What is a callback in C and how are they implemented

Question

From the reading that I have done  Core Audio relies heavily on callbacks  and C    but that s another story      I understand the concept  sort of  of setting up a function that is called by another function repeatedly to accomplish a task   I just don t understand how they get set up and how they actually work   Any examples would be appreciated

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A simple call back program  Hope it answers your question    include  lt stdio h gt   include  lt stdlib h gt   include  lt unistd h gt   include  lt fcntl h gt   include  lt string h gt   include        common typedef h   typedef void   call back   S32  S32    void test call back S32 a  S32 b        printf  In call back function  a  d  t b  d  n   a  b      void call callback func call back back        S32 a   5      S32 b   7       back a  b      S32 main S32 argc  S8  argv          S32 ret   SUCCESS       call back back       back   test call back       call callback func back        return ret

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Callbacks in C are usually implemented using function pointers and an associated data pointer  You pass your function on event   and data pointers to a framework function watch events    for example   When an event happens  your function is called with your data and some event-specific data   Callbacks are also used in GUI programming  The GTK  tutorial has a nice section on the theory of signals and callbacks

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It is lot easier to understand an idea through example  What have been told about callback function in C so far are great answers  but probably the biggest benefit of using the feature is to keep the code clean and uncluttered   Example  The following C code implements quick sorting  The most interesting line in the code below is this one  where we can see the callback function in action   qsort arr N sizeof int  compare s2b     The compare s2b is the name of function which qsort   is using to call the function  This keeps qsort   so uncluttered  hence easier to maintain   You just call a function by name from inside another function  of course  the function prototype declaration  at the least  must precde before it can be called from another function    The Complete Code   include  lt stdio h gt   include  lt stdlib h gt   int arr    56 90 45 1234 12 3 7 18     function prototype declaration   int compare s2b const void  a const void  b    int compare b2s const void  a const void  b      arranges the array number from the smallest to the biggest int compare s2b const void  a  const void  b        const int  p  const int  a      const int  q  const int  b       return  p- q       arranges the array number from the biggest to the smallest int compare b2s const void  a  const void  b        const int  p  const int  a      const int  q  const int  b       return  q- p     int main         printf  Before sorting n n         int N sizeof arr  sizeof int        for int i 0 i lt N i                  printf   d t  arr i               printf   n         qsort arr N sizeof int  compare s2b        printf   nSorted small to big n n         for int j 0 j lt N j                  printf   d t  arr j               qsort arr N sizeof int  compare b2s        printf   nSorted big to small n n         for int j 0 j lt N j                  printf   d t  arr j               exit 0

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A callback in C is a function that is provided to another function to  call back to  at some point when the other function is doing its task   There are two ways that a callback is used  synchronous callback and asynchronous callback  A synchronous callback is provided to another function which is going to do some task and then return to the caller with the task completed  An asynchronous callback is provided to another function which is going to start a task and then return to the caller with the task possibly not completed   A synchronous callback is typically used to provide a delegate to another function to which the other function delegates some step of the task  Classic examples of this delegation are the functions bsearch   and qsort   from the C Standard Library  Both of these functions take a callback which is used during the task the function is providing so that the type of the data being searched  in the case of bsearch    or sorted  in the case of qsort    does not need to be known by the function being used   For instance here is a small sample program with bsearch   using different comparison functions  synchronous callbacks  By allowing us to delegate the data comparison to a callback function  the bsearch   function allows us to decide at run time what kind of comparison we want to use  This is synchronous because when the bsearch   function returns the task is complete    include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   typedef struct       int iValue      int kValue      char label 6     MyData   int cmpMyData iValue  MyData  item1  MyData  item2        if  item1- gt iValue  lt  item2- gt iValue  return -1      if  item1- gt iValue  gt  item2- gt iValue  return 1      return 0     int cmpMyData kValue  MyData  item1  MyData  item2        if  item1- gt kValue  lt  item2- gt kValue  return -1      if  item1- gt kValue  gt  item2- gt kValue  return 1      return 0     int cmpMyData label  MyData  item1  MyData  item2        return strcmp  item1- gt label  item2- gt label      void bsearch results  MyData  srch  MyData  found            if  found                printf   found - iValue    d  kValue    d  label    s n   found- gt iValue  found- gt kValue  found- gt label             else               printf   item not found  iValue    d  kValue    d  label    s n   srch- gt iValue  srch- gt kValue  srch- gt label                int main          MyData dataList 256     0                  int i          for  i   0  i  lt  20  i                  dataList i  iValue   i   100              dataList i  kValue   i   1000              sprintf  dataList i  label    2 2d   i   10                            some code then we do a search               MyData srchItem     105  1018   13            MyData  foundItem   bsearch   amp srchItem  dataList  20  sizeof MyData   cmpMyData iValue             bsearch results   amp srchItem  foundItem            foundItem   bsearch   amp srchItem  dataList  20  sizeof MyData   cmpMyData kValue            bsearch results   amp srchItem  foundItem            foundItem   bsearch   amp srchItem  dataList  20  sizeof MyData   cmpMyData label            bsearch results   amp srchItem  foundItem             An asynchronous callback is different in that when the called function to which we provide a callback returns  the task may not be completed  This type of callback is often used with asynchronous I O in which an I O operation is started and then when it is completed  the callback is invoked   In the following program we create a socket to listen for TCP connection requests and when a request is received  the function doing the listening then invokes the callback function provided  This simple application can be exercised by running it in one window while using the telnet utility or a web browser to attempt to connect in another window   I lifted most of the WinSock code from the example Microsoft provides with the accept   function at https   msdn microsoft com en-us library windows desktop ms737526 v vs 85  aspx  This application starts a listen   on the local host  127 0 0 1  using port 8282 so you could use either telnet 127 0 0 1 8282 or http   127 0 0 1 8282    This sample application was created as a console application with Visual Studio 2017 Community Edition and it is using the Microsoft WinSock version of sockets  For a Linux application the WinSock functions would need to be replaced with the Linux alternatives and the Windows threads library would use pthreads instead    include  lt stdio h gt   include  lt winsock2 h gt   include  lt stdlib h gt   include  lt string h gt    include  lt Windows h gt      Need to link with Ws2 32 lib  pragma comment lib   Ws2 32 lib       function for the thread we are going to start up with  beginthreadex       this function thread will create a listen server waiting for a TCP    connection request to come into the designated port      stdcall modifier required by  beginthreadex    int  stdcall ioThread void   pOutput             ----------------------        Initialize Winsock      WSADATA wsaData      int iResult   WSAStartup MAKEWORD 2  2    amp wsaData       if  iResult    NO ERROR            printf  WSAStartup failed with error   ld n   iResult           return 1              ----------------------        Create a SOCKET for listening for        incoming connection requests      SOCKET ListenSocket      ListenSocket   socket AF INET  SOCK STREAM  IPPROTO TCP       if  ListenSocket    INVALID SOCKET            wprintf L socket failed with error   ld n   WSAGetLastError             WSACleanup            return 1              ----------------------        The sockaddr in structure specifies the address family         IP address  and port for the socket that is being bound      struct sockaddr in service      service sin family   AF INET      service sin addr s addr   inet addr  127 0 0 1        service sin port   htons 8282        if  bind ListenSocket   SOCKADDR    amp  service  sizeof service      SOCKET ERROR            printf  bind failed with error   ld n   WSAGetLastError             closesocket ListenSocket           WSACleanup            return 1              ----------------------        Listen for incoming connection requests         on the created socket     if  listen ListenSocket  1     SOCKET ERROR            printf  listen failed with error   ld n   WSAGetLastError             closesocket ListenSocket           WSACleanup            return 1              ----------------------        Create a SOCKET for accepting incoming requests      SOCKET AcceptSocket      printf  Waiting for client to connect    n           ----------------------        Accept the connection      AcceptSocket   accept ListenSocket  NULL  NULL       if  AcceptSocket    INVALID SOCKET            printf  accept failed with error   ld n   WSAGetLastError             closesocket ListenSocket           WSACleanup            return 1            else         pOutput          we have a connection request so do the callback         No longer need server socket     closesocket ListenSocket        WSACleanup        return 0        our callback which is invoked whenever a connection is made  void printOut void        printf  connection received  n        include  lt process h gt   int main             start up our listen server and provide a callback      beginthreadex NULL  0  ioThread  printOut  0  NULL          do other things while waiting for a connection  In this case        just sleep for a while      Sleep 30000

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There is no  callback  in C - not more than any other generic programming concept   They re implemented using function pointers  Here s an example   void populate array int  array  size t arraySize  int   getNextValue  void         for  size t i 0  i lt arraySize  i            array i    getNextValue       int getNextRandomValue void        return rand       int main void        int myarray 10       populate array myarray  10  getNextRandomValue               Here  the populate array function takes a function pointer as its third parameter  and calls it to get the values to populate the array with  We ve written the callback getNextRandomValue  which returns a random-ish value  and passed a pointer to it to populate array  populate array will call our callback function 10 times and assign the returned values to the elements in the given array

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Here is an example of callbacks in C   Let s say you want to write some code that allows registering callbacks to be called when some event occurs   First define the type of function used for the callback   typedef void   event cb t  const struct event  evt  void  userdata     Now  define a function that is used to register a callback   int event cb register event cb t cb  void  userdata     This is what code would look like that registers a callback   static void my event cb const struct event  evt  void  data           do stuff and things with the event              event cb register my event cb   amp my custom data         In the internals of the event dispatcher  the callback may be stored in a struct that looks something like this   struct event cb       event cb t cb      void  data       This is what the code looks like that executes a callback   struct event cb  callback           Get the event cb that you want to execute     callback- gt cb event  callback- gt data

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A callback function in C is the equivalent of a function parameter   variable assigned to be used within another function Wiki Example  In the code below     include  lt stdio h gt   include  lt stdlib h gt      The calling function takes a single callback as a parameter     void PrintTwoNumbers int   numberSource  void         printf   d and  d n   numberSource    numberSource           A possible callback    int overNineThousand void        return  rand     1000    9001        Another possible callback     int meaningOfLife void        return 42        Here we call PrintTwoNumbers   with three different callbacks     int main void        PrintTwoNumbers  amp rand       PrintTwoNumbers  amp overNineThousand       PrintTwoNumbers  amp meaningOfLife       return 0      The function   numberSource  inside the function call PrintTwoNumbers is a function to  call back    execute from inside  PrintTwoNumbers as dictated by the code as it runs    So if you had something like a pthread function you could assign another function to run inside the loop from its instantiation

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Usually this can be done by using a function pointer  that is a special variable that points to the memory location of a function  You can then use this to call the function with specific arguments  So there will probably be a function that sets the callback function  This will accept a function pointer and then store that address somewhere where it can be used  After that when the specified event is triggered  it will call that function

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This wikipedia article has an example in C   A good example is that new modules written to augment the Apache Web server register with the main apache process by passing them function pointers so those functions are called back to process web page requests

[c] What is a "callback" in C and how are they implemented?

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