#define _XOPEN_SOURCE 600 - where to define?
in [Unix Programming]
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From: Poster Matt on 7 Mar 2010 08:15 Yesterday I split one large C file and its header into a more manageable modular design consisting of 5 C files and headers. In my single C file approach it was simple, I defined _XOPEN_SOURCE at the top of the C file like this: #ifndef _XOPEN_SOURCE #define _XOPEN_SOURCE 600 #endif Having split the project into multiple .c and .h files, I received a confusing compiler warning (one of the modules uses nftw): "warning: 'struct FTW' declared inside parameter list" "warning: its scope is only this definition or declaration, which is probably not what you want" At first I thought this was a failure to #include <ftw.h> in the correct places but it wasn't. Eventually after a lot of frustrating attempts to get rid of the compiler warning I discovered that if I added this... #ifndef _XOPEN_SOURCE #define _XOPEN_SOURCE 600 #endif ....to the top of the C file which contains the (only) call to nftw() the warning is no longer shown. I suppose I am a bit confused about exactly where and when I should define "#define _XOPEN_SOURCE 600"? [and do I need the #ifndef...#endif around the #define?] Can someone explain this to me please? Thanks and regards, from a somewhat confused, Matt
From: Jens Thoms Toerring on 7 Mar 2010 09:06 Poster Matt <postermatt(a)no_spam_for_me.org> wrote: > Yesterday I split one large C file and its header into a more manageable modular > design consisting of 5 C files and headers. > In my single C file approach it was simple, I defined _XOPEN_SOURCE at the top > of the C file like this: > #ifndef _XOPEN_SOURCE > #define _XOPEN_SOURCE 600 > #endif > Having split the project into multiple .c and .h files, I received a confusing > compiler warning (one of the modules uses nftw): > "warning: 'struct FTW' declared inside parameter list" > "warning: its scope is only this definition or declaration, which is probably > not what you want" > At first I thought this was a failure to #include <ftw.h> in the correct places > but it wasn't. Eventually after a lot of frustrating attempts to get rid of the > compiler warning I discovered that if I added this... > #ifndef _XOPEN_SOURCE > #define _XOPEN_SOURCE 600 > #endif > ...to the top of the C file which contains the (only) call to nftw() the warning > is no longer shown. > I suppose I am a bit confused about exactly where and when I should define > "#define _XOPEN_SOURCE 600"? [and do I need the #ifndef...#endif around the > #define?] You should define _XOPEN_SOURCE before you include <ftw.h> since what gets included from <ftw.h> already may depend on this macro being set or not (e.g. nftw() or 'struct FTW' may not become de- clared if it's not set). So the best place is probably in the header file that includes <ftw.h>, before the line where you include <ftw.h> (or in the C file if you include <ftw.h> there and not in one of your header files). The '#ifndef _XOPEN_SOURCE' is useful if there's a chance that _XOPEN_SOURCE has already been defined somewhere else (e.g. in some other header file). In that case trying to define it anew would upset the compiler. BTW, according to my (Linux) man page for nftw() you need to set _XOPEN_SOURCE to 500 only. Regards, Jens -- \ Jens Thoms Toerring ___ jt(a)toerring.de \__________________________ http://toerring.de
From: David Schwartz on 7 Mar 2010 09:42 On Mar 7, 5:15 am, Poster Matt <postermatt(a)no_spam_for_me.org> wrote: > I suppose I am a bit confused about exactly where and when I should define > "#define _XOPEN_SOURCE 600"? [and do I need the #ifndef...#endif around the > #define?] You have a lot of choices, but the short version is that any C/C++ files that uses POSIX features should specify the version of the POSIX standard that it was coded to. You can add it to the compile command line with "-D_XOPEN_SOURCE=600". You can put it in each C/C++ file before any files are included. You can put it in your include files before any system headers are included. > Can someone explain this to me please? You would be pretty angry if you had included 'ftw.h' to get the prototype for 'ftw' and had your own function called 'nftw'. The compiler has no way to know that you've seen the 2008 (or later) POSIX specification unless you tell it. DS
From: Ben Bacarisse on 7 Mar 2010 09:46 Poster Matt <postermatt(a)no_spam_for_me.org> writes: > Yesterday I split one large C file and its header into a more > manageable modular design consisting of 5 C files and headers. > > In my single C file approach it was simple, I defined _XOPEN_SOURCE at > the top of the C file like this: > > #ifndef _XOPEN_SOURCE > #define _XOPEN_SOURCE 600 > #endif <snip> > I suppose I am a bit confused about exactly where and when I should > define "#define _XOPEN_SOURCE 600"? [and do I need the > #ifndef...#endif around the #define?] I'd say you don't need the #ifndef. In fact I'd say it might even be wrong to have it (unless you know why it was written like that in the first place). There macros are state what features (usually from the system headers) your code needs. If your code needs _XOPEN_SOURCE to be 600, the honouring some other setting may not be correct. If you just #define _XOPEN_SOURCE then you'll helpfully get an error if some other part of the build sets it to something else. Alternatively, you can silently force it to be 600 by using #undef first. This is just a rule of thumb. The lines you used might come from a build system where only acceptable values for _XOPEN_SOURCE can be chosen but where 600 should be used as a default. My point is that it is probably wrong unless you know why it is written like that. -- Ben.
From: Poster Matt on 7 Mar 2010 10:18
Jens Thoms Toerring wrote: > Poster Matt <postermatt(a)no_spam_for_me.org> wrote: >> Yesterday I split one large C file and its header into a more manageable modular >> design consisting of 5 C files and headers. > >> In my single C file approach it was simple, I defined _XOPEN_SOURCE at the top >> of the C file like this: > >> #ifndef _XOPEN_SOURCE >> #define _XOPEN_SOURCE 600 >> #endif > >> Having split the project into multiple .c and .h files, I received a confusing >> compiler warning (one of the modules uses nftw): > >> "warning: 'struct FTW' declared inside parameter list" >> "warning: its scope is only this definition or declaration, which is probably >> not what you want" > >> At first I thought this was a failure to #include <ftw.h> in the correct places >> but it wasn't. Eventually after a lot of frustrating attempts to get rid of the >> compiler warning I discovered that if I added this... > >> #ifndef _XOPEN_SOURCE >> #define _XOPEN_SOURCE 600 >> #endif > >> ...to the top of the C file which contains the (only) call to nftw() the warning >> is no longer shown. > >> I suppose I am a bit confused about exactly where and when I should define >> "#define _XOPEN_SOURCE 600"? [and do I need the #ifndef...#endif around the >> #define?] > > You should define _XOPEN_SOURCE before you include <ftw.h> since > what gets included from <ftw.h> already may depend on this macro > being set or not (e.g. nftw() or 'struct FTW' may not become de- > clared if it's not set). So the best place is probably in the > header file that includes <ftw.h>, before the line where you > include <ftw.h> (or in the C file if you include <ftw.h> there > and not in one of your header files). > > The '#ifndef _XOPEN_SOURCE' is useful if there's a chance that > _XOPEN_SOURCE has already been defined somewhere else (e.g. in > some other header file). In that case trying to define it anew > would upset the compiler. > > BTW, according to my (Linux) man page for nftw() you need to set > _XOPEN_SOURCE to 500 only. > Regards, Jens Okay. Many thanks Jens. |