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pragma?

Started by kelaklub Jul 23, 2003 at 2:02 PM 10 replies 600+ views

Original Post

Jul 23, 2003 at 2:02 PM

I was looking at the code from Watt''s and Policarpo''s 3d games book and in his vector/matrix code I saw something that went like this in the very top line of his code...

#pragma(16)     // 16 byte allocation

what the heck is "pragma"? Thank you.

felisandria

143

felisandria

Jul 23, 2003 at 2:20 PM

"pragma" is a compiler directive used by the preprocessor. It''s a macro definition like, say, #define.

If you want to know what a specific one does you have to look it up.

-fel

Sneftel

296

Sneftel

Jul 23, 2003 at 2:27 PM

A bit more info: the C/C++ standards define #pragma as completely implementation specific. That is to say, each compiler developer is free to do whatever they want with #pragma; it''s used for "extensions" to the language that are not in the standard, stuff like structure padding, optimization hints, etc. So you''ll need to check the documentation for your individual compiler for more info.

How appropriate. You fight like a cow.

Jul 23, 2003 at 2:50 PM

And if you want to get rid of those annoying compiler warnings you can use:

#pragma warning(disable:4244)

kelaklub

Author

kelaklub

Jul 23, 2003 at 2:51 PM

The reason I ask is because the vector/matrix class they have implemented uses SIMD assembly instructions and if your compiler does not support them, then regular old c++ instructions are used to compute a vector times a scalar, etc. Does it really matter these days, or can you just code using c++, or would assembly really make a significant boost in a part of your code that is probably going to be called alot?

[edited by - kelaklub on July 23, 2003 3:52:05 PM]

Sneftel

296

Sneftel

Jul 23, 2003 at 2:53 PM

You''ll never know until you profile your code.

RapidStunna

Jul 23, 2003 at 2:54 PM

Hmm that made me wonder. Does the library linking (#pragma comment(lib,"")) work on most compilers? My game engine, which will later be open-source, uses that compiler directive. Is that safe?

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Brent Gunning | My Site

Yohumbus

Jul 23, 2003 at 2:58 PM

Is could be considered safe and you can certainly keep them there (the pragma''s shouldn''t do anything if they are not supported) but be sure to tell how to link those libraries via command line.

zealouselixir

157

zealouselixir

Jul 23, 2003 at 3:03 PM

Dev-C++ ignores "#pragma comment (lib, ...)", so be careful.

RapidStunna

Jul 23, 2003 at 3:09 PM

Okay I understand. So I''ll include the pragma directive, but I will make sure that non-VC++ users must link the libraries in manually. Are there any MINGW or BORLAND compiler directives to do the same? just so it makes engine usage easiest.

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Brent Gunning | My Site

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