Understanding typedefs for function pointers in C

Question

I have always been a bit stumped when I read other peoples  code which had typedefs for pointers to functions with arguments  I recall that it took me a while to get around to such a definition while trying to understand a numerical algorithm written in C a while ago  So  could you share your tips and thoughts on how to write good typedefs for pointers to functions  Do s and Do not s   as to why are they useful and how to understand others  work  Thanks

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This is the simplest example of function pointers and function pointer arrays that I wrote as an exercise       typedef double   pf  double x      this defines a type pf         double f1 double x    return x x        double f2 double x    return x x         pf pa      f1  f2         main                 pf p           p   pa 0           printf   f n   p 3 0            p   pa 1           printf   f n   p 3 0

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int add int a  int b      return  a b     int minus int a  int b      return  a-b      typedef int   math func  int  int     declaration of function pointer  int main       math func addition   add     typedef assigns a new variable i e   addition  to original function  add    math func substract   minus    typedef assigns a new variable i e   substract  to original function  minus     int c   addition 11  11       calling function via new variable   printf   d n  c     c   substract 11  5       calling function via new variable   printf   d  c     return 0      Output of this is    22  6  Note that  same math func definer has been used for the declaring both the function   Same approach of typedef may be used for extern struct  using sturuct in other file

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A very easy way to understand typedef of function pointer   int add int a  int b        return  a b      typedef int   add integer  int  int     declaration of function pointer  int main         add integer addition   add    typedef assigns a new variable i e   addition  to original function  add      int c   addition 11  11       calling function via new variable     printf   d  c       return 0

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Consider the signal   function from the C standard  extern void   signal int  void    int    int    Perfectly obscurely obvious - it s a function that takes two arguments  an integer and a pointer to a function that takes an integer as an argument and returns nothing  and it  signal    returns a pointer to a function that takes an integer as an argument and returns nothing  If you write  typedef void   SignalHandler  int signum    then you can instead declare signal   as  extern  SignalHandler signal int signum  SignalHandler handler    This means the same thing  but is usually regarded as somewhat easier to read   It is clearer that the function takes an int and a SignalHandler and returns a SignalHandler  It takes a bit of getting used to  though   The one thing you can t do  though is write a signal handler function using the SignalHandler typedef in the function definition  I m still of the old-school that prefers to invoke a function pointer as    functionpointer  arg1  arg2         Modern syntax uses just  functionpointer arg1  arg2         I can see why that works - I just prefer to know that I need to look for where the variable is initialized rather than for a function called functionpointer   Sam commented   I have seen this explanation before  And then  as is the case now  I think what I didn t get was the connection between the two statements      extern void   signal int  void   int    int      and        typedef void   SignalHandler  int signum       extern SignalHandler signal int signum  SignalHandler handler    Or  what I want to ask is  what is the underlying concept that one can use to come up with the second version you have  What is the fundamental that connects  quot SignalHandler quot  and the first typedef  I think what needs to be explained here is what is typedef is actually doing here   Let s try again   The first of these is lifted straight from the C standard - I retyped it  and checked that I had the parentheses right  not until I corrected it - it is a tough cookie to remember   First of all  remember that typedef introduces an alias for a type   So  the alias is SignalHandler  and its type is   a pointer to a function that takes an integer as an argument and returns nothing   The  returns nothing  part is spelled void  the argument that is an integer is  I trust  self-explanatory   The following notation is simply  or not  how C spells pointer to function taking arguments as specified and returning the given type  type   function  argtypes    After creating the signal handler type  I can use it to declare variables and so on   For example  static void alarm catcher int signum        fprintf stderr   quot  s   called   d  n quot     func    signum      static void signal catcher int signum        fprintf stderr   quot  s   called   d  - exiting n quot     func    signum       exit 1      static struct Handlers       int              signum      SignalHandler    handler    handler             SIGALRM    alarm catcher           SIGINT     signal catcher          SIGQUIT    signal catcher        int main void        size t num handlers   sizeof handler    sizeof handler 0        size t i       for  i   0  i  lt  num handlers  i                  SignalHandler old handler   signal handler i  signum  SIG IGN           if  old handler    SIG IGN              old handler   signal handler i  signum  handler i  handler           assert old handler    SIG IGN                 continue with ordinary processing         return EXIT SUCCESS      Please note How to avoid using printf   in a signal handler  So  what have we done here - apart from omit 4 standard headers that would be needed to make the code compile cleanly  The first two functions are functions that take a single integer and return nothing   One of them actually doesn t return at all thanks to the exit 1   but the other does return after printing a message   Be aware that the C standard does not permit you to do very much inside a signal handler  POSIX is a bit more generous in what is allowed  but officially does not sanction calling fprintf     I also print out the signal number that was received   In the alarm handler   function  the value will always be SIGALRM as that is the only signal that it is a handler for  but signal handler   might get SIGINT or SIGQUIT as the signal number because the same function is used for both  Then I create an array of structures  where each element identifies a signal number and the handler to be installed for that signal   I ve chosen to worry about 3 signals  I d often worry about SIGHUP  SIGPIPE and SIGTERM too and about whether they are defined   ifdef conditional compilation   but that just complicates things   I d also probably use POSIX sigaction   instead of signal    but that is another issue  let s stick with what we started with  The main   function iterates over the list of handlers to be installed   For each handler  it first calls signal   to find out whether the process is currently ignoring the signal  and while doing so  installs SIG IGN as the handler  which ensures that the signal stays ignored   If the signal was not previously being ignored  it then calls signal   again  this time to install the preferred signal handler    The other value is presumably SIG DFL  the default signal handler for the signal    Because the first call to  signal    set the handler to SIG IGN and signal   returns the previous error handler  the value of old after the if statement must be SIG IGN - hence the assertion    Well  it could be SIG ERR if something went dramatically wrong - but then I d learn about that from the assert firing   The program then does its stuff and exits normally  Note that the name of a function can be regarded as a pointer to a function of the appropriate type   When you do not apply the function-call parentheses - as in the initializers  for example - the function name becomes a function pointer   This is also why it is reasonable to invoke functions via the pointertofunction arg1  arg2  notation  when you see alarm handler 1   you can consider that alarm handler is a pointer to the function and therefore alarm handler 1  is an invocation of a function via a function pointer  So  thus far  I ve shown that a SignalHandler variable is relatively straight-forward to use  as long as you have some of the right type of value to assign to it - which is what the two signal handler functions provide  Now we get back to the question - how do the two declarations for signal   relate to each other  Let s review the second declaration   extern SignalHandler signal int signum  SignalHandler handler    If we changed the function name and the type like this   extern double function int num1  double num2    you would have no problem interpreting this as a function that takes an int and a double as arguments and returns a double value  would you  maybe you d better not  fess up if that is problematic - but maybe you should be cautious about asking questions as hard as this one if it is a problem   Now  instead of being a double  the signal   function takes a SignalHandler as its second argument  and it returns one as its result  The mechanics by which that can also be treated as  extern void   signal int signum  void  handler  int signum    int signum    are tricky to explain - so I ll probably screw it up   This time I ve given the parameters names - though the names aren t critical  In general  in C  the declaration mechanism is such that if you write  type var   then when you write var it represents a value of the given type   For example  int     i                i is an int int     ip                ip is an int  so ip is a pointer to an integer int     abs int val      abs -1  is an int  so abs is a  pointer to a                           function returning an int and taking an int argument  In the standard  typedef is treated as a storage class in the grammar  rather like static and extern are storage classes  typedef void   SignalHandler  int signum    means that when you see a variable of type SignalHandler  say alarm handler  invoked as    alarm handler  -1    the result has type void - there is no result   And   alarm handler  -1   is an invocation of alarm handler   with argument -1  So  if we declared  extern SignalHandler alt signal void    it means that    alt signal      represents a void value   And therefore  extern void   alt signal void   int signum    is equivalent   Now  signal   is more complex because it not only returns a SignalHandler  it also accepts both an int and a SignalHandler as arguments  extern void   signal int signum  SignalHandler handler   int signum    extern void   signal int signum  void   handler  int signum    int signum    If that still confuses you  I m not sure how to help - it is still at some levels mysterious to me  but I ve grown used to how it works and can therefore tell you that if you stick with it for another 25 years or so  it will become second nature to you  and maybe even a bit quicker if you are clever

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cdecl is a great tool for deciphering weird syntax like function pointer declarations   You can use it to generate them as well   As far as tips for making complicated declarations easier to parse for future maintenance  by yourself or others   I recommend making typedefs of small chunks and using those small pieces as building blocks for larger and more complicated expressions   For example   typedef int   FUNC TYPE 1  void   typedef double   FUNC TYPE 2  void   typedef FUNC TYPE 1   FUNC TYPE 3  FUNC TYPE 2     rather than   typedef int     FUNC TYPE 3  double     void    void     cdecl can help you out with this stuff   cdecl gt  explain int   FUNC TYPE 1  void  declare FUNC TYPE 1 as pointer to function  void  returning int cdecl gt  explain double   FUNC TYPE 2  void  declare FUNC TYPE 2 as pointer to function  void  returning double cdecl gt  declare FUNC TYPE 3 as pointer to function  pointer to function  void  returning double  returning pointer to function  void  returning int int     FUNC TYPE 3  double     void     void     And is  in fact  exactly how I generated that crazy mess above

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A function pointer is like any other pointer  but it points to the address of a function instead of the address of data  on heap or stack   Like any pointer  it needs to be typed correctly  Functions are defined by their return value and the types of parameters they accept  So in order to fully describe a function  you must include its return value and the type of each parameter is accepts  When you typedef such a definition  you give it a  friendly name  which makes it easier to create and reference pointers using that definition   So for example assume you have a function   float doMultiplication  float num1  float num2         return num1   num2      then the following typedef   typedef float  pt2Func  float  float     can be used to point to this doMulitplication function  It is simply defining a pointer to a function which returns a float and takes two parameters  each of type float  This definition has the friendly name pt2Func  Note that pt2Func can point to ANY function which returns a float and takes in 2 floats   So you can create a pointer which points to the doMultiplication function as follows   pt2Func  myFnPtr    amp doMultiplication    and you can invoke the function using this pointer as follows   float result     myFnPtr  2 0  5 1     This makes good reading  http   www newty de fpt index html

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Use typedefs to define more complicated types i e function pointers  I will take the example of defining a state-machine in C      typedef  int   action handler t  void  ctx  void  data     now we have defined a type called action handler that takes two pointers and returns a int  define your state-machine      typedef struct             state t curr state       Enum for the Current state          event t event      Enum for the event          state t next state       Enum for the next state          action handler t event handler     Function-pointer to the action           state element    The function pointer to the action looks like a simple type and typedef primarily serves this purpose   All my event handlers now should adhere to the type defined by action handler      int handle event a void  fsm ctx  void  in msg         int handle event b void  fsm ctx  void  in msg      References   Expert C programming by Linden

[c] Understanding typedefs for function pointers in C

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