C Programming Language :: help: define problem and meaning of {{{ and }}}.


	help: define problem and meaning of {{{ and }}}. for struct: struct in6_addr { uint8_t s6_addr[16]; }; is provided a costant: #define IN6ADDR_LOOPBACK_INIT {{{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1}}} what does means {{{, and }}}? why can be used in a declaration only? thankyou in advance, MArio. -------- I refer to: http://www.faqs.org/rfcs/rfc3493.html "[...] The symbolic constant is named IN6ADDR_LOOPBACK_INIT and is defined in <netinet/in.h>. It can be used at declaration time ONLY; for example: struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT; Like IN6ADDR_ANY_INIT, this constant cannot be used in an assignment to a previously declared IPv6 address variable. [...] " _mario.lat skrev: > for struct: > struct in6_addr { > uint8_t s6_addr[16]; > }; > is provided a costant: this is not a konstant, it is macro _ meaning that it will replate your initialization in pre-compiler > #define IN6ADDR_LOOPBACK_INIT {{{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1}}} #include <stdio.h> #include <stdlib.h> struct in6_addr{ unsigned int s6_addr[16]; }; /* you had to may brackets; the firs pair {} is for the struck initialization, the second pair {}, inside is for array initialization / #define IN6ADDR_LOOPBACK_INIT {{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1}} int main(void){ int i; struct in6_addr a = IN6ADDR_LOOPBACK_INIT ; / pre-compiler will replace it with {{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1}} / for(i=0;i<16;i++){ printf("%d ", a.s6_addr[i]); } return EXIT_SUCCESS; - Hide quoted text - } > what does means {{{, and }}}? > why can be used in a declaration only? > thankyou in advance, > MArio. > -------- > I refer to: > http://www.faqs.org/rfcs/rfc3493.html > "[...] > The symbolic constant is named IN6ADDR_LOOPBACK_INIT and is > defined in <netinet/in.h>. > It can be used at declaration time ONLY; for example: > struct in6_addr loopbackaddr = IN6ADDR_LOOPBACK_INIT; > Like IN6ADDR_ANY_INIT, this constant cannot be used in an assignment > to a previously declared IPv6 address variable. > [...] > " "_mario.lat" <n@libero.it> writes: > for struct: > struct in6_addr { > uint8_t s6_addr[16]; > }; > is provided a costant: > #define IN6ADDR_LOOPBACK_INIT {{{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1}}} > what does means {{{, and }}}? {{{ is simply a sequence of three distinct { tokens. Likewise for }}}. > why can be used in a declaration only? [...] > I refer to: > http://www.faqs.org/rfcs/rfc3493.html That RFC specifies the IN6ADDR_LOOPBACK_INIT, but it doesn't specify how it's defined; the triple curly braces don't appear anywhere in the RFC itself. Some implementations do use triple braces in their definition of IN6ADDR_LOOPBACK_INIT. Others don't. IN6ADDR_LOOPBACK_INIT is intended to expand to an initializer for an object of type struct in6_addr. If you look at the definition of type "struct in6_addr", you'll probably see that it consists of an array within a union within a structure. The definition uses one level of braces for each level of the type definition. Note that C is fairly lax about braces in initializers. You can legally omit some braces; you can even add them in some cases: int i = { 42 }; But IMHO it's best to have the structure of the initializer match the structure of the type. -- Keith Thompson (The_Other_Keith) k@mib.org <http://www.ghoti.net/~kst> San Diego Supercomputer Center <> <http://users.sdsc.edu/~kst> "We must do something. This is something. Therefore, we must do this." -- Antony Jay and Jonathan Lynn, "Yes Minister" In article <ln1wh2c92y.@nuthaus.mib.org>, Keith Thompson <k@mib.org> wrote: >> struct in6_addr { >> uint8_t s6_addr[16]; >> }; >> is provided a costant: >> #define IN6ADDR_LOOPBACK_INIT {{{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1}}} >> what does means {{{, and }}}? >IN6ADDR_LOOPBACK_INIT is intended to expand to an initializer for an >object of type struct in6_addr. If you look at the definition of type >"struct in6_addr", you'll probably see that it consists of an array >within a union within a structure. The OP quoted the definition of struct in6_addr (see above), and it just consists of an array within the struct. So the mystery is why there are 3 rather than 2 levels of brace. Some implementations certainly do define it as a struct containing a union as you suggested; perhaps the OP has a buggy implementation, or has misread some #ifdefs or something like that. -- Richard -- "Consideration shall be given to the need for as many as 32 characters in some alphabets" - X3.4, 1963. - Hide quoted text - rich@cogsci.ed.ac.uk (Richard Tobin) writes: > In article <ln1wh2c92y.@nuthaus.mib.org>, > Keith Thompson <k@mib.org> wrote: >>> struct in6_addr { >>> uint8_t s6_addr[16]; >>> }; >>> is provided a costant: >>> #define IN6ADDR_LOOPBACK_INIT {{{0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1}}} >>> what does means {{{, and }}}? >>IN6ADDR_LOOPBACK_INIT is intended to expand to an initializer for an >>object of type struct in6_addr. If you look at the definition of type >>"struct in6_addr", you'll probably see that it consists of an array >>within a union within a structure. > The OP quoted the definition of struct in6_addr (see above), and it > just consists of an array within the struct. So the mystery is why > there are 3 rather than 2 levels of brace. > Some implementations certainly do define it as a struct containing a > union as you suggested; perhaps the OP has a buggy implementation, or > has misread some #ifdefs or something like that. The quoted definition of struct s6_addr is from the RFC. Reading on in the RFC: The structure in6_addr above is usually implemented with an embedded union with extra fields that force the desired alignment level in a manner similar to BSD implementations of "struct in_addr". Those additional implementation details are omitted here for simplicity. An example is as follows: struct in6_addr { union { uint8_t _S6_u8[16]; uint32_t _S6_u32[4]; uint64_t _S6_u64[2]; } _S6_un; }; #define s6_addr _S6_un._S6_u8 Given such a definition, the triple braces are appropriate (though single braces would also be legal). -- Keith Thompson (The_Other_Keith) k@mib.org <http://www.ghoti.net/~kst> San Diego Supercomputer Center <> <http://users.sdsc.edu/~kst> "We must do something. This is something. Therefore, we must do this." -- Antony Jay and Jonathan Lynn, "Yes Minister"