c-preprocessor

Possible Duplicate: Why are there sometimes meaningless do/while and if/else statements in C/C++ macros? I met code like below: #define ev_io_init(ev,cb,fd,events) \ do { \ ev_init ((ev), (cb)); \ ev_io_set ((ev),(fd),(events)); \ } while (0) I want to know why the author use do { } while (0) here. Is there any difference with this? #define ev_io_init(ev,cb,fd,events) { \ ev_init ((ev), (cb)); \ ev_io_set ((ev),(fd),(events)); \ } BTW: the code is from libev, ev_local.h Consider if( something ) function1(); else function2(); If function1() is actually a macro, just using { } requires you to

I have several obj-c classes, each of which require a number of constants that are used in switch statements. I had tried defining these numerical constants in the .m file using the #define preprocessor instruction. All these constants begin with 'kCell'. This seems to work well but Xcode's code sense is presenting me with every 'kCell' prefixed constant no matter where I am in the project. Do #define constants have global scope? If so, what is the best way to define purely local class constants that won't class with similarly named constants in other classes? Manjunath Have your own constant

Reverse engineering code and I'm kind of appalled at the style, but I wanted to make sure there's no good reason for doing these things.... Is it just me or is this a horrible coding style if ( pwbuf ) sprintf(username,"%s",pwbuf->pw_name); else sprintf(username,"%d",user_id); And why wrap code not intended for compilation in an #if 0 .... #endif Instead of comments? EDIT: So as some explained below, this is due to the possibility to flummox /* */ which I didn't realize. But I still don't understand, why not just use your programming environment tools or favorite text editor's macro's to block

I have been looking at the Boost libraries source code, and I have noticed that often there are single pound signs without any preprocessor directives attached to them. I read through the GCC preprocessor manual and specification guide and can't find anything about it. (1) #ifndef BOOST_CONFIG_HPP (2) # include <boost/config.hpp> (3) #endif (4) # (5) #if defined(BOOST_HAS_PRAGMA_ONCE) (6) # pragma once (7) #endif On line 4, there is nothing after the pound sign. What effect does this have? Is it defined in the C preprocessor (CPP) specification? As Boost is a cross-platform library, I would

What is the difference (if any) between the two following preprocessor control statements. #if and #ifdef You can demonstrate the difference by doing: #define FOO 0 #if FOO // won't compile this #endif #ifdef FOO // will compile this #endif #if checks for the value of the symbol, while #ifdef checks the existence of the symbol (regardless of its value). #ifdef FOO is a shortcut for: #if defined(FOO) #if can also be used for other tests or for more complex preprocessor conditions. #if defined(FOO) || defined(BAR) 来源： https://stackoverflow.com/questions/3802988/difference-between-preprocessor

After I were able to successfully reduce code duplication with this I want to further reduce my code duplication with totally different problem #include <iostream> #include <array> #define A1 #define B2 #define C3 #define D4 #define E5 enum Enum { #ifdef A1 one, #endif #ifdef B2 two, #endif #ifdef C3 three, #endif #ifdef D4 four, #endif #ifdef E5 five, #endif end }; const char* map(Enum e) { switch(e) { #ifdef A1 case one : return "one"; #endif #ifdef B2 case two: return "two"; #endif #ifdef C3 case three : return "three"; #endif #ifdef D4 case four: return "four"; #endif #ifdef E5 case five:

What C macro is in your opinion is the most useful? I have found the following one, which I use to do vector arithmetic in C : #define v3_op_v3(x, op, y, z) {z[0]=x[0] op y[0]; \ z[1]=x[1] op y[1]; \ z[2]=x[2] op y[2];} It works like that: v3_op_v3(vectorA, +, vectorB, vectorC); v3_op_v3(vectorE, *, vectorF, vectorJ); ... for-each loop in C99: #define foreach(item, array) \ for(int keep=1, \ count=0,\ size=sizeof (array)/sizeof *(array); \ keep && count != size; \ keep = !keep, count++) \ for(item = (array)+count; keep; keep = !keep) int main() { int a[] = { 1, 2, 3 }; int sum = 0; foreach(int