What is your favorite C programming trick

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For example  I recently came across this in the linux kernel       Force a compilation error if condition is true     define BUILD BUG ON condition    void sizeof char 1 - 2    condition       So  in your code  if you have some structure which must be  say a multiple of 8 bytes in size  maybe because of some hardware constraints  you can do    BUILD BUG ON  sizeof struct mystruct    8     0     and it won t compile unless the size of struct mystruct is a multiple of 8  and if it is a multiple of 8  no runtime code is generated at all   Another trick I know is from the book  Graphics Gems  which allows a single header file to both declare and initialize variables in one module while in other modules using that module  merely declare them as externs     ifdef DEFINE MYHEADER GLOBALS  define GLOBAL  define INIT x  y   x     y   else  define GLOBAL extern  define INIT x  y   endif  GLOBAL int INIT x  0   GLOBAL int somefunc int a  int b     With that  the code which defines x and somefunc does     define DEFINE MYHEADER GLOBALS  include  the above header file h    while code that s merely using x and somefunc   does     include  the above header file h    So you get one header file that declares both instances of globals and function prototypes where they are needed  and the corresponding extern declarations   So  what are your favorite C programming tricks along those lines

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I m a fan of xor hacks   Swap 2 pointers without third temp pointer   int   a  int   b  a    b  b    a  a    b    Or I really like the xor linked list with only one pointer   http   en wikipedia org wiki XOR linked list   Each node in the linked list is the Xor of the previous node and the next node   To traverse forward  the address of the nodes are found in the following manner    LLNode   first   head  LLNode   second   first linked nodes  LLNode   third   second linked nodes   first  LLNode   fourth   third linked nodes   second    etc   or to traverse backwards   LLNode   last   tail  LLNode   second to last   last linked nodes  LLNode   third to last   second to last linked nodes   last  LLNode   fourth to last   third to last linked nodes   second to last    etc   While not terribly useful  you can t start traversing from an arbitrary node  I find it to be very cool

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C99 offers some really cool stuff using anonymous arrays   Removing pointless variables        int yes 1      setsockopt yourSocket  SOL SOCKET  SO REUSEADDR   amp yes  sizeof int        becomes  setsockopt yourSocket  SOL SOCKET  SO REUSEADDR   int    1   sizeof int      Passing a Variable Amount of Arguments  void func type  values        while  values            x    values               do whatever with x             func  type    val1 val2 val3 val4 0      Static linked lists  int main         struct llist   int a  struct llist  next         define cons x y   struct llist     x y       struct llist  list cons 1  cons 2  cons 3  cons 4  NULL          struct llist  p   list      while p    0            printf   d n   p- gt a           p   p- gt next            Any I m sure many other cool techniques I haven t thought of

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Not specific to C  but I ve always liked the XOR operator  One cool thing it can do is  quot swap without a temp value quot   int a   1  int b   2   printf  quot a    d  b    d n quot   a  b    a    b  b    a  a    b   printf  quot a    d  b    d n quot   a  b    Result   a   1  b   2 a   2  b   1

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I always liked dumb preprocessor tricks to make generic container types      list h     ifndef CONTAINER TYPE  define CONTAINER TYPE VALUE TYPE    List  endif typedef struct CONTAINER TYPE       CONTAINER TYPE  next      VALUE TYPE v    CONTAINER TYPE     Possibly Lots of functions for manipulating Lists     undef VALUE TYPE  undef CONTAINER TYPE   Then you can do e g     define VALUE TYPE int  include  list h  typedef struct myFancyStructure  myFancyStructureP   define VALUE TYPE myFancyStructureP  define CONTAINER TYPE mfs  include  list h    And never write a linked list again   If VALUE TYPE is always going to be a pointer  then this is an overkill  since a void   would work just as well   But there are often very small structures for which the overhead of indirection often doesn t make sense   Also  you gain type checking  i e  you might not want to concatenate a linked list of strings with a linked list of doubles  even though both would work in a void   linked list

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For creating a variable which is read-only in all modules except the one it s declared in      Header1 h    ifndef SOURCE1 C    extern const int MyVar   endif        Source1 c    define SOURCE1 C  include Header1 h    MyVar isn t seen in the header  int MyVar     Declared in this file  and is writeable        Source2 c   include Header1 h    MyVar is seen as a constant  declared elsewhere

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Declaring array s of pointer to functions for implementing finite state machines       int    fsm    void              The most pleasing advantage is that it is simple to force each stimulus state to check all code paths   In an embedded system  I ll often map an ISR to point to such a table and revector it as needed  outside the ISR

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This one comes from the book  Enough rope to shoot yourself in the foot    In the header declare   ifndef RELEASE    define D x  do   x    while  0   else    define D x   endif   In your code place testing statements eg   D printf  Test statement n       The do while helps in case the contents of the macro expand to multiple statements    The statement will only be printed if  -D RELEASE  flag for compiler is not used   You can then eg  pass the flag to your makefile etc    Not sure how this works in windows but in  nix it works well

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fill in the blanks to print both  correct  and  wrong  below     if --------  printf  correct    else printf  wrong      The answer is  printf  correct

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I think the use of userdata pointers is pretty neat  A fashion losing ground nowdays  It s not so much a C feature but is pretty easy to use in C

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I use X-Macros to to let the pre-compiler generate code  They are especially useful for defining error values and associated error strings in one place  but they can go far beyond that

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Once a mate of mine and I redefined return to find a tricky stack corruption bug   Something like    define return DoSomeStackCheckStuff  return

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Fun with macros    define SOME ENUMS F        F ZERO  zero        F ONE  one        F TWO  two      Now define the constant values   See how succinct this is      enum Constants    define DEFINE ENUM A  B  A      SOME ENUMS DEFINE ENUMS   undef DEFINE ENUM        Now a function to return the name of an enum      const char  ToString int c        switch  c        default  return NULL     Or whatever      define CASE MACRO A  B  case A  return  b       SOME ENUMS CASE MACRO   undef CASE MACRO

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if ---------    printf  hello      else    printf  hi      Fill in the blanks so that neither hello nor hi would appear in output  ans  fclose stdout

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Using a stupid macro trick to make record definitions easier to maintain    define COLUMNS S E    E  -  S    1   typedef struct       char studentNumber COLUMNS  1   9       char firstName     COLUMNS 10  30       char lastName      COLUMNS 31  51      StudentRecord

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If we are talking about c tricks my favourite has to be Duff s Device for loop unrolling  I m just waiting for the right opportunity to come along for me to actually use it in anger

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I like the concept of container of used for example in lists  Basically  you do not need to specify next and last fields for each structure which will be in the list  Instead  you append the list structure header to actual linked items   Have a look on include linux list h for real-life examples

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While reading Quake 2 source code I came up with something like this   double normals            include  normals txt        more or less  I don t have the code handy to check it now    Since then  a new world of creative use of the preprocessor opened in front of my eyes  I no longer include just headers  but entire chunks of code now and then  it improves reusability a lot   -p  Thanks John Carmack  xD

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Another nice pre-processor  trick  is to use the     character to print debugging expressions  For example    define MY ASSERT cond      do         if    cond              printf  MY ASSERT  s  failed n    cond           exit -1                 while  0     edit  the code below only works on C    Thanks to smcameron and Evan Teran   Yes  the compile time assert is always great  It can also be written as    define COMPILE ASSERT cond        typedef char   compile time assert   cond    0   -1

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if TESTMODE    1         debug 1      while 0          Get attention  endif   The while 0   has no effect on the program  but the compiler will issue a warning about  this does nothing   which is enough to get me to go look at the offending line and then see the real reason I wanted to call attention to it

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Bit-shifts are only defined up to a shift-amount of 31  on a 32 bit integer      What do you do if you want to have a computed shift that need to work with higher shift-values as well  Here is how the Theora vide-codec does it   unsigned int shiftmystuff  unsigned int a  unsigned int v      return  a gt  gt  v gt  gt 1   gt  gt   v 1  gt  gt 1       Or much more readable    unsigned int shiftmystuff  unsigned int a  unsigned int v      unsigned int halfshift   v gt  gt 1    unsigned int otherhalf    v 1  gt  gt 1     return  a  gt  gt  halfshift   gt  gt  otherhalf       Performing the task the way shown above is a good deal faster than using a branch like this    unsigned int shiftmystuff  unsigned int a  unsigned int v      if  v lt  31      return a gt  gt v    else     return 0

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using    FILE   and   LINE   for debugging   define WHERE fprintf stderr   LOG  s  d n    FILE     LINE

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I wouldn t really call it a favorite trick  since I ve never used it  but the mention of Duff s Device reminded me of this article about implementing Coroutines in C   It always gives me a chuckle  but I m sure it could be useful some time

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I like the  struct hack  for having a dynamically sized object  This site explains it pretty well too  though they refer to the C99 version where you can write  str    as the last member of a struct   you could make a string  object  like this   struct X       int len      char str 1       int n   strlen  hello world    struct X  string   malloc sizeof struct X    n   strcpy string- gt str   hello world    string- gt len   n    here  we ve allocated a structure of type X on the heap that is the size of an int  for len   plus the length of  hello world   plus 1  since str1 is included in the sizeof X    It is generally useful when you want to have a  header  right before some variable length data in the same block

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Here is an example how to make C code completly unaware about what is actually used of HW for running the app  The main c does the setup and then the free layer can be implemented on any compiler arch  I think it is quite neat for abstracting C code a bit  so it does not get to be to spesific   Adding a complete compilable example here      free h     ifndef  FREE H   define  FREE H   include  lt stdio h gt   include  lt string h gt  typedef unsigned char ubyte   typedef void   F ParameterlessFunction      typedef void   F CommandFunction  ubyte byte     void F SetupLowerLayer   F ParameterlessFunction initRequest  F CommandFunction sending command  F CommandFunction  receiving command    endif     free c    static F ParameterlessFunction Init Lower Layer   NULL  static F CommandFunction Send Command   NULL  static ubyte init   0  void recieve value ubyte my input        if init    0                Init Lower Layer            init   1            printf  Receiving 0x 02x n  my input       Send Command   my input      void F SetupLowerLayer       F ParameterlessFunction initRequest      F CommandFunction sending command      F CommandFunction  receiving command        Init Lower Layer   initRequest      Send Command   sending command       receiving command    amp recieve value        main c    int my hw do init         printf  Doing HW init n        return 0    int my hw do sending ubyte send this        printf  doing HW sending 0x 02x n  send this       return 0    F CommandFunction my hw send to read   NULL   int main  void        ubyte rx   0x40      F SetupLowerLayer my hw do init my hw do sending  amp my hw send to read        my hw send to read rx       getchar        return 0

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Instead of  printf  counter  d n  counter     Use   define print dec var   printf   s  d n   var var   print dec counter

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Two good source books for this sort of stuff are The Practice of Programming and Writing Solid Code  One of them  I don t remember which  says  Prefer enum to  define where you can  because enum gets checked by the compiler

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Object oriented code with C  by emulating classes     Simply  create a struct and a set of functions that take a pointer to that struct as a first parameter

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Our codebase has a trick similar to   ifdef DEBUG   define my malloc amt  my malloc debug amt    FILE      LINE    void   my malloc debug int amt  char  file  int line   else void   my malloc int amt   endif         remember file and line no  for this malloc in debug mode     which allows for the tracking of memory leaks in debug mode   I always thought this was cool

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Rusty actually produced a whole set of build conditionals in ccan  check out the build assert module    include  lt stddef h gt   include  lt ccan build assert build assert h gt   struct foo           char string 5           int x      char  foo string struct foo  foo               This trick requires that the string be first in the structure         BUILD ASSERT offsetof struct foo  string     0           return  char   foo      There are lots of other helpful macros in the actual header  which are easy to drop into place   I try  with all of my might to resist the pull of the dark side  and preprocessor abuse  by sticking mostly to inline functions  but I do enjoy clever  useful macros like the ones you described

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I love empty if-else and while 0  operators   For example    define CMD1 X  do   foo x   bar x     while  0   define CMD2 X  if  1    foo x   bar x     else

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Adding two numbers  a and b  without using any operators   printf   d   printf    s  s  a   r  b   r        it prints a b

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I m fond of using    0   to initialize structures without needing to call memset   struct something X    0     This will initialize all of the members of the struct  or array  to zero  but not any padding bytes - use memset if you need to zero those as well    But you should be aware there are some issues with this for large  dynamically allocated structures

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In C99 you can directly embed URLs into source code inside functions  Example    include  lt stdio h gt   int main int argc  char   argv        http   stackoverflow com      printf  Hello World

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See  Hidden features of C  question

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Using the otherwise pointless     operator to initialise a const variable  const int bytesPerPixel   isAlpha     4   3

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In C99  typedef struct      int value      int otherValue    s   s test     value   15   otherValue   16       or    int a 100     1 2  50  3 4 5  23  6 7

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I don t know if it s a trick   But when I was a junior in university a friend of mine and I were completing a lab in our intro C   course   We had to take a person s name and capitalize it  display it back  and then give them the option of displaying their name  last  first    For this lab we were prohibited from using array notation    He showed me this code  I thought it was the coolest thing I d seen at the time   char   ptr    first name      flies to the end of the array  regardless of length while   ptr

[c] What is your favorite C programming trick?

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