Main use of function pointers (from what I gather) is to pass them along with some variables to a function which will then call the function the pointer points to. To achieve similar results one can pass an arbitrary integer instead of a function pointer and let a switch case call the appropriate function; which will also bypass the restriction that is innate to function pointers, that a function you are passing the pointer to, needs to know exactly what kind of function is coming to it (what it returns and what variables it expects.)
Is there an advantage to using function pointers over the switch case method proposed above? I'm interested in technical capabilities of function pointers that I might be missing and preferably some examples too.
If the functions each have different signatures, you'll need to have a separate case for each one in order to call the right parameters and get the right return type.
If the signatures are the same, you could instead create an array of function pointers and just index the array. This avoids branches and is simpler to maintain. For example:
int add(int, int);
int sub(int, int);
typedef int (*math_func)(int, int);
math_func arr[] = { add, sub };
int call_math_func(int idx, int a, int b)
{
return math_func[idx](a, b);
}
I prefer using function pointers when possible because imho they make intents more explicit at the calling site, e.g.:
#include <stdio.h>
typedef void (*HelloWorld)(void);
void english(void) { printf("Hello, World!\n"); }
void italian(void) { printf("Ciao, Mondo!\n"); }
void greet_world_in(HelloWorld hello_world) { hello_world(); }
int main(void)
{
greet_world_in(english);
greet_world_in(italian);
return 0;
}
Assuming that the function pointers have exactly the same kind of prototype, a switch case statement is often implemented as a jump table:
void conceptual_dispatcher_emulating_switch_case(int n) {
static const jmp_ptr_t jmp_table[5]={label_0,label_1,label_2,label_3,label_4};
if (n < 5) {
goto jmp_table[n];
}
return;
label_0 : return myfunc_1();
label_1 : return myfunc_2();
label_2 : return myfunc_3();
label_3 : return myfunc_4();
label_4 : return myfunc_5();
}
From this it's pretty easy to make an optimization:
void conceptual_dispatcher_better(int n) {
static const function_ptr_t jmp_table[5]={myfunc_1, myfunc_2, myfunc_3, myfunc_4, myfunc_5};
if (n < 5) {
goto jmp_table[n];
}
return;
}
And from this the next logical step is to
void conceptual_dispatcher_even_better(function_ptr_t *ptr) {
ptr();
}
And from this the next step is to make that inline or just call the ptr without the dispatcher.
I would like this to work, but it does not:
#include <stdio.h>
typedef struct closure_s {
void (*incrementer) ();
void (*emitter) ();
} closure;
closure emit(int in) {
void incrementer() {
in++;
}
void emitter() {
printf("%d\n", in);
}
return (closure) {
incrementer,
emitter
};
}
main() {
closure test[] = {
emit(10),
emit(20)
};
test[0] . incrementer();
test[1] . incrementer();
test[0] . emitter();
test[1] . emitter();
}
It actually does compile and does work for 1 instance ... but the second one fails. Any idea how to get closures in C?
It would be truly awesome!
Using FFCALL,
#include <callback.h>
#include <stdio.h>
static void incrementer_(int *in) {
++*in;
}
static void emitter_(int *in) {
printf("%d\n", *in);
}
int main() {
int in1 = 10, in2 = 20;
int (*incrementer1)() = alloc_callback(&incrementer_, &in1);
int (*emitter1)() = alloc_callback(&emitter_, &in1);
int (*incrementer2)() = alloc_callback(&incrementer_, &in2);
int (*emitter2)() = alloc_callback(&emitter_, &in2);
incrementer1();
incrementer2();
emitter1();
emitter2();
free_callback(incrementer1);
free_callback(incrementer2);
free_callback(emitter1);
free_callback(emitter2);
}
But usually in C you end up passing extra arguments around to fake closures.
Apple has a non-standard extension to C called blocks, which do work much like closures.
The ANSI C has not a support for closure, as well as nested functions. Workaround for it is usage simple "struct".
Simple example closure for sum two numbers.
// Structure for keep pointer for function and first parameter
typedef struct _closure{
int x;
char* (*call)(struct _closure *str, int y);
} closure;
// An function return a result call a closure as string
char *
sumY(closure *_closure, int y) {
char *msg = calloc(20, sizeof(char));
int sum = _closure->x + y;
sprintf(msg, "%d + %d = %d", _closure->x, y, sum);
return msg;
}
// An function return a closure for sum two numbers
closure *
sumX(int x) {
closure *func = (closure*)malloc(sizeof(closure));
func->x = x;
func->call = sumY;
return func;
}
Usage:
int main (int argv, char **argc)
{
closure *sumBy10 = sumX(10);
puts(sumBy10->call(sumBy10, 1));
puts(sumBy10->call(sumBy10, 3));
puts(sumBy10->call(sumBy10, 2));
puts(sumBy10->call(sumBy10, 4));
puts(sumBy10->call(sumBy10, 5));
}
Result:
10 + 1 = 11
10 + 3 = 13
10 + 2 = 12
10 + 4 = 14
10 + 5 = 15
On C++11 it will be achived by use lambda expression.
#include <iostream>
int main (int argv, char **argc)
{
int x = 10;
auto sumBy10 = [x] (int y) {
std::cout << x << " + " << y << " = " << x + y << std::endl;
};
sumBy10(1);
sumBy10(2);
sumBy10(3);
sumBy10(4);
sumBy10(5);
}
A result, after compilation with a flag -std=c++11.
10 + 1 = 11
10 + 2 = 12
10 + 3 = 13
10 + 4 = 14
10 + 5 = 15
A Working Definition of a Closure with a JavaScript Example
A closure is a kind of object that contains a pointer or reference of some kind to a function to be executed along with the an instance of the data needed by the function.
An example in JavaScript from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures is
function makeAdder(x) {
return function(y) { // create the adder function and return it along with
return x + y; // the captured data needed to generate its return value
};
}
which could then be used like:
var add5 = makeAdder(5); // create an adder function which adds 5 to its argument
console.log(add5(2)); // displays a value of 2 + 5 or 7
Some of the Obstacles to Overcome with C
The C programming language is a statically typed language, unlike JavaScript, nor does it have garbage collection, and some other features that make it easy to do closures in JavaScript or other languages with intrinsic support for closures.
One large obstacle for closures in Standard C is the lack of language support for the kind of construct in the JavaScript example in which the closure includes not only the function but also a copy of data that is captured when the closure is created, a way of saving state which can then be used when the closure is executed along with any additional arguments provided at the time the closure function is invoked.
However C does have some basic building blocks which can provide the tools for creating a kind of closure. Some of the difficulties are (1) memory management is the duty of the programmer, no garbage collection, (2) functions and data are separated, no classes or class type mechanics, (3) statically typed so no run time discovery of data types or data sizes, and (4) poor language facilities for capturing state data at the time the closure is created.
One thing that makes something of a closure facility possible with C is the void * pointer and using unsigned char as a kind of general purpose memory type which is then transformed into other types through casting.
An update with new approach
My original posted answer seems to have been helpful enough that people have upvoted it however it had a constraint or two that I didn't like.
Getting a notification of a recent upvote, I took a look at some of the other posted answers and realized that I could provide a second approach that would overcome the problem that bothered me.
A new approach that removes a problem of the original approach
The original approach required function arguments to be passed on the stack. This new approach eliminates that requirement. It also seems much cleaner. I'm keeping the original approach below.
The new approach uses a single struct, ClosureStruct, along with two functions to build the closure, makeClosure() and pushClosureArg().
This new approach also uses the variable argument functionality of stdarg.h to process the captured arguments in the closure data.
Using the following in a C source code file requires the following includes:
#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <stdarg.h>
typedef struct {
void (*p)(); // pointer to the function of this closure
size_t sargs; // size of the memory area allocated for closure data
size_t cargs; // current memory area in use for closure data
unsigned char * args; // pointer to the allocated closure data area
} ClosureStruct;
void * makeClosure(void (*p)(), size_t sargs)
{
// allocate the space for the closure management data and the closure data itself.
// we do this with a single call to calloc() so that we have only one pointer to
// manage.
ClosureStruct* cp = calloc(1, sizeof(ClosureStruct) + sargs);
if (cp) {
cp->p = p; // save a pointer to the function
cp->sargs = sargs; // save the total size of the memory allocated for closure data
cp->cargs = 0; // initialize the amount of memory used
cp->args = (unsigned char *)(cp + 1); // closure data is after closure management block
}
return cp;
}
void * pushClosureArg(void* cp, size_t sarg, void* arg)
{
if (cp) {
ClosureStruct* p = cp;
if (p->cargs + sarg <= p->sargs) {
// there is room in the closure area for this argument so make a copy
// of the argument and remember our new end of memory.
memcpy(p->args + p->cargs, arg, sarg);
p->cargs += sarg;
}
}
return cp;
}
This code is then used similar to the following:
// example functions that we will use with closures
// funcadd() is a function that accepts a closure with two int arguments
// along with three additional int arguments.
// it is similar to the following function declaration:
// void funcadd(int x1, int x2, int a, int b, int c);
//
void funcadd(ClosureStruct* cp, int a, int b, int c)
{
// using the variable argument functionality we will set our
// variable argument list address to the closure argument memory area
// and then start pulling off the arguments that are provided by the closure.
va_list jj;
va_start(jj, cp->args); // get the address of the first argument
int x1 = va_arg(jj, int); // get the first argument of the closure
int x2 = va_arg(jj, int);
printf("funcadd() = %d\n", a + b + c + x1 + x2);
}
int zFunc(ClosureStruct* cp, int j, int k)
{
va_list jj;
va_start(jj, cp->args); // get the address of the first argument
int i = va_arg(jj, int);
printf("zFunc() i = %d, j = %d, k = %d\n", i, j, k);
return i + j + k;
}
typedef struct { char xx[24]; } thing1;
int z2func( ClosureStruct* cp, int i)
{
va_list jj;
va_start(jj, cp->args); // get the address of the first argument
thing1 a = va_arg(jj, thing1);
printf("z2func() i = %d, %s\n", i, a.xx);
return 0;
}
int mainxx(void)
{
ClosureStruct* p;
int x;
thing1 xpxp = { "1234567890123" };
p = makeClosure(funcadd, 256);
x = 4; pushClosureArg(p, sizeof(int), &x);
x = 10; pushClosureArg(p, sizeof(int), &x);
p->p(p, 1, 2, 3);
free(p);
p = makeClosure(z2func, sizeof(thing1));
pushClosureArg(p, sizeof(thing1), &xpxp);
p->p(p, 45);
free(p);
p = makeClosure(zFunc, sizeof(int));
x = 5; pushClosureArg(p, sizeof(int), &x);
p->p(p, 12, 7);
return 0;
}
The output from the above usage is:
funcadd() = 20
z2func() i = 45, 1234567890123
zFunc() i = 5, j = 12, k = 7
However there is an issue with the above implementation, you have no way of getting the return value of a function that returns a value. In other words, the function zFunc() used in a closure above returns an int value which is ignored. If you try to capture the return value with something like int k = pint->p(pint, 12, 7); you will get an error message because the function pointer argument of ClosureStruct is void (*p)(); rather than int (*p)();.
To work around this restraint, we will add two C Preprocessor macros to help us create individual versions of the ClosureStruct struct that specify a function return type other than void.
#define NAME_CLOSURE(t) ClosureStruct_ ## t
#define DEF_CLOSURE(t) \
typedef struct { \
t (*p)(); \
size_t sargs; \
size_t cargs; \
unsigned char* args; \
} NAME_CLOSURE(t);
We then redefine the two functions, zFunc() and z2func(), as follows using the macros.
DEF_CLOSURE(int) // define closure struct that returns an int
int zFunc(NAME_CLOSURE(int)* cp, int j, int k)
{
va_list jj;
va_start(jj, cp->args); // get the address of the first argument
int i = va_arg(jj, int);
printf("zFunc() i = %d, j = %d, k = %d\n", i, j, k);
return i + j + k;
}
typedef struct { char xx[24]; } thing1;
int z2func( NAME_CLOSURE(int) * cp, int i)
{
va_list jj;
va_start(jj, cp->args); // get the address of the first argument
thing1 a = va_arg(jj, thing1);
printf("z2func() i = %d, %s\n", i, a.xx);
return 0;
}
And we use this as follows:
int mainxx(void)
{
ClosureStruct* p;
NAME_CLOSURE(int) *pint;
int x;
thing1 xpxp = { "1234567890123" };
p = makeClosure(funcadd, 256);
x = 4; pushClosureArg(p, sizeof(int), &x);
x = 10; pushClosureArg(p, sizeof(int), &x);
p->p(p, 1, 2, 3);
free(p);
pint = makeClosure(z2func, sizeof(thing1));
pushClosureArg(pint, sizeof(thing1), &xpxp);
int k = pint->p(pint, 45);
free(pint);
pint = makeClosure(zFunc, sizeof(int));
x = 5; pushClosureArg(pint, sizeof(int), &x);
k = pint->p(pint, 12, 7);
return 0;
}
First Implementation With Standard C and a Bit of Stretching Here and There
NOTE: The following example depends on a stack based argument passing convention as is used with most x86 32 bit compilers. Most compilers also allow for a calling convention to be specified other than stack based argument passing such as the __fastcall modifier of Visual Studio. The default for x64 and 64 bit Visual Studio is to use the __fastcall convention by default so that function arguments are passed in registers and not on the stack. See Overview of x64 Calling Conventions in the Microsoft MSDN as well as How to set function arguments in assembly during runtime in a 64bit application on Windows? as well as the various answers and comments in How are variable arguments implemented in gcc? .
One thing that we can do is to solve this problem of providing some kind of closure facility for C is to simplify the problem. Better to provide an 80% solution that is useful for a majority of applications than no solution at all.
One such simplification is to only support functions that do not return a value, in other words functions declared as void func_name(). We are also going to give up compile time type checking of the function argument list since this approach builds the function argument list at run time. Neither one of these things that we are giving up are trivial so the question is whether the value of this approach to closures in C outweighs what we are giving up.
First of all lets define our closure data area. The closure data area represents the memory area we are going to use to contain the information we need for a closure. The minimum amount of data I can think of is a pointer to the function to execute and a copy of the data to be provided to the function as arguments.
In this case we are going to provide any captured state data needed by the function as an argument to the function.
We also want to have some basic safe guards in place so that we will fail reasonably safely. Unfortunately the safety rails are a bit weak with some of the work arounds we are using to implement a form of closures.
The Source Code
The following source code was developed using Visual Studio 2017 Community Edition in a .c C source file.
The data area is a struct that contains some management data, a pointer to the function, and an open ended data area.
typedef struct {
size_t nBytes; // current number of bytes of data
size_t nSize; // maximum size of the data area
void(*pf)(); // pointer to the function to invoke
unsigned char args[1]; // beginning of the data area for function arguments
} ClosureStruct;
Next we create a function that will initialize a closure data area.
ClosureStruct * beginClosure(void(*pf)(), int nSize, void *pArea)
{
ClosureStruct *p = pArea;
if (p) {
p->nBytes = 0; // number of bytes of the data area in use
p->nSize = nSize - sizeof(ClosureStruct); // max size of the data area
p->pf = pf; // pointer to the function to invoke
}
return p;
}
This function is designed to accept a pointer to a data area which gives flexibility as to how the user of the function wants to manage memory. They can either use some memory on the stack or static memory or they can use heap memory via the malloc() function.
unsigned char closure_area[512];
ClosureStruct *p = beginClosure (xFunc, 512, closure_area);
or
ClosureStruct *p = beginClosure (xFunc, 512, malloc(512));
// do things with the closure
free (p); // free the malloced memory.
Next we provide a function that allows us to add data and arguments to our closure. The purpose of this function is to build up the closure data so that when closure function is invoked, the closure function will be provided any data it needs to do its job.
ClosureStruct * pushDataClosure(ClosureStruct *p, size_t size, ...)
{
if (p && p->nBytes + size < p->nSize) {
va_list jj;
va_start(jj, size); // get the address of the first argument
memcpy(p->args + p->nBytes, jj, size); // copy the specified size to the closure memory area.
p->nBytes += size; // keep up with how many total bytes we have copied
va_end(jj);
}
return p;
}
And to make this a bit simpler to use lets provide a wrapping macro which is generally handy but does have limitations since it is C Processor text manipulation.
#define PUSHDATA(cs,d) pushDataClosure((cs),sizeof(d),(d))
so we could then use something like the following source code:
unsigned char closurearea[256];
int iValue = 34;
ClosureStruct *dd = PUSHDATA(beginClosure(z2func, 256, closurearea), iValue);
dd = PUSHDATA(dd, 68);
execClosure(dd);
Invoking the Closure: The execClosure() Function
The last piece to this is the execClosure() function to execute the closure function with its data. What we are doing in this function is to copy the argument list supplied in the closure data structure onto the stack as we invoke the function.
What we do is cast the args area of the closure data to a pointer to a struct containing an unsigned char array and then dereference the pointer so that the C compiler will put a copy of the arguments onto the stack before it calls the function in the closure.
To make it easier to create the execClosure() function, we will create a macro that makes it easy to create the various sizes of structs we need.
// helper macro to reduce type and reduce chance of typing errors.
#define CLOSEURESIZE(p,n) if ((p)->nBytes < (n)) { \
struct {\
unsigned char x[n];\
} *px = (void *)p->args;\
p->pf(*px);\
}
Then we use this macro to create a series of tests to determine how to call the closure function. The sizes chosen here may need tweaking for particular applications. These sizes are arbitrary and since the closure data will rarely be of the same size, this is not efficiently using stack space. And there is the possibility that there may be more closure data than we have allowed for.
// execute a closure by calling the function through the function pointer
// provided along with the created list of arguments.
ClosureStruct * execClosure(ClosureStruct *p)
{
if (p) {
// the following structs are used to allocate a specified size of
// memory on the stack which is then filled with a copy of the
// function argument list provided in the closure data.
CLOSEURESIZE(p,64)
else CLOSEURESIZE(p, 128)
else CLOSEURESIZE(p, 256)
else CLOSEURESIZE(p, 512)
else CLOSEURESIZE(p, 1024)
else CLOSEURESIZE(p, 1536)
else CLOSEURESIZE(p, 2048)
}
return p;
}
We return the pointer to the closure in order to make it easily available.
An Example Using the Library Developed
We can use the above as follows. First a couple of example functions that don't really do much.
int zFunc(int i, int j, int k)
{
printf("zFunc i = %d, j = %d, k = %d\n", i, j, k);
return i + j + k;
}
typedef struct { char xx[24]; } thing1;
int z2func(thing1 a, int i)
{
printf("i = %d, %s\n", i, a.xx);
return 0;
}
Next we build our closures and execute them.
{
unsigned char closurearea[256];
thing1 xpxp = { "1234567890123" };
thing1 *ypyp = &xpxp;
int iValue = 45;
ClosureStruct *dd = PUSHDATA(beginClosure(z2func, 256, malloc(256)), xpxp);
free(execClosure(PUSHDATA(dd, iValue)));
dd = PUSHDATA(beginClosure(z2func, 256, closurearea), *ypyp);
dd = PUSHDATA(dd, 68);
execClosure(dd);
dd = PUSHDATA(beginClosure(zFunc, 256, closurearea), iValue);
dd = PUSHDATA(dd, 145);
dd = PUSHDATA(dd, 185);
execClosure(dd);
}
Which gives an output of
i = 45, 1234567890123
i = 68, 1234567890123
zFunc i = 45, j = 145, k = 185
Well What About Currying?
Next we could make a modification to our closure struct to allow us to do currying of functions.
typedef struct {
size_t nBytes; // current number of bytes of data
size_t nSize; // maximum size of the data area
size_t nCurry; // last saved nBytes for curry and additional arguments
void(*pf)(); // pointer to the function to invoke
unsigned char args[1]; // beginning of the data area for function arguments
} ClosureStruct;
with the supporting functions for currying and resetting of a curry point being
ClosureStruct *curryClosure(ClosureStruct *p)
{
p->nCurry = p->nBytes;
return p;
}
ClosureStruct *resetCurryClosure(ClosureStruct *p)
{
p->nBytes = p->nCurry;
return p;
}
The source code for testing this could be:
{
unsigned char closurearea[256];
thing1 xpxp = { "1234567890123" };
thing1 *ypyp = &xpxp;
int iValue = 45;
ClosureStruct *dd = PUSHDATA(beginClosure(z2func, 256, malloc(256)), xpxp);
free(execClosure(PUSHDATA(dd, iValue)));
dd = PUSHDATA(beginClosure(z2func, 256, closurearea), *ypyp);
dd = PUSHDATA(dd, 68);
execClosure(dd);
dd = PUSHDATA(beginClosure(zFunc, 256, closurearea), iValue);
dd = PUSHDATA(dd, 145);
dd = curryClosure(dd);
dd = resetCurryClosure(execClosure(PUSHDATA(dd, 185)));
dd = resetCurryClosure(execClosure(PUSHDATA(dd, 295)));
}
with the output of
i = 45, 1234567890123
i = 68, 1234567890123
zFunc i = 45, j = 145, k = 185
zFunc i = 45, j = 145, k = 295
GCC and clang have the blocks extension, which is essentially closures in C.
GCC supports inner functions, but not closures. C++0x will have closures. No version of C that I'm aware of, and certainly no standard version, provides that level of awesome.
Phoenix, which is part of Boost, provides closures in C++.
On this page you can find a description on how to do closures in C:
http://brodowsky.it-sky.net/2014/06/20/closures-in-c-and-scala/
The idea is that a struct is needed and that struct contains the function pointer, but gets provided to the function as first argument. Apart from the fact that it requires a lot of boiler plate code and the memory management is off course an issue, this works and provides the power and possibilities of other languages' closures.
You can achieve this with -fblocks flag, but it does not look so nice like in JS or TS:
#include <stdio.h>
#include <stdlib.h>
#include <Block.h>
#define NEW(T) ({ \
T* __ret = (T*)calloc(1, sizeof(T)); \
__ret; \
})
typedef struct data_t {
int value;
} data_t;
typedef struct object_t {
int (^get)(void);
void (^set)(int);
void (^free)(void);
} object_t;
object_t const* object_create(void) {
data_t* priv = NEW(data_t);
object_t* pub = NEW(object_t);
priv->value = 123;
pub->get = Block_copy(^{
return priv->value;
});
pub->set = Block_copy(^(int value){
priv->value = value;
});
pub->free = Block_copy(^{
free(priv);
free(pub);
});
return pub;
}
int main() {
object_t const* obj = object_create();
printf("before: %d\n", obj->get());
obj->set(321);
printf("after: %d\n", obj->get());
obj->free();
return 0;
}
clang main.c -o main.o -fblocks -fsanitize=address; ./main.o
before: 123
after: 321
The idiomatic way of doing it in is C is passing a function pointer and a void pointer to the context.
However, some time ago I came up with a different approach. Surprisingly, there is a family of builtin types in C that carries both a data and the code itself. Those are pointers to a function pointer.
The trick is use this single object to pass both the code by dereferencing a function pointer. And next passing the very same double function pointer as the context as a first argument. It looks a bit convoluted by actually it results in very flexible and readable machanism for closures.
See the code:
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
// typedefing functions makes usually makes code more readable
typedef double double_fun_t(void*, double);
struct exponential {
// closure must be placed as the first member to allow safe casting
// between a pointer to `closure` and `struct exponential`
double_fun_t *closure;
double temperature;
};
double exponential(void *ctx_, double x) {
struct exponential *ctx = ctx_;
return exp(x / ctx->temperature);
}
// the "constructor" of the closure for exponential
double_fun_t **make_exponential(double temperature) {
struct exponential *e = malloc(sizeof *e);
e->closure = exponential;
e->temperature = temperature;
return &e->closure;
}
// now simple closure with no context, a pure x -> x*x mapping
double square(void *_unused, double x){
(void)_unused;
return x*x;
}
// use compound literal to transform a function to a closure
double_fun_t **square_closure = & (double_fun_t*) { square };
// the worker that process closures, note that `double_fun_t` is not used
// because `double(**)(void*,double)` is builtin type
double somme(double* liste, int length, double (**fun)(void*,double)){
double poids = 0;
for(int i=0;i<length;++i)
// calling a closure, note that `fun` is used for both obtaing
// the function pointer and for passing the context
poids = poids + (*fun)(fun, liste[i]);
return poids;
}
int main(void) {
double list[3] = { 1, 2, 3 };
printf("%g\n", somme(list, 3, square_closure));
// a dynamic closure
double_fun_t **exponential = make_exponential(42);
printf("%g\n", somme(list, 3, exponential));
free(exponential);
return 0;
}
The advantage of this approach is that the closure exports a pure interface for calling double->double functions. There is no need to introduce any boxing structures used by all clients of the closure. The only requirement is the "calling convention" which is very natural and does not require sharing any code.
Answer
#include <stdio.h>
#include <stdlib.h>
/*
File Conventions
----------------
alignment: similar statements only
int a = 10;
int* omg = {120, 5};
functions: dofunction(a, b, c);
macros: _do_macro(a, b, c);
variables: int dovariable=10;
*/
////Macros
#define _assert(got, expected, teardownmacro) \
do { \
if((got)!=(expected)) { \
fprintf(stderr, "line %i: ", __LINE__); \
fprintf(stderr, "%i != %i\n", (got), (expected)); \
teardownmacro; \
return EXIT_FAILURE; \
} \
} while(0);
////Internal Helpers
static void istarted() {
fprintf(stderr, "Start tests\n");
}
static void iended() {
fprintf(stderr, "End tests\n");
}
////Tests
int main(void)
{
///Environment
int localvar = 0;
int* localptr = NULL;
///Closures
#define _setup_test(mvar, msize) \
do { \
localptr=calloc((msize), sizeof(int)); \
localvar=(mvar); \
} while(0);
#define _teardown_test() \
do { \
free(localptr); \
localptr=NULL; \
} while(0);
///Tests
istarted();
_setup_test(10, 2);
_assert(localvar, 10, _teardown_test());
_teardown_test();
_setup_test(100, 5);
_assert(localvar, 100, _teardown_test());
_teardown_test();
iended();
return EXIT_SUCCESS;
}
Context
I was curious about how others accomplished this in C. I wasn't totally surprised when I didn't see this answer. Warning: This answer is not for beginners.
I live a lot more in the Unix style of thinking: lots of my personal programs and libraries are small and do one thing very well. Macros as "closures" are much safer in this context. I believe all the organization and specified conventions for readability is super important, so the code is readable by us later, and a macro looks like a macro and a function looks like a function. To clarify, not literally these personal conventions, just having some, that are specified and followed to distinguish different language constructs (macros and functions). We all should be doing that anyway.
Don't do afraid of macros. When it makes sense: use them. The advanced part is the when. My example is one example of the whens. They are ridiculously powerful and not that scary.
Rambling
I sometimes use a proper closure/lambda in other languages to execute a set of expressions over and over within a function. It's a little context aware private helper function. Regardless of its proper definition, that's something a closure can do. It helps me write less code. Another benefit of this is you don't need to reference a struct to know how to use it or understand what it's doing. The other answers do not have this benefit, and, if it wasn't obvious I hold readability very highly. I strive for simple legible solutions. This one time I wrote an iOS app and it was wonderful and as simple as I could get it. Then I wrote the same "app" in bash in like 5 lines of code and cursed.
Also embedded systems.
I currently use three different functions to return a numeric value (one returns a double, the other two return a long):
int main(void)
{
// lots of code
dRate = funcGetInterestRate();
lMonths = funcGetTerm();
lPrincipal = funcGetPrincipal();
// lots of code
return 0;
}
The three functions code is about 90% the same so I would like to consolidate into 1 function. I want to pass a value flag to a single function something like this:
if "1" passed, determine interest rate, return a double
if "2" passed, determine term of loan, return a long
if "3" passed, determine principal of loan, return a long
I only want to return 1 value ever from the function when it is called, but the value I want to return can be either a double or a long. I want to do something like this:
void funcFunction(value passed to determine either long or double)
{
// lots of code
if (foo)
return double value;
else
return long value;
}
Is there an easy way to do this?
A function's return type is fixed at compile time. You can't change the return type based on the parameters you pass in.
However, since your main goal is to remove repeated code in the functions and consolidate them, this can be addressed in a different way. In other words, this is an XY problem.
What you can do in your case is extract the common code in each of your three functions into a separate function, then the three original functions can call the common function to do most of the work, then extract the part they need and return that.
For example:
struct loan {
double rate;
long term;
long principal;
};
void calcLoan(struct loan *loan)
{
// do stuff
loan->rate = // some value
loan->term = // some value
loan->principal = // some value
}
double funcGetInterestRate()
{
struct loan loan;
calcLoan(&loan);
return loan.rate;
}
long funcGetTerm()
{
struct loan loan;
calcLoan(&loan);
return loan.term;
}
long funcGetPrincipal()
{
struct loan loan;
calcLoan(&loan);
return loan.principal;
}
No, C does not allow this. The return type is in the function declaration (which you have as void).
Slightly easier is to provide two pointers to variables and indicate which one to use in the return value:
int funcFunction(yourArgument, long *longResult, double *dblResult)
{
// lots of code
if (foo)
{
*dblResult = value;
return 1;
} else
{
*longResult = otherValue;
return 0;
}
}
(And possibly you can even use a union.)
However ... I had to use value and otherValue as inside the function you cannot use the same variable to hold either a long or a double. You can – again, with a union – but this is stressing the eaze of having only one single function to the breaking point.
You might consider returning some tagged union. The Glib GVariant type could be inspirational, and since Glib is free software, you could study its source code. See also this answer.
So you would declare some public struct with an anonymous union inside:
struct choice_st {
bool islong;
union {
long i; // valid when islong is true
double d; // valid when islong is false
}
}
and you could return a value of that struct choice_st.
struct choice_st res;
if (foo) {
res.islong = true;
res.i = somelong;
return res;
}
else {
res.islong = false;
res.d = somedouble;
return res;
}
You might also decide to use C dynamic memory allocation, return a freshly malloc-ed pointer to struct choice_st, and adopt a convention about who is responsible of free-ing that pointer. (BTW, GVariant is doing something similar to this).
You sort of can do this. It's easier to show than explain, but I'll add an explanation if this isn't clear:
void add_or_divide_by_xor(unsigned int a, unsigned int b, unsigned short c,
unsigned long *add, unsigned double *divide) {
unsigned int xor = a ^ b;
if (add != NULL) {
*add = xor + c;
}
if (divide != NULL) {
*divide = (double)xor / (double)c;
}
}
Called:
unsigned long add;
unsigned double divide;
add_or_divide_by_xor(5, 17, 4, &add, NULL);
add_or_divide_by_xor(17, 6, 4, NULL, ÷);
Depending on your platform, double might be a superset of long. You should already know whether this is true (because you know what your algorithms do, and what their possible output values are); if you don't, consider the following:
double can represent integers up to 253 exactly
long is either a 32-bit or a 64-bit type
So if your long values are 32-bit, you can just always return double from your functions, and cast it to long outside your functions, where needed.
You could try something like this.
#include <stdio.h>
void* func1(int key){
static double interest;
static long term;
static long principle;
//your common code to modify values of interest, term and principle
//
//Let us give them some dummy values for demonstration
interest = 34.29347;
term = 5843535;
principle = 7397930;
//conditions
if(key == 1){
return &interest;
}
else if(key == 2){
return &term;
}else if(key == 3){
return &principle;
}else printf("%s\n","error!!");
}
int main()
{
printf("%f\n",*(double*)func1(1));
printf("%ld\n",*(long*)func1(2));
printf("%ld\n",*(long*)func1(3));
func1(4);
return 0;
}
Output
34.293470
5843535
7397930
error!!
I want to pass greater than (>) and less than (<) operators as arguments to a function,how is it possible..is there any way to pass those operators as arguments..please any one can help me.
You can do terrible things with macros, but in general, no, you can't do this. You typically accept a two argument function and call it, and that function can use > or < as appropriate, see the sort docs for an example.
That said, it's not super efficient (calling a function through a pointer can't be inlined, and for cheap operations like a > or < comparison, the function call overhead outweighs the comparison work). Making it efficient requires:
Multiple copies of the code, one for each possible operator (possibly generated via macros)
Moving to C++ and using templated code with functors/lambdas that can be inlined properly
There is no way to pass a 'raw' operator, but there are ways to achieve the same result.
The simplest would be a char
int func(char op, int a, int b)
{
if (op == '<')
{
return a < b;
}
else if (op == '>')
{
return a > b;
}
return -l; /* error */
}
A more complex solution would be to use a function pointer to a function that does the operation (similar to the comparator used by the sort method).
You can create a enum and pass it.
Or you can pass in a pointer to a comparison function like this:
#include <stdio.h>
int max(int a, int b, int (*comp)(int, int)) {
if (comp(a, b) < 0) {
return b;
} else {
return a;
}
}
int mycomp(int a, int b) {
return a < b ? -1 : 1;
}
int main() {
printf("%d %d\n", max(1, 2, mycomp), max(2, 1, mycomp));
}
You can write this function by using #define and #. Character #, changes an operator to a string. (for example in #define, #+ = "+").
Sample code:
#include <stdio.h>
#define great(c) ((#c==">")? (1):(0))
int main()
{
printf ("%d", great(>));
return 0;
}
In this code, I passed > as a function argument.
Consider the following code segment written in S-expr notation:
(lambda (x) (lambda (y) (+ x y)))
or in Javascript:
function(x) { return function(y) { return x+y; }; }
How do I write this in C?
This is difficult to do in C, since it relies on closures. With C you have to pass an explicit context, so you might end up with something like this.
#include <stdio.h>
struct closure {
int saved_x;
int (*function)(struct closure, int);
};
int second_half_add(struct closure context, int y) {
return context.saved_x + y;
}
struct closure curried_add(int x) {
struct closure ret;
ret.saved_x = x;
ret.function = second_half_add;
return ret;
}
int main() {
struct closure context = curried_add(3);
printf("%d\n", context.function(context, 4));
}
It's really ugly, and you lose almost all benefit of currying, but it is possible
C doesn't have first class functions, so the answer is: Nohow.
This depends on what you mean when you say “C.”
Supposing that you really want to get something back which you can call by simple function application, ISO C makes such a thing pretty hard to do—probably impossible if you really want to stay within the confounds of the standard and not use some low-level assembly tricks.
Clang, however, implements non-standard support for closures that actually makes such things possible. Using this feature, your example could be implemented in the following way:
#include <stdio.h>
#include <stdlib.h>
#include <Block.h>
int (^plus(int x))() {
return Block_copy(^(int y) {
return x + y;
});
}
int main() {
int (^plus2)(int) = plus(2);
printf("2 + 3 = %d\n", plus2(3));
Block_release(plus2);
return EXIT_SUCCESS;
}
Well, you can do it. It's not pretty. It goes something like this:
typedef int (*intfuncint)(Env*, int);
// this is the "closure" block
typedef struct Env {
int x;
intfuncint f;
} env_t;
// this is the internal function
int sum(Env* me, int y){return me->x + y;}
// this is the external function
Env* foo(int x){
Env* result = malloc(sizeof(*result));
result->x = x;
result->f = sum;
return result;
}
Using it to get the sum of 3 and 5 would look something like this:
Env* p = foo(3); p->f(p, 5)
You can't write it in c, for several reasons, mostly that c just doesn't work like that.
If this is a homework question your teacher may say otherwise... something like:
struct functor {
int x;
functiontype* f;
}
int dofunctor(functor*, y) { ... }
But it generalises so poorly that it's not worth doing.
You can write it in some other c-like languages - such as perl.
You can do something like this in c++ - see the following answer:
C++ Functors - and their uses