C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  Size of a struct - totally confused          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message James Gregory Guest Posted: Fri Jan 30, 2004 11:29 pm    Post subject: Size of a struct - totally confused I'm pretty sure this is non-totally-trivial enough and standard-C++ enough to post here, however much it may initially look like a "how do I make a computer game?" question, though I may be wrong. I'm loading a bitmap, and as I'm using Linux I've had to define BITMAPFILEHEADER myself (maybe better practice would be to give my version a new name or whatever, I dunno). Windows uses WORDs and DWORDs to define it, I've used some standard-size types provided by SDL (a graphics/sound library thing) to define it thus: struct BITMAPFILEHEADER { Uint16 bfType; Uint32 bfSize; Uint16 bfReserved1; Uint16 bfReserved2; Uint32 bfOffBits; }; For some reason I utterly can't fathom, however, a debugger shows that my bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16. Surely that should be 14? I then tried redefining it a couple of times: BITMAPFILEHEADER { Uint32 bfType; //changed from 16 to 32 Uint32 bfSize; Uint16 bfReserved1; Uint16 bfReserved2; Uint32 bfOffBits; }; This gives sizeof(BITMAPFILEHEADER) == 16, which is now correct. BITMAPFILEHEADER { Uint32 bfType; Uint32 bfSize; Uint32 bfReserved1; Uint32 bfReserved2; Uint32 bfOffBits; }; This gives sizeof(BITMAPFILEHEADER) == 20, which is also correct. Most likely I just misunderstand something somewhere, but I've no idea what. Thanks, James Back to top Rolf Magnus Guest Posted: Fri Jan 30, 2004 11:42 pm    Post subject: Re: Size of a struct - totally confused James Gregory wrote: Quote: I'm pretty sure this is non-totally-trivial enough and standard-C++ enough to post here, however much it may initially look like a "how do I make a computer game?" question, though I may be wrong. I'm loading a bitmap, You mean a .bmp file? The term "bitmap" has a different meaning outside the Microsoft world. Quote: and as I'm using Linux I've had to define BITMAPFILEHEADER myself (maybe better practice would be to give my version a new name or whatever, I dunno). Windows uses WORDs and DWORDs to define it, I've used some standard-size types provided by SDL (a graphics/sound library thing) to define it thus: struct BITMAPFILEHEADER { Uint16 bfType; Uint32 bfSize; Uint16 bfReserved1; Uint16 bfReserved2; Uint32 bfOffBits; }; For some reason I utterly can't fathom, however, a debugger shows that my bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16. What do you mean by "believes"? Quote: Surely that should be 14? Nope. You seem to be getting alignment. For faster access (and on many hardware platforms for being able to access it at all), 32bit variables are often alinged on a 32bit basis. So your compiler probably added two bytes of empty space between the first and second member to align it. Quote: I then tried redefining it a couple of times: BITMAPFILEHEADER { Uint32 bfType; //changed from 16 to 32 Uint32 bfSize; Uint16 bfReserved1; Uint16 bfReserved2; Uint32 bfOffBits; }; This gives sizeof(BITMAPFILEHEADER) == 16, which is now correct. BITMAPFILEHEADER { Uint32 bfType; Uint32 bfSize; Uint32 bfReserved1; Uint32 bfReserved2; Uint32 bfOffBits; }; This gives sizeof(BITMAPFILEHEADER) == 20, which is also correct. Most likely I just misunderstand something somewhere, but I've no idea what. The above shows one reason why it's never a good idea to read binary data directly into a struct. It's not portable. Also think what happens if your program will be compiled e.g. on linux/ppc. Your program will stop to work because of endianness problems. And as I wrote above, some systems can't even access unaligned memory. So better read each value separately, and if you want portability, consider things like endianness conversion. Back to top White Wolf Guest Posted: Fri Jan 30, 2004 11:45 pm    Post subject: Re: Size of a struct - totally confused James Gregory wrote: Quote: I'm pretty sure this is non-totally-trivial enough and standard-C++ enough to post here, however much it may initially look like a "how do I make a computer game?" question, though I may be wrong. I'm loading a bitmap, and as I'm using Linux I've had to define BITMAPFILEHEADER myself (maybe better practice would be to give my version a new name or whatever, I dunno). Windows uses WORDs and DWORDs to define it, I've used some standard-size types provided by SDL (a graphics/sound library thing) to define it thus: struct BITMAPFILEHEADER { Uint16 bfType; Uint32 bfSize; Uint16 bfReserved1; Uint16 bfReserved2; Uint32 bfOffBits; }; For some reason I utterly can't fathom, however, a debugger shows that my bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16. Surely that should be 14? [SNIP] Not necessarily: bfType: 2 Padding to get bfSize to start again on int boundary: 2 bfSize: 4 Res1: 2 Res2: 2 bfOffBits: 4 That is 16. You may want to look at the gcc/g++ docs how to ask packing for a struct. -- WW aka Attila ::: Be bold in what you stand for and careful what you fall for. Back to top Phlip Guest Posted: Fri Jan 30, 2004 11:50 pm    Post subject: Re: Size of a struct - totally confused James Gregory wrote: Quote: I'm pretty sure this is non-totally-trivial enough and standard-C++ enough to post here, however much it may initially look like a "how do I make a computer game?" question, though I may be wrong. Relax - padding is on-topic. Quote: I'm loading a bitmap, and as I'm using Linux I've had to define BITMAPFILEHEADER myself (maybe better practice would be to give my version a new name or whatever, I dunno). There must be a graphics library you can use. Start with ImageMagick. Quote: Windows uses WORDs and DWORDs to define it, I've used some standard-size types provided by SDL (a graphics/sound library thing) to define it thus: struct BITMAPFILEHEADER { Uint16 bfType; Uint32 bfSize; Uint16 bfReserved1; Uint16 bfReserved2; Uint32 bfOffBits; }; For some reason I utterly can't fathom, however, a debugger shows that my bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16. Surely that should be 14? Plenty of hardware works best reading integers aligned on quad-word boundaries. Put another way, the address of a bfSize can evenly divide by 4 with 0 remainder. The compiler inserted an invisible 16-bit padding element between bfType and bfSize. Compilers are allowed to do this, and required to give notice how they do it. It's "implementation specified". Now ask the documentation or forum that covers your compiler how to turn it off. They will probably point out a library that already reads BMP files, too. -- Phlip http://www.xpsd.org/cgi-bin/wiki?TestFirstUserInterfaces Back to top Victor Bazarov Guest Posted: Fri Jan 30, 2004 11:50 pm    Post subject: Re: Size of a struct - totally confused "James Gregory" <jamesg (AT) f2s (DOT) com> wrote... Quote: I'm pretty sure this is non-totally-trivial enough and standard-C++ enough to post here, however much it may initially look like a "how do I make a computer game?" question, though I may be wrong. I'm loading a bitmap, and as I'm using Linux I've had to define BITMAPFILEHEADER myself (maybe better practice would be to give my version a new name or whatever, I dunno). Windows uses WORDs and DWORDs to define it, I've used some standard-size types provided by SDL (a graphics/sound library thing) to define it thus: struct BITMAPFILEHEADER { Uint16 bfType; Uint32 bfSize; Uint16 bfReserved1; Uint16 bfReserved2; Uint32 bfOffBits; }; For some reason I utterly can't fathom, however, a debugger shows that my bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16. Surely that should be 14? I then tried redefining it a couple of times: BITMAPFILEHEADER { Uint32 bfType; //changed from 16 to 32 Uint32 bfSize; Uint16 bfReserved1; Uint16 bfReserved2; Uint32 bfOffBits; }; This gives sizeof(BITMAPFILEHEADER) == 16, which is now correct. BITMAPFILEHEADER { Uint32 bfType; Uint32 bfSize; Uint32 bfReserved1; Uint32 bfReserved2; Uint32 bfOffBits; }; This gives sizeof(BITMAPFILEHEADER) == 20, which is also correct. Most likely I just misunderstand something somewhere, but I've no idea what. Sizes of object can include so called "padding" to ensure that the objects in an array begin at a particular multiple of bytes boundary. For example, on some platforms it should be multiple of 4 to make memory access faster. 14 is not multiple of 4, so it chooses the next larger multiple of 4. There probably is a [compiler-specific] way to control the padding or the alignment factor, consult your compiler documentation to find out how to do that. Victor Back to top E. Robert Tisdale Guest Posted: Sat Jan 31, 2004 12:07 am    Post subject: Re: Size of a struct - totally confused James Gregory wrote: Quote: I'm pretty sure this is non-totally-trivial enough and standard-C++ enough to post here, however much it may initially look like a "how do I make a computer game?" question, though I may be wrong. I'm loading a bitmap, and as I'm using Linux I've had to define BITMAPFILEHEADER myself (maybe better practice would be to give my version a new name or whatever, I dunno). Windows uses WORDs and DWORDs to define it, I've used some standard-size types provided by SDL (a graphics/sound library thing) to define it thus: #include<stdint.h> Quote: struct BITMAPFILEHEADER { uint16_t bfType; uint32_t bfSize; uint16_t bfReserved1; uint16_t bfReserved2; uint32_t bfOffBits; }; For some reason I utterly can't fathom, however, a debugger shows that my bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16. Surely that should be 14? Apparently, your compiler and/or machine architecture requires unint32_t alignment on 32 bit word boundaries. Try this: struct X { int i; double x; int j; }; and this: struct Y { double x; int i; int j; }; A double probably requires a 64 bit word alignment. If this is important to you, you should try place the objects with the coarsest alignment requirements first. Back to top Default User Guest Posted: Sat Jan 31, 2004 12:14 am    Post subject: Re: Size of a struct - totally confused James Gregory wrote: Quote: For some reason I utterly can't fathom, however, a debugger shows that my bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16. Surely that should be 14? http://www.eskimo.com/~scs/C-faq/q2.13.html Brian Rodenborn Back to top Gianni Mariani Guest Posted: Sat Jan 31, 2004 12:30 am    Post subject: Re: Size of a struct - totally confused James Gregory wrote: .... Quote: For some reason I utterly can't fathom, however, a debugger shows that my bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16. Surely that should be 14? So many people have already answered why it's not. However - try this: #pragma pack(1) struct BITMAPFILEHEADER { Uint16 bfType; Uint32 bfSize; Uint16 bfReserved1; Uint16 bfReserved2; Uint32 bfOffBits; }; #pragma pack() The "pack" pragma is not standard but I have yet to meet a compiler that does not implement it (in some way). .... Back to top Jack Klein Guest Posted: Sat Jan 31, 2004 4:27 am    Post subject: Re: Size of a struct - totally confused On Fri, 30 Jan 2004 23:50:33 GMT, "Phlip" <phlip_cpp (AT) yahoo (DOT) com> wrote in comp.lang.c++: [snip] Quote: Plenty of hardware works best reading integers aligned on quad-word boundaries. Put another way, the address of a bfSize can evenly divide by 4 with 0 remainder. The compiler inserted an invisible 16-bit padding element between bfType and bfSize. Compilers are allowed to do this, and required to give notice how they do it. It's "implementation specified". Both the C and C++ standards define the term "implementation-defined", not "implementation specified". And both spell it with the hyphen. -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html Back to top Jack Klein Guest Posted: Sat Jan 31, 2004 4:28 am    Post subject: Re: Size of a struct - totally confused On Fri, 30 Jan 2004 16:07:58 -0800, "E. Robert Tisdale" <E.Robert.Tisdale (AT) jpl (DOT) nasa.gov> wrote in comp.lang.c++: Quote: James Gregory wrote: I'm pretty sure this is non-totally-trivial enough and standard-C++ enough to post here, however much it may initially look like a "how do I make a computer game?" question, though I may be wrong. I'm loading a bitmap, and as I'm using Linux I've had to define BITMAPFILEHEADER myself (maybe better practice would be to give my version a new name or whatever, I dunno). Windows uses WORDs and DWORDs to define it, I've used some standard-size types provided by SDL (a graphics/sound library thing) to define it thus: #include Non standard in C++. Be all right if you were posting in comp.lang.c, though. -- Jack Klein Home: http://JK-Technology.Com FAQs for comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html comp.lang.c++ http://www.parashift.com/c++-faq-lite/ alt.comp.lang.learn.c-c++ http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html Back to top James Gregory Guest Posted: Sat Jan 31, 2004 11:14 am    Post subject: Re: Size of a struct - totally confused On Fri, 30 Jan 2004 23:29:26 +0000, James Gregory wrote: Quote: This gives sizeof(BITMAPFILEHEADER) == 20, which is also correct. Most likely I just misunderstand something somewhere, but I've no idea what. Thanks, Wow, thanks for all the replies, most helpful. James Back to top EventHelix.com Guest Posted: Sat Jan 31, 2004 1:21 pm    Post subject: Re: Size of a struct - totally confused The compiler is adding pad bytes. The following article will help: http://www.eventhelix.com/RealtimeMantra/ByteAlignmentAndOrdering.htm Sandeep -- http://www.EventHelix.com/EventStudio EventStudio 2.0 - System Architecture Design CASE Tool Back to top Display posts from previous: All Posts1 Day7 Days2 Weeks1 Month3 Months6 Months1 Year Oldest FirstNewest First         C++Talk.NET Forum Index -> C++ language (comp.lang.c++) All times are GMT Page 1 of 1   Jump to: Select a forum C/C++----------------C LanguageC Language (Moderated)C++ Language (Moderated)C++ language (comp.lang.c++)C++ language, library and standards Non-English C/C++ Newsgroups----------------C++ (German)C++ (French)  You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum Powered by phpBB © 2001, 2006 phpBB Group SEO toolkit © 2004-2006 webmedic.