2012-02-07 21:58 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <jgso8d$ass$1@dont-email.me> (permalink) |
| References | <abb85a15-f68c-4e2f-8985-fe8b4945aca0@t8g2000yqg.googlegroups.com> <jgsdla$ol5$1@dont-email.me> |
On 2/7/2012 18:58, Shao Miller wrote:
>>
>> typedef struct x { int var0; char var1[20]; } X;
>>
>> X gX[30];
>>
>> void f(int slot)
>> {
>> X init = {0};
>>
>> gX[slot] = init;
>>
>> ...
>> }
>>
>> vs.
>> void f(int slot)
>> {
>> memset(&gX[slot], 0, sizeof(X));
>>
>> ...
>> }
>>
>
> Well these aren't the same. The former initializes all sub-objects to
> the "zeroey" values that would initialize a 'static'-storage-duration
> object having the same type as the sub-object and having no explicit
> initializer.
>
> The latter fills the object with bytes with the 'unsigned char' value
> '0', which is all-bits-zero.
>
> In your example, the 'struct' type 'X' has an 'int' member. The object
> representation of an 'int' can have padding bits that can be used any
> way the implementation pleases.
>
> If filling the padding bits with zeroes results in a trap representation
> for an 'int', then you might be in for a surprise.
>
Ben Bacarisse proved in another thread that my claim for a potential
surprise is false; there is no potential for all-zero-bits in an
integer's object representation to be a trap representation. Sorry
about that!
> There are similar concerns for other types, including pointers, where a
> null pointer value might not be all-bits-zero.
>
Still applies for other things, like pointers. :)
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a = b or memset/cpy? nroberts <roberts.noah@gmail.com> - 2012-02-07 09:02 -0800
Re: a = b or memset/cpy? Jens Gustedt <jens.gustedt@loria.fr> - 2012-02-07 18:41 +0100
Re: a = b or memset/cpy? James Kuyper <jameskuyper@verizon.net> - 2012-02-07 13:40 -0500
Re: a = b or memset/cpy? Jens Gustedt <jens.gustedt@loria.fr> - 2012-02-07 20:09 +0100
Re: a = b or memset/cpy? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2012-02-07 11:37 -0800
Re: a = b or memset/cpy? Shao Miller <sha0.miller@gmail.com> - 2012-02-07 18:35 -0500
Re: a = b or memset/cpy? nroberts <roberts.noah@gmail.com> - 2012-02-07 11:07 -0800
Re: a = b or memset/cpy? blp@cs.stanford.edu (Ben Pfaff) - 2012-02-07 11:14 -0800
Re: a = b or memset/cpy? nroberts <roberts.noah@gmail.com> - 2012-02-07 11:41 -0800
Re: a = b or memset/cpy? Shao Miller <sha0.miller@gmail.com> - 2012-02-07 18:39 -0500
Re: a = b or memset/cpy? "Scott Fluhrer" <sfluhrer@ix.netcom.com> - 2012-02-07 14:14 -0500
Re: a = b or memset/cpy? Shao Miller <sha0.miller@gmail.com> - 2012-02-07 18:58 -0500
Re: a = b or memset/cpy? Shao Miller <sha0.miller@gmail.com> - 2012-02-07 21:58 -0500
Re: a = b or memset/cpy? Jens Gustedt <jens.gustedt@loria.fr> - 2012-02-09 21:34 +0100
Re: a = b or memset/cpy? Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-07 20:34 -0500
Re: a = b or memset/cpy? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-02-08 18:43 +0000
Re: a = b or memset/cpy? jgk@panix.com (Joe keane) - 2012-02-08 20:05 +0000
Re: a = b or memset/cpy? Ian Collins <ian-news@hotmail.com> - 2012-02-09 09:18 +1300
Re: a = b or memset/cpy? Tim Prince <tim@nospamcomputer.org> - 2012-02-14 08:30 -0500
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