c - Why we cannot dynamically allocate a pointer to a function? -


i tried dynamically allocate pointer function that

void (*f)(void*); f = (*f)malloc(sizeof(f) * 2);

and got error.

error: expected ';' before 'malloc'

after searched, discovered cannot dynamically allocate pointer function, can declare array. in first step, declare array of functions

void (f[2])(void*);

and saw got error, cannot declare array of functions, must declare array of pointers functions.

void (*f[2])(void*);

in search discovered too, pointer function doesn't point data, points code. if can explain me why cannot dynamically allocate pointer function , why cannot have array of function instead of array of pointers functions.

my code following:

#include <stdio.h> #include <stdlib.h>  void printnr(void *nr) {     printf("%i\n", nr); }  void printchar(void *ch) {     printf("%c\n", ch); }  int  main(void) {     void (*f)(void*);     f = (*f)malloc(sizeof(f) * 2);     f = printnr;     (f  + 1) = printchar;     (*f)(15);     (*(f  + 1))('t');     return (0); }

there's sorts of ill defined (if not undefined) behaviour in 2 functions you've defined. you're living dangerously (at best) code. define functions take int , ok (changing function pointer type too).

however, guess that's tangential immediate problem — (*f) before malloc() spurious. if want cast there, you'd need (void (**)(void *)) specify pointer pointer function returning void , taking void * argument.

here's fixed code type of function changed suggested:

#include <stdio.h> #include <stdlib.h>  static void printnr(int nr) {     printf("%i\n", nr); }  static void printch(int ch) {     printf("%c\n", ch); }  int main(void) {     void (**f)(int);     f = (void(**)(int))malloc(sizeof(*f) * 2);     f[0] = printnr;     f[1] = printch;     (*f)(15);     (*(f  + 1))('t');     free(f);     return(0); }

note assignments (to f[0] , f[1]) fixed too. if must use non-array notation, *(f + 0) , *(f + 1) self-consistent , *f , *(f+1) work.

it easier use typedef pointer function type.

#include <stdio.h> #include <stdlib.h>  typedef void (*functionptr)(int);  static void printnr(int nr) {     printf("%i\n", nr); }  static void printch(int ch) {     printf("%c\n", ch); }  int main(void) {     functionptr *f = (functionptr *)malloc(sizeof(*f) * 2);     f[0] = printnr;     f[1] = printch;     (*f)(15);     (*(f  + 1))('t');     free(f);     return(0); }

note can't use typedef while defining functions. it's curious quirk of c can't that, if define function type instead of function pointer type. example, given typedef void function(int);, can't define static function printnr { … } or similar , have mean same functions defined. trying static function *returnsptr(int n) { … } compiles (if body appropriate) returns function pointer (function * or functionptr).

the cast malloc() isn't needed if you're using c compiler. if you're using c++ compiler, needed. code (both programs) frees allocated memory. neither program checks allocation — slap wrist.


Comments

Post a Comment

Popular posts from this blog

android - InAppBilling registering BroadcastReceiver in AndroidManifest -

i'm implementing inapp billing in android app. works well, however, i'm trying decouple broadcast receiver activity manifest. suggestion in android's trivialdrive sample: // important: dynamically register broadcast messages updated purchases. // register receiver here instead of <receiver> in manifest // because call getpurchases() @ startup, therefore can ignore // broadcasts sent while app isn't running. // note: registering listener in activity bad idea, done here // because sample. regardless, receiver must registered after // iabhelper setup, before first call getpurchases(). currently there's class extends broadcastreceiver : public class iabbroadcastreceiver extends broadcastreceiver { /** * intent action receiver should filter for. */ public static final string action = "com.android.vending.billing.purchases_updated"; private final iabbroadcastlistener mlistener; public iabbroadcastreceiver(iabbroadcastlistener listener) { m...
Read more

python Tkinter Capturing keyboard events save as one single string -

is possible store keyboard events 1 string? code below stores 1 char , prints it. card reader or bar code reader, contains collection of character/ string not 1 character @ time. goal save char pressed text variable. from tkinter import * root = tk() def key(event): text= event.char text+= event.char print ("pressed", text) def callback(event): frame.focus_set() print ("clicked at", event.x, event.y) frame = frame(root, width=100, height=100) frame.bind("<key>", key) frame.bind("<button-1>", callback) frame.pack() root.mainloop() currently, creating text variable , everytime key() function gets called, , text stores last character typed. you can define text module level variable , , use module level text inside key function - text = '' def key(event): global text text+= event.char print("pressed", text)
Read more

sql server - Why does Linq-to-SQL add unnecessary COUNT()? -

here tables: create table customer ( cid int primary key, name varchar(32) ) create table ord ( oid int primary key, cid int foreign key references customer, address varchar(20) ) here's linq-to-sql statement: var aaa = c in db.customer select new { c, o = c.ord.tolist() }; and here's inexplicable query generated linq-to-sql: select [t0].[cid] [cid], [t0].[name] [name], [t1].[oid] [oid], [t1].[cid] [cid2], [t1].[address] [address], (select count(*) [dbo].[ord] [t2] [t2].[cid] = [t0].[cid]) [value] [dbo].[customer] [t0] left outer join [dbo].[ord] [t1] on [t1].[cid] = [t0].[cid] order [t0].[cid], [t1].[oid] i want understand how rid of count(*) part. it's entirely uncalled for! linq needs count(*) in order determine number of entries returned o = c.ord.tolist() .
Read more