By Yuval Adam

2009-05-08 15:49:17 8 Comments

I had some experience lately with function pointers in C.

So going on with the tradition of answering your own questions, I decided to make a small summary of the very basics, for those who need a quick dive-in to the subject.


@Mohit Dabas 2014-09-26 08:09:51

Starting from scratch function has Some Memory Address From Where They start executing. In Assembly Language They Are called as (call "function's memory address").Now come back to C If function has a memory address then they can be manipulated by Pointers in C.So By the rules of C

1.First you need to declare a pointer to function 2.Pass the Address of the Desired function

****Note->the functions should be of same type****

This Simple Programme will Illustrate Every Thing.

void (*print)() ;//Declare a  Function Pointers
void sayhello();//Declare The Function Whose Address is to be passed
                //The Functions should Be of Same Type
int main()
 print=sayhello;//Addressof sayhello is assigned to print
 print();//print Does A call To The Function 
 return 0;

void sayhello()
 printf("\n Hello World");

enter image description hereAfter That lets See How machine Understands Them.Glimpse of machine instruction of the above programme in 32 bit architecture.

The red mark area is showing how the address is being exchanged and storing in eax. Then their is a call instruction on eax. eax contains the desired address of the function.

@Eric Wang 2014-11-10 08:50:57

Function pointer is usually defined by typedef, and used as param & return value.

Above answers already explained a lot, I just give a full example:

#include <stdio.h>

#define NUM_A 1
#define NUM_B 2

// define a function pointer type
typedef int (*two_num_operation)(int, int);

// an actual standalone function
static int sum(int a, int b) {
    return a + b;

// use function pointer as param,
static int sum_via_pointer(int a, int b, two_num_operation funp) {
    return (*funp)(a, b);

// use function pointer as return value,
static two_num_operation get_sum_fun() {
    return &sum;

// test - use function pointer as variable,
void test_pointer_as_variable() {
    // create a pointer to function,
    two_num_operation sum_p = &sum;
    // call function via pointer
    printf("pointer as variable:\t %d + %d = %d\n", NUM_A, NUM_B, (*sum_p)(NUM_A, NUM_B));

// test - use function pointer as param,
void test_pointer_as_param() {
    printf("pointer as param:\t %d + %d = %d\n", NUM_A, NUM_B, sum_via_pointer(NUM_A, NUM_B, &sum));

// test - use function pointer as return value,
void test_pointer_as_return_value() {
    printf("pointer as return value:\t %d + %d = %d\n", NUM_A, NUM_B, (*get_sum_fun())(NUM_A, NUM_B));

int main() {

    return 0;

@Vamsi 2014-11-17 18:24:09

One of the big uses for function pointers in C is to call a function selected at run-time. For example, the C run-time library has two routines, qsort and bsearch, which take a pointer to a function that is called to compare two items being sorted; this allows you to sort or search, respectively, anything, based on any criteria you wish to use.

A very basic example, if there is one function called print(int x, int y) which in turn may require to call a function (either add() or sub(), which are of the same type) then what we will do, we will add one function pointer argument to the print() function as shown below:

#include <stdio.h>

int add()
   return (100+10);

int sub()
   return (100-10);

void print(int x, int y, int (*func)())
    printf("value is: %d\n", (x+y+(*func)()));

int main()
    int x=100, y=200;

    return 0;

The output is:

value is: 410
value is: 390

@Lee 2011-04-09 00:51:16

The guide to getting fired: How to abuse function pointers in GCC on x86 machines by compiling your code by hand:

These string literals are bytes of 32-bit x86 machine code. 0xC3 is an x86 ret instruction.

You wouldn't normally write these by hand, you'd write in assembly language and then use an assembler like nasm to assemble it into a flat binary which you hexdump into a C string literal.

  1. Returns the current value on the EAX register

    int eax = ((int(*)())("\xc3 <- This returns the value of the EAX register"))();
  2. Write a swap function

    int a = 10, b = 20;
    ((void(*)(int*,int*))"\x8b\x44\x24\x04\x8b\x5c\x24\x08\x8b\x00\x8b\x1b\x31\xc3\x31\xd8\x31\xc3\x8b\x4c\x24\x04\x89\x01\x8b\x4c\x24\x08\x89\x19\xc3 <- This swaps the values of a and b")(&a,&b);
  3. Write a for-loop counter to 1000, calling some function each time

    ((int(*)())"\x66\x31\xc0\x8b\x5c\x24\x04\x66\x40\x50\xff\xd3\x58\x66\x3d\xe8\x03\x75\xf4\xc3")(&function); // calls function with 1->1000
  4. You can even write a recursive function that counts to 100

    const char* lol = "\x8b\x5c\x24\x4\x3d\xe8\x3\x0\x0\x7e\x2\x31\xc0\x83\xf8\x64\x7d\x6\x40\x53\xff\xd3\x5b\xc3\xc3 <- Recursively calls the function at address lol.";
    i = ((int(*)())(lol))(lol);

Note that compilers place string literals in the .rodata section (or .rdata on Windows), which is linked as part of the text segment (along with code for functions).

The text segment has Read+Exec permission, so casting string literals to function pointers works without needing mprotect() or VirtualProtect() system calls like you'd need for dynamically allocated memory. (Or gcc -z execstack links the program with stack + data segment + heap executable, as a quick hack.)

To disassemble these, you can compile this to put a label on the bytes, and use a disassembler.

// at global scope
const char swap[] = "\x8b\x44\x24\x04\x8b\x5c\x24\x08\x8b\x00\x8b\x1b\x31\xc3\x31\xd8\x31\xc3\x8b\x4c\x24\x04\x89\x01\x8b\x4c\x24\x08\x89\x19\xc3 <- This swaps the values of a and b";

Compiling with gcc -c -m32 foo.c and disassembling with objdump -D -rwC -Mintel, we can get the assembly, and find out that this code violates the ABI by clobbering EBX (a call-preserved register) and is generally inefficient.

00000000 <swap>:
   0:   8b 44 24 04             mov    eax,DWORD PTR [esp+0x4]   # load int *a arg from the stack
   4:   8b 5c 24 08             mov    ebx,DWORD PTR [esp+0x8]   # ebx = b
   8:   8b 00                   mov    eax,DWORD PTR [eax]       # dereference: eax = *a
   a:   8b 1b                   mov    ebx,DWORD PTR [ebx]
   c:   31 c3                   xor    ebx,eax                # pointless xor-swap
   e:   31 d8                   xor    eax,ebx                # instead of just storing with opposite registers
  10:   31 c3                   xor    ebx,eax
  12:   8b 4c 24 04             mov    ecx,DWORD PTR [esp+0x4]  # reload a from the stack
  16:   89 01                   mov    DWORD PTR [ecx],eax     # store to *a
  18:   8b 4c 24 08             mov    ecx,DWORD PTR [esp+0x8]
  1c:   89 19                   mov    DWORD PTR [ecx],ebx
  1e:   c3                      ret    

  not shown: the later bytes are ASCII text documentation
  they're not executed by the CPU because the ret instruction sends execution back to the caller

This machine code will (probably) work in 32-bit code on Windows, Linux, OS X, and so on: the default calling conventions on all those OSes pass args on the stack instead of more efficiently in registers. But EBX is call-preserved in all the normal calling conventions, so using it as a scratch register without saving/restoring it can easily make the caller crash.

@SecurityMatt 2013-02-12 05:53:45

Note: this doesn't work if Data Execution Prevention is enabled (e.g. on Windows XP SP2+), because C strings are not normally marked as executable.

@roboto1986 2013-07-03 18:42:09

will this work on Visual Studio? I get:error C2440: 'type cast' : cannot convert from 'const char [68]' to 'void (__cdecl *)(int *,int *)' . Any ideas?

@Lee 2014-01-02 06:20:51

Hi Matt! Depending on the optimization level, GCC will often inline string constants into the TEXT segment, so this will work even on newer version of windows provided that you don't disallow this type of optimization. (IIRC, the MINGW version at the time of my post over two years ago inlines string literals at the default optimization level)

@ajay 2014-01-20 10:17:40

could someone please explain what's happening here? What are those weird looking string literals?

@ejk314 2014-02-21 21:27:59

@ajay It looks like he's writing raw hexidecimal values (for instance '\x00' is the same as '/0', they're both equal to 0) into a string, then casting the string into a C function pointer, then executing the C function pointer because he's the devil.

@fuz 2014-02-22 11:52:02

@LeeGao In linux, strings go into the .rodata (read-only data) section which the operating system marks as read-only non-executable. Executing the strings won't work there either.

@Lee 2014-03-13 00:48:21

hi FUZxxl, I think it might vary based on the compiler and the operating system version. The above code seems to run fine on;

@Pervy Sage 2014-07-20 20:02:20

@JimBalter - Thanks for correction. Could you please explain this poor guy what is this about?

@Joey van Hummel 2015-04-22 12:10:05

Hi there, I fixed a little thingy in the swap function: it accepts pointers, so I edited the post to reflect this in the function prototype.

@Destructor 2015-12-01 11:17:41

@LeeGao: your second trick about swap function invokes UB both in C & C++. Here it result in segmentation faults: &

@Lee 2015-12-11 23:52:11

@PravasiMeet: Calling char* is definitely undefined behavior no matter what language you're in ;) I wrote this back in 2011; back then, most of the major gcc ports to Windows and distributions to mainstream *nix distros used the TEXT relocation trick for constant strings. Within the last 5 years, both the Operating System and the compilers' static analysers have gotten a lot smarter. For a flavor of what it looked like while I was testing this snippet, check out

@Fabio says Reinstate Monica 2017-03-09 18:51:01

This is interesting and fun, but it doesn't belong here.

@FrancescoMM 2017-07-20 07:45:33

You'll get fired, not only loosing your job but they will actually cover you with gasoline and burn you, then shoot you, after. :) Anyway, clever.

@Lee 2017-07-26 03:25:52

I've been on the run for the last 4 years from angry employers armed with tanks of gasoline :P

@0decimal0 2017-10-01 11:02:54

This kind of code is probably the reason why my love for C sometimes whittles down .

@supercat 2017-10-25 21:14:11

Such constructs may be icky, but in many cases they can be more portable than any other alternative. They are obviously dependent upon the target platform, but they avoid reliance upon things like "asm" directives whose behavior will often vary widely even among implementations targeting the same platform.

@ceilingcat 2018-03-02 18:54:23

@Destructor it is written for x86 and not x86_64, you'll have to add the -m32 flag to the compiler if you're on x86_64.

@ceilingcat 2018-03-03 23:59:52

@Lee here is a shorter, hopefully simpler variable swap ((void(*)(int*,int*))"\x8B\x44\x24\x04\x8B\x5C\x24\x08\x8B\x‌​08\x8B\x13\x89\x10\x‌​89\x0B\xC3 <- This swaps the values of a and b")(&a,&b); Try it online!

@Lee 2018-03-05 18:39:33

@ceilingcat Good eyes! A friend noticed a while ago that I used an unnecessarily complicated prologue just to setup the swap using xors, I was waiting for someone else to call me out on this :P

@Peter Cordes 2018-10-09 14:03:48

@ceilingcat: that's more efficient, but you still destroy EBX, which is likely to crash the caller in a 32-bit PIE executable. (gcc often uses EBX to hold a PC-relative reference point.) Probably the best bet for 32-bit code is to save/restore a register. Or for minimal code-size at the cost of a very slow atomic exchange, use xchg [ecx], eax so you only need 2 pointers + 1 value in registers, so you can get by with only call-clobbered EAX, ECX, and EDX. Or just reload one of the pointer args.

@Peter Cordes 2018-10-09 14:13:29

@ceilingcat: yup, looks like reloading the b pointer only costs 1 extra instruction, so it's better than a save/restore to do this with only 3 regs without using xchg. (commented asm source). Obviously a non-inline integer swap function is totally ridiculous, though, and passing args + the call instruction takes about as much code-size as just inlining the swap into a function that probably already has the pointers in registers.

@Richard Chambers 2018-08-20 05:47:15

A function pointer is a variable that contains the address of a function. Since it is a pointer variable though with some restricted properties, you can use it pretty much like you would any other pointer variable in data structures.

The only exception I can think of is treating the function pointer as pointing to something other than a single value. Doing pointer arithmetic by incrementing or decrementing a function pointer or adding/subtracting an offset to a function pointer isn't really of any utility as a function pointer only points to a single thing, the entry point of a function.

The size of a function pointer variable, the number of bytes occupied by the variable, may vary depending on the underlying architecture, e.g. x32 or x64 or whatever.

The declaration for a function pointer variable needs to specify the same kind of information as a function declaration in order for the C compiler to do the kinds of checks that it normally does. If you don't specify a parameter list in the declaration/definition of the function pointer, the C compiler will not be able to check the use of parameters. There are cases when this lack of checking can be useful however just remember that a safety net has been removed.

Some examples:

int func (int a, char *pStr);    // declares a function

int (*pFunc)(int a, char *pStr);  // declares or defines a function pointer

int (*pFunc2) ();                 // declares or defines a function pointer, no parameter list specified.

int (*pFunc3) (void);             // declares or defines a function pointer, no arguments.

The first two declararations are somewhat similar in that:

  • func is a function that takes an int and a char * and returns an int
  • pFunc is a function pointer to which is assigned the address of a function that takes an int and a char * and returns an int

So from the above we could have a source line in which the address of the function func() is assigned to the function pointer variable pFunc as in pFunc = func;.

Notice the syntax used with a function pointer declaration/definition in which parenthesis are used to overcome the natural operator precedence rules.

int *pfunc(int a, char *pStr);    // declares a function that returns int pointer
int (*pFunc)(int a, char *pStr);  // declares a function pointer that returns an int

Several Different Usage Examples

Some examples of usage of a function pointer:

int (*pFunc) (int a, char *pStr);    // declare a simple function pointer variable
int (*pFunc[55])(int a, char *pStr); // declare an array of 55 function pointers
int (**pFunc)(int a, char *pStr);    // declare a pointer to a function pointer variable
struct {                             // declare a struct that contains a function pointer
    int x22;
    int (*pFunc)(int a, char *pStr);
} thing = {0, func};                 // assign values to the struct variable
char * xF (int x, int (*p)(int a, char *pStr));  // declare a function that has a function pointer as an argument
char * (*pxF) (int x, int (*p)(int a, char *pStr));  // declare a function pointer that points to a function that has a function pointer as an argument

You can use variable length parameter lists in the definition of a function pointer.

int sum (int a, int b, ...);
int (*psum)(int a, int b, ...);

Or you can not specify a parameter list at all. This can be useful but it eliminates the opportunity for the C compiler to perform checks on the argument list provided.

int  sum ();      // nothing specified in the argument list so could be anything or nothing
int (*psum)();
int  sum2(void);  // void specified in the argument list so no parameters when calling this function
int (*psum2)(void);

C style Casts

You can use C style casts with function pointers. However be aware that a C compiler may be lax about checks or provide warnings rather than errors.

int sum (int a, char *b);
int (*psplsum) (int a, int b);
psplsum = sum;               // generates a compiler warning
psplsum = (int (*)(int a, int b)) sum;   // no compiler warning, cast to function pointer
psplsum = (int *(int a, int b)) sum;     // compiler error of bad cast generated, parenthesis are required.

Compare Function Pointer to Equality

You can check that a function pointer is equal to a particular function address using an if statement though I am not sure how useful that would be. Other comparison operators would seem to have even less utility.

static int func1(int a, int b) {
    return a + b;

static int func2(int a, int b, char *c) {
    return c[0] + a + b;

static int func3(int a, int b, char *x) {
    return a + b;

static char *func4(int a, int b, char *c, int (*p)())
    if (p == func1) {
        p(a, b);
    else if (p == func2) {
        p(a, b, c);      // warning C4047: '==': 'int (__cdecl *)()' differs in levels of indirection from 'char *(__cdecl *)(int,int,char *)'
    } else if (p == func3) {
        p(a, b, c);
    return c;

An Array of Function Pointers

And if you want to have an array of function pointers each of the elements of which the argument list has differences then you can define a function pointer with the argument list unspecified (not void which means no arguments but just unspecified) something like the following though you may see warnings from the C compiler. This also works for a function pointer parameter to a function:

int(*p[])() = {       // an array of function pointers
    func1, func2, func3
int(**pp)();          // a pointer to a function pointer

p[0](a, b);
p[1](a, b, 0);
p[2](a, b);      // oops, left off the last argument but it compiles anyway.

func4(a, b, 0, func1);
func4(a, b, 0, func2);  // warning C4047: 'function': 'int (__cdecl *)()' differs in levels of indirection from 'char *(__cdecl *)(int,int,char *)'
func4(a, b, 0, func3);

    // iterate over the array elements using an array index
for (i = 0; i < sizeof(p) / sizeof(p[0]); i++) {
    func4(a, b, 0, p[i]);
    // iterate over the array elements using a pointer
for (pp = p; pp < p + sizeof(p)/sizeof(p[0]); pp++) {
    (*pp)(a, b, 0);          // pointer to a function pointer so must dereference it.
    func4(a, b, 0, *pp);     // pointer to a function pointer so must dereference it.

C style namespace Using Global struct with Function Pointers

You can use the static keyword to specify a function whose name is file scope and then assign this to a global variable as a way of providing something similar to the namespace functionality of C++.

In a header file define a struct that will be our namespace along with a global variable that uses it.

typedef struct {
   int (*func1) (int a, int b);             // pointer to function that returns an int
   char *(*func2) (int a, int b, char *c);  // pointer to function that returns a pointer
} FuncThings;

extern const FuncThings FuncThingsGlobal;

Then in the C source file:

#include "header.h"

// the function names used with these static functions do not need to be the
// same as the struct member names. It's just helpful if they are when trying
// to search for them.
// the static keyword ensures these names are file scope only and not visible
// outside of the file.
static int func1 (int a, int b)
    return a + b;

static char *func2 (int a, int b, char *c)
    c[0] = a % 100; c[1] = b % 50;
    return c;

const FuncThings FuncThingsGlobal = {func1, func2};

This would then be used by specifying the complete name of global struct variable and member name to access the function. The const modifier is used on the global so that it can not be changed by accident.

int abcd = FuncThingsGlobal.func1 (a, b);

Application Areas of Function Pointers

A DLL library component could do something similar to the C style namespace approach in which a particular library interface is requested from a factory method in a library interface which supports the creation of a struct containing function pointers.. This library interface loads the requested DLL version, creates a struct with the necessary function pointers, and then returns the struct to the requesting caller for use.

typedef struct {
    HMODULE  hModule;
    int (*Func1)();
    int (*Func2)();
    int(*Func3)(int a, int b);
} LibraryFuncStruct;

int  LoadLibraryFunc LPCTSTR  dllFileName, LibraryFuncStruct *pStruct)
    int  retStatus = 0;   // default is an error detected

    pStruct->hModule = LoadLibrary (dllFileName);
    if (pStruct->hModule) {
        pStruct->Func1 = (int (*)()) GetProcAddress (pStruct->hModule, "Func1");
        pStruct->Func2 = (int (*)()) GetProcAddress (pStruct->hModule, "Func2");
        pStruct->Func3 = (int (*)(int a, int b)) GetProcAddress(pStruct->hModule, "Func3");
        retStatus = 1;

    return retStatus;

void FreeLibraryFunc (LibraryFuncStruct *pStruct)
    if (pStruct->hModule) FreeLibrary (pStruct->hModule);
    pStruct->hModule = 0;

and this could be used as in:

LibraryFuncStruct myLib = {0};
LoadLibraryFunc (L"library.dll", &myLib);
//  ....
//  ....
FreeLibraryFunc (&myLib);

The same approach can be used to define an abstract hardware layer for code that uses a particular model of the underlying hardware. Function pointers are filled in with hardware specific functions by a factory to provide the hardware specific functionality that implements functions specified in the abstract hardware model. This can be used to provide an abstract hardware layer used by software which calls a factory function in order to get the specific hardware function interface then uses the function pointers provided to perform actions for the underlying hardware without needing to know implementation details about the specific target.

Function Pointers to create Delegates, Handlers, and Callbacks

You can use function pointers as a way to delegate some task or functionality. The classic example in C is the comparison delegate function pointer used with the Standard C library functions qsort() and bsearch() to provide the collation order for sorting a list of items or performing a binary search over a sorted list of items. The comparison function delegate specifies the collation algorithm used in the sort or the binary search.

Another use is similar to applying an algorithm to a C++ Standard Template Library container.

void * ApplyAlgorithm (void *pArray, size_t sizeItem, size_t nItems, int (*p)(void *)) {
    unsigned char *pList = pArray;
    unsigned char *pListEnd = pList + nItems * sizeItem;
    for ( ; pList < pListEnd; pList += sizeItem) {
        p (pList);

    return pArray;

int pIncrement(int *pI) {

    return 1;

void * ApplyFold(void *pArray, size_t sizeItem, size_t nItems, void * pResult, int(*p)(void *, void *)) {
    unsigned char *pList = pArray;
    unsigned char *pListEnd = pList + nItems * sizeItem;
    for (; pList < pListEnd; pList += sizeItem) {
        p(pList, pResult);

    return pArray;

int pSummation(int *pI, int *pSum) {
    (*pSum) += *pI;

    return 1;

// source code and then lets use our function.
int intList[30] = { 0 }, iSum = 0;

ApplyAlgorithm(intList, sizeof(int), sizeof(intList) / sizeof(intList[0]), pIncrement);
ApplyFold(intList, sizeof(int), sizeof(intList) / sizeof(intList[0]), &iSum, pSummation);

Another example is with GUI source code in which a handler for a particular event is registered by providing a function pointer which is actually called when the event happens. The Microsoft MFC framework with its message maps uses something similar to handle Windows messages that are delivered to a window or thread.

Asynchronous functions that require a callback are similar to an event handler. The user of the asynchronous function calls the asynchronous function to start some action and provides a function pointer which the asynchronous function will call once the action is complete. In this case the event is the asynchronous function completing its task.

@Yuval Adam 2009-05-08 15:49:59

Function pointers in C

Let's start with a basic function which we will be pointing to:

int addInt(int n, int m) {
    return n+m;

First thing, let's define a pointer to a function which receives 2 ints and returns an int:

int (*functionPtr)(int,int);

Now we can safely point to our function:

functionPtr = &addInt;

Now that we have a pointer to the function, let's use it:

int sum = (*functionPtr)(2, 3); // sum == 5

Passing the pointer to another function is basically the same:

int add2to3(int (*functionPtr)(int, int)) {
    return (*functionPtr)(2, 3);

We can use function pointers in return values as well (try to keep up, it gets messy):

// this is a function called functionFactory which receives parameter n
// and returns a pointer to another function which receives two ints
// and it returns another int
int (*functionFactory(int n))(int, int) {
    printf("Got parameter %d", n);
    int (*functionPtr)(int,int) = &addInt;
    return functionPtr;

But it's much nicer to use a typedef:

typedef int (*myFuncDef)(int, int);
// note that the typedef name is indeed myFuncDef

myFuncDef functionFactory(int n) {
    printf("Got parameter %d", n);
    myFuncDef functionPtr = &addInt;
    return functionPtr;

@Rich.Carpenter 2009-05-08 15:55:02

Thanks for the great info. Could you add some insight on where function pointers are used or happen to be particularly useful?

@hlovdal 2009-05-09 14:39:44

"functionPtr = &addInt;" can also be written (and often is) as " functionPtr = addInt;" which is also valid since the standard says that a function name in this context is converted to the address of the function.

@Johannes Schaub - litb 2009-05-10 17:54:30

hlovdal, in this context it's interesting to explain that this is what enables one to write functionPtr = ******************addInt;

@giant91 2013-10-13 02:28:01

@Rich.Carpenter I know this is 4 years too late, but I figure other people might benefit from this: Function pointers are useful for passing functions as parameters to other functions. It took a lot of searching for me to find that answer for some odd reason. So basically, it gives C pseudo first-class functionality.

@Peter Cordes 2015-08-30 02:22:31

@Rich.Carpenter: function pointers are nice for runtime CPU detection. Have multiple versions of some functions to take advantage of SSE, popcnt, AVX, etc. At startup, set your function pointers to the best version of each function for the current CPU. In your other code, just call through the function pointer instead of having conditional branches on the CPU features everywhere. Then you can do complicated logic about deciding that well, even though this CPU supports pshufb, it's slow, so the earlier implementation is still faster. x264/x265 use this extensively, and are open source.

@Rich.Carpenter 2015-08-31 13:14:15

@Peter Cordes: Thanks. That's what I was looking for. I kind of understood what they were. I was just a big foggy on where they would be used.

@Peter Cordes 2015-08-31 18:33:22

@Rich.Carpenter and other future readers: Another major use is things like the standard library qsort(3) function, where you have to pass a comparison function as an argument. This is what giant91 was talking about.

@vesperto 2017-08-09 14:03:45

If you're treating similar structs and want to handle them with a single function handle(), you can init function pointers according to struct types (some #define) and call them in handle().

@userNaN 2018-05-22 01:02:27

consider clarifying add2to3 -- i could just as well pass any function with the same signature but which returns, say, n*m. in fact, you might want to just scrap add2to3 in favor of something with utility... i'm thinking of something along the lines of map.

@Rafael Eyng 2020-01-29 21:51:06

Only thing that is missing is how to call add2to3. Should it be add2to3(addInt), or add2to3(&addInt)? Both seem to work, which is the root of my confusion.

@coobird 2009-05-08 16:32:47

Function pointers in C can be used to perform object-oriented programming in C.

For example, the following lines is written in C:

String s1 = newString();
s1->set(s1, "hello");

Yes, the -> and the lack of a new operator is a dead give away, but it sure seems to imply that we're setting the text of some String class to be "hello".

By using function pointers, it is possible to emulate methods in C.

How is this accomplished?

The String class is actually a struct with a bunch of function pointers which act as a way to simulate methods. The following is a partial declaration of the String class:

typedef struct String_Struct* String;

struct String_Struct
    char* (*get)(const void* self);
    void (*set)(const void* self, char* value);
    int (*length)(const void* self);

char* getString(const void* self);
void setString(const void* self, char* value);
int lengthString(const void* self);

String newString();

As can be seen, the methods of the String class are actually function pointers to the declared function. In preparing the instance of the String, the newString function is called in order to set up the function pointers to their respective functions:

String newString()
    String self = (String)malloc(sizeof(struct String_Struct));

    self->get = &getString;
    self->set = &setString;
    self->length = &lengthString;

    self->set(self, "");

    return self;

For example, the getString function that is called by invoking the get method is defined as the following:

char* getString(const void* self_obj)
    return ((String)self_obj)->internal->value;

One thing that can be noticed is that there is no concept of an instance of an object and having methods that are actually a part of an object, so a "self object" must be passed in on each invocation. (And the internal is just a hidden struct which was omitted from the code listing earlier -- it is a way of performing information hiding, but that is not relevant to function pointers.)

So, rather than being able to do s1->set("hello");, one must pass in the object to perform the action on s1->set(s1, "hello").

With that minor explanation having to pass in a reference to yourself out of the way, we'll move to the next part, which is inheritance in C.

Let's say we want to make a subclass of String, say an ImmutableString. In order to make the string immutable, the set method will not be accessible, while maintaining access to get and length, and force the "constructor" to accept a char*:

typedef struct ImmutableString_Struct* ImmutableString;

struct ImmutableString_Struct
    String base;

    char* (*get)(const void* self);
    int (*length)(const void* self);

ImmutableString newImmutableString(const char* value);

Basically, for all subclasses, the available methods are once again function pointers. This time, the declaration for the set method is not present, therefore, it cannot be called in a ImmutableString.

As for the implementation of the ImmutableString, the only relevant code is the "constructor" function, the newImmutableString:

ImmutableString newImmutableString(const char* value)
    ImmutableString self = (ImmutableString)malloc(sizeof(struct ImmutableString_Struct));

    self->base = newString();

    self->get = self->base->get;
    self->length = self->base->length;

    self->base->set(self->base, (char*)value);

    return self;

In instantiating the ImmutableString, the function pointers to the get and length methods actually refer to the String.get and String.length method, by going through the base variable which is an internally stored String object.

The use of a function pointer can achieve inheritance of a method from a superclass.

We can further continue to polymorphism in C.

If for example we wanted to change the behavior of the length method to return 0 all the time in the ImmutableString class for some reason, all that would have to be done is to:

  1. Add a function that is going to serve as the overriding length method.
  2. Go to the "constructor" and set the function pointer to the overriding length method.

Adding an overriding length method in ImmutableString may be performed by adding an lengthOverrideMethod:

int lengthOverrideMethod(const void* self)
    return 0;

Then, the function pointer for the length method in the constructor is hooked up to the lengthOverrideMethod:

ImmutableString newImmutableString(const char* value)
    ImmutableString self = (ImmutableString)malloc(sizeof(struct ImmutableString_Struct));

    self->base = newString();

    self->get = self->base->get;
    self->length = &lengthOverrideMethod;

    self->base->set(self->base, (char*)value);

    return self;

Now, rather than having an identical behavior for the length method in ImmutableString class as the String class, now the length method will refer to the behavior defined in the lengthOverrideMethod function.

I must add a disclaimer that I am still learning how to write with an object-oriented programming style in C, so there probably are points that I didn't explain well, or may just be off mark in terms of how best to implement OOP in C. But my purpose was to try to illustrate one of many uses of function pointers.

For more information on how to perform object-oriented programming in C, please refer to the following questions:

@Alexei Averchenko 2012-09-16 14:30:47

This answer is horrible! Not only it implies that OO somehow depends on dot notation, it also encourages putting junk into your objects!

@Reinstate Monica 2013-03-18 21:53:15

This is OO all right, but not anywhere near the C-style OO. What you have brokenly implemented is Javascript-style prototype-based OO. To get C++/Pascal-style OO, you'd need to: 1. Have a const struct for a virtual table of each class with virtual members. 2. Have pointer to that struct in polymorphic objects. 3. Call virtual methods via the virtual table, and all other methods directly -- usually by sticking to some ClassName_methodName function naming convention. Only then you get the same runtime and storage costs as you do in C++ and Pascal.

@rbaleksandar 2013-07-04 15:21:34

Working OO with a language that is not intended to be OO is always a bad idea. If you want OO and still have C just work with C++.

@Jonathon Reinhart 2015-04-30 12:31:24

@rbaleksandar Tell that to the Linux kernel developers. "always a bad idea" is strictly your opinion, with which I firmly disagree.

@Nathan FD 2016-07-19 20:50:08

Your ImmutableString violates the Liskov substitution principle

@Madhusoodan P 2016-07-23 10:36:46

In this implementation each instance created will waste a lot of memory. Hence providing a struct such as static String_Funcs with pointers to functions.

@cat 2016-09-29 14:41:26

I like this answer but don't cast malloc

@Zack Sheffield 2013-06-10 19:56:41

Another good use for function pointers:
Switching between versions painlessly

They're very handy to use for when you want different functions at different times, or different phases of development. For instance, I'm developing an application on a host computer that has a console, but the final release of the software will be put on an Avnet ZedBoard (which has ports for displays and consoles, but they are not needed/wanted for the final release). So during development, I will use printf to view status and error messages, but when I'm done, I don't want anything printed. Here's what I've done:


// First, undefine all macros associated with version.h

// Define which version we want to use
#define DEBUG_VERSION       // The current version
// #define RELEASE_VERSION  // To be uncommented when finished debugging

#ifndef __VERSION_H_      /* prevent circular inclusions */
    #define __VERSION_H_  /* by using protection macros */
    void board_init();
    void noprintf(const char *c, ...); // mimic the printf prototype

// Mimics the printf function prototype. This is what I'll actually 
// use to print stuff to the screen
void (* zprintf)(const char*, ...); 

// If debug version, use printf
    #include <stdio.h>

// If both debug and release version, error

// If neither debug or release version, error

    // Won't allow compilation without a valid version define
    #error "Invalid version definition"

In version.c I will define the 2 function prototypes present in version.h


#include "version.h"

* @name board_init
* Sets up the application based on the version type defined in version.h.
* Includes allowing or prohibiting printing to STDOUT.
* @return    None
void board_init()
    // Assign the print function to the correct function pointer
    #ifdef DEBUG_VERSION
        zprintf = &printf;
        // Defined below this function
        zprintf = &noprintf;

* @name noprintf
* simply returns with no actions performed
* @return   None
void noprintf(const char* c, ...)

Notice how the function pointer is prototyped in version.h as

void (* zprintf)(const char *, ...);

When it is referenced in the application, it will start executing wherever it is pointing, which has yet to be defined.

In version.c, notice in the board_init()function where zprintf is assigned a unique function (whose function signature matches) depending on the version that is defined in version.h

zprintf = &printf; zprintf calls printf for debugging purposes


zprintf = &noprint; zprintf just returns and will not run unnecessary code

Running the code will look like this:


#include "version.h"
#include <stdlib.h>
int main()
    // Must run board_init(), which assigns the function
    // pointer to an actual function

    void *ptr = malloc(100); // Allocate 100 bytes of memory
    // malloc returns NULL if unable to allocate the memory.

    if (ptr == NULL)
        zprintf("Unable to allocate memory\n");
        return 1;

    // Other things to do...
    return 0;

The above code will use printf if in debug mode, or do nothing if in release mode. This is much easier than going through the entire project and commenting out or deleting code. All that I need to do is change the version in version.h and the code will do the rest!

@AlphaGoku 2018-05-11 13:41:45

U stand to lose a lot of performance time. Instead you could use a macro that enables and disables a section of code based on Debug / Release.

@Nick Van Brunt 2009-05-08 16:26:09

One of my favorite uses for function pointers is as cheap and easy iterators -

#include <stdio.h>
#define MAX_COLORS  256

typedef struct {
    char* name;
    int red;
    int green;
    int blue;
} Color;

Color Colors[MAX_COLORS];

void eachColor (void (*fp)(Color *c)) {
    int i;
    for (i=0; i<MAX_COLORS; i++)

void printColor(Color* c) {
    if (c->name)
        printf("%s = %i,%i,%i\n", c->name, c->red, c->green, c->blue);

int main() {


@Alexei Averchenko 2012-09-16 14:32:52

You should also pass a pointer to user-specified data if you want to somehow extract any output from iterations (think closures).

@Jonathon Reinhart 2015-04-30 12:35:25

Agreed. All of my iterators look like this: int (*cb)(void *arg, ...). The return value of the iterator also lets me stop early (if nonzero).

@Tim Post 2009-05-09 13:56:12

Since function pointers are often typed callbacks, you might want to have a look at type safe callbacks. The same applies to entry points, etc of functions that are not callbacks.

C is quite fickle and forgiving at the same time :)

@Johannes Schaub - litb 2009-05-09 02:05:44

Function pointers become easy to declare once you have the basic declarators:

  • id: ID: ID is a
  • Pointer: *D: D pointer to
  • Function: D(<parameters>): D function taking <parameters> returning

While D is another declarator built using those same rules. In the end, somewhere, it ends with ID (see below for an example), which is the name of the declared entity. Let's try to build a function taking a pointer to a function taking nothing and returning int, and returning a pointer to a function taking a char and returning int. With type-defs it's like this

typedef int ReturnFunction(char);
typedef int ParameterFunction(void);
ReturnFunction *f(ParameterFunction *p);

As you see, it's pretty easy to build it up using typedefs. Without typedefs, it's not hard either with the above declarator rules, applied consistently. As you see i missed out the part the pointer points to, and the thing the function returns. That's what appears at the very left of the declaration, and is not of interest: It's added at the end if one built up the declarator already. Let's do that. Building it up consistently, first wordy - showing the structure using [ and ]:

function taking 
    [pointer to [function taking [void] returning [int]]] 
    [pointer to [function taking [char] returning [int]]]

As you see, one can describe a type completely by appending declarators one after each other. Construction can be done in two ways. One is bottom-up, starting with the very right thing (leaves) and working the way through up to the identifier. The other way is top-down, starting at the identifier, working the way down to the leaves. I'll show both ways.

Bottom Up

Construction starts with the thing at the right: The thing returned, which is the function taking char. To keep the declarators distinct, i'm going to number them:


Inserted the char parameter directly, since it's trivial. Adding a pointer to declarator by replacing D1 by *D2. Note that we have to wrap parentheses around *D2. That can be known by looking up the precedence of the *-operator and the function-call operator (). Without our parentheses, the compiler would read it as *(D2(char p)). But that would not be a plain replace of D1 by *D2 anymore, of course. Parentheses are always allowed around declarators. So you don't make anything wrong if you add too much of them, actually.


Return type is complete! Now, let's replace D2 by the function declarator function taking <parameters> returning, which is D3(<parameters>) which we are at now.


Note that no parentheses are needed, since we want D3 to be a function-declarator and not a pointer declarator this time. Great, only thing left is the parameters for it. The parameter is done exactly the same as we've done the return type, just with char replaced by void. So i'll copy it:

(*D3(   (*ID1)(void)))(char)

I've replaced D2 by ID1, since we are finished with that parameter (it's already a pointer to a function - no need for another declarator). ID1 will be the name of the parameter. Now, i told above at the end one adds the type which all those declarator modify - the one appearing at the very left of every declaration. For functions, that becomes the return type. For pointers the pointed to type etc... It's interesting when written down the type, it will appear in the opposite order, at the very right :) Anyway, substituting it yields the complete declaration. Both times int of course.

int (*ID0(int (*ID1)(void)))(char)

I've called the identifier of the function ID0 in that example.

Top Down

This starts at the identifier at the very left in the description of the type, wrapping that declarator as we walk our way through the right. Start with function taking <parameters> returning


The next thing in the description (after "returning") was pointer to. Let's incorporate it:


Then the next thing was functon taking <parameters> returning. The parameter is a simple char, so we put it in right away again, since it's really trivial.


Note the parentheses we added, since we again want that the * binds first, and then the (char). Otherwise it would read function taking <parameters> returning function .... Noes, functions returning functions aren't even allowed.

Now we just need to put <parameters>. I will show a short version of the deriveration, since i think you already by now have the idea how to do it.

pointer to: *ID1
... function taking void returning: (*ID1)(void)

Just put int before the declarators like we did with bottom-up, and we are finished

int (*ID0(int (*ID1)(void)))(char)

The nice thing

Is bottom-up or top-down better? I'm used to bottom-up, but some people may be more comfortable with top-down. It's a matter of taste i think. Incidentally, if you apply all the operators in that declaration, you will end up getting an int:

int v = (*ID0(some_function_pointer))(some_char);

That is a nice property of declarations in C: The declaration asserts that if those operators are used in an expression using the identifier, then it yields the type on the very left. It's like that for arrays too.

Hope you liked this little tutorial! Now we can link to this when people wonder about the strange declaration syntax of functions. I tried to put as little C internals as possible. Feel free to edit/fix things in it.

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