I have an array of structures as a function parameter and the size of the array is dynamic. My coworker said that I'll have to use a double pointer since the values contained in the array of struct will be overwritten.
The parameter that will become a double pointer is the following :
xPIDConfig_t **pxPIDConfig
Here is what the structure looks like for the xPIDConfig_t :
typedef struct
{
ePIDType_t ePIDType;
/* Common fields for the different types of PID */
float fLowerSaturationLimit;
float fUpperSaturationLimit;
float fOldInput;
float fIError;
uint32_t ulDeltaTime;
eBool_t bSaturationEnable;
eBool_t bAntiWindupEnable;
eBool_t bNegativeErrorEmptyIError;
union
{
/* Parallel PID fields */
struct
{
float fProportionalGain;
float fIntegralGain;
float fDerivativeGain;
}xParallelPID;
/* Non-interactive PID fields */
struct
{
float fControllerGain;
uint32_t ulIntegralTime;
uint32_t ulDerivativeTime;
}xNonInteractivePID;
}xUniqueFields;
}xPIDConfig_t;
The size of the array of pxPIDConfig will vary.
But I am not sure how to malloc that double pointer or even how to use the function containing the double pointer.
I was just wondering if anyone had a good example of code of how to use a function with a double pointer array of variating size? and how to properly change the values contained in the array itself inside a function?
Right now this is how I change the values within the function :
pxPIDConfig->ePIDType = ePIDType;
pxPIDConfig->fOldInput = 0;
pxPIDConfig->fIError = 0;
pxPIDConfig->ulDeltaTime = ulDeltaTime;
pxPIDConfig->bSaturationEnable = bIsSaturationEnable;
pxPIDConfig->bAntiWindupEnable = bIsAntiWindupEnable;
pxPIDConfig->bNegativeErrorEmptyIError = bNegativeErrorEmptyIError;
when the pointer is double do I have to use double '->'? This is very confusing for me.
Thank you all for the help
/***************** EDIT ************************************
My function is working right now, but I got told I need to use memory allocation since the size of my arrays varies according to the number of loops I want to implement.
Here are the parameters of my function :
eError_t eControlCascadeInit( uint8_t ucNumberOfLoops, ePIDType_t *pePIDType, xPIDConfig_t **pxPIDConfig, float *pfLowerLimit, float *pfUpperLimit, uint32_t *pulDeltaTime, \
eBool_t *pbIsSaturationEnable, eBool_t *pbIsAntiWindupEnable, eBool_t *pbNegativeErrorEmptyIError, \
float *pfPGain, float *pfIGain, float *pfDGain, float *pfCGain, uint32_t *pulITime, uint32_t *pulDTime )
They're all arrays of size ucNumberOfLoops. All of them are read-only arrays, except for the pxPIDConfig one that is write-only. The function initializes all the xPIDConfig_t present in the array with the parameters passed to the function through array.
array[ 0 ] contains the parameters for the first PID controller being initialized.
array[ 1 ] contains the parameters for the second PID controller being initialized and so on...
It's like that for all the parameters in the function.
Hope it makes my question more clear?
Here you have an example of how to use double-pointer, to change the pointer in the function:
void allocate(xPIDConfig_t **array, size_t size)
{
*array = malloc(sizeof(**array) * size);
/* some examples how to access the struct members vi double pointer -*
(*array) -> ulDeltaTime = 100;
(**array).ulDeltaTime = 100;
(*(array + 5)) -> ulDeltaTime = 100;
array[5] -> ulDeltaTime = 100;
(*array[5]).ulDeltaTime = 100;
}
int main(void)
{
xPIDConfig_t *array;
allocate(&array, 100);
printf("%s\n", array ? "success" : "failure");
free(array);
}
You would only need a double pointer if the function reallocates the array to a different size. If the size isn't changing, you can just pass a pointer to (usually the first) element of the array, along with any size or index required by the function. For example:
extern void frobPidConfig(xPIDConfig_t *);
// 'frob' the xPIDConfig_t array elements from index a to b
void frobSomePidConfigs(xPIDConfig_t *pidconfigs, unsigned int a, unsigned int b)
{
unsigned int i;
for (i = a; i <= b; i++)
{
frobPidConfig(&pidConfigs[i]);
// Example of member access:
pidConfigs[i].ulDeltaTime = 42;
}
}
Example of calling code:
xPIDConfig_t *pidConfigs;
unsigned int n = 10; // or whatever...
pidConfigs = calloc(sizeof *pidConfigs, n);
if (!pidConfigs)
{
// Allocation error
exit(1);
}
/* ... */
frobSomePidConfigs(pidConfigs, 2, 5);
On the other hand, if the function needs to reallocate the array and initialize any new elements, it could be done using a double pointer like this:
extern void initPidConfig(xPIDConfig_t *);
void reallocPidConfigs(xPIDConfig_t **pidConfigs, unsigned int oldSize, unsigned int newSize)
{
unsigned int i;
// Reallocate to new size
xPIDConfig_t *realloced = realloc(*pidConfigs, sizeof **pidConfigs * newSize);
if (newSize && !realloced)
{
// allocation error
exit(EXIT_FAILURE);
}
*pidConfigs = realloced;
// Initialize any additional elements
for (i = oldSize; i < newSize; i++)
{
initPidConfig(*pidConfigs + i); // or: initPidConfig(&(*pidConfigs)[i]);
// Examples of member access:
(*pidConfigs)[i].bSaturationEnable = true;
(*pidConfigs + i)->bAntiWindupEnable = true;
}
}
Example of calling code:
xPIDConfig_t *pidConfigs = NULL;
// Note: realloc of the NULL pointer in *pidConfigs is OK.
reallocPidConfigs(&pidConfigs, 0, 10);
frobSomePidConfigs(pidConfigs, 2, 5);
Limited to addressing assumptions and questions regarding your title question:
"How to use double pointers (pointer to pointer) for an array of structures properly in standard C"
First, just because the function argument might have a double pointer (i.e. xPIDConfig_t **pxPIDConfig) does not mean that the variable need to be allocated memory with a double pointer, i.e. if the function eg is called like this: funcChangeParam(&pxPIDConfig); this often means that the object being passed needs to be changed in some way, requiring that the address of be passed, not the object itself. Also, if the object itself is a pointer, (such as a pointer to several instances of a struct object.) then the function used to pass the object for modification will be prototyped with arguments such as void funcChangeParam(xPIDConfig_t **pxPIDConfig); (Note the double pointer here.)
So with this function prototype Making the allocation of memory look like this:
void funcChangeParam(xPIDConfig_t **pxPIDConfig);
//allocate memory for multiple instances of struct
xPIDConfig_t *pxPIDConfig = malloc(countOfInstances * sizeof(*pxPIDConfig);
if(pxPIDConfig)
{
//use pxPIDConfig
funcChangeParam(&pxPIDConfig);pass pointer to multiple instances of struct
And references to the object members inside the calling function could use the following notation. Eg:
//in a loop or other construct where i is defined from 0 to countOfInstances - 1
(*pxPIDConfig)[i].ePIDType = ePIDType;//modification of assignment per your example
//etc.
//The following is a trivial example for illustration purposes.
//Code here uses a simplified struct, function
//prototype, and simple calling example, the concept
//of which easily translates to what you are
//asking about.
typedef struct {
int num;
}test_s;
void change(test_s **new);
int main(){
test_s *test = malloc(10*sizeof *test);
change(&test);
return 0;
}
void change(test_s **new)
{
for(int i=0;i<10;i++)
{
(*new)[i].num = (i+1)*3; //init all instances to some value
}
}
I need to change the properties of the structure within a function, so I could do a calculation with the properties and set new values,but when i try to change the value of a structure i am getting the following error: cannot convert 'data specs (*) [2]' to 'data specs' for argument '1' to 'void changeValues (data specs)' changeValues (& stats);
Does anyone know how to fix it?
typedef struct
{
char nome;
int vida;
int dano;
int x;
int y;
} dadospecas;
void changeValues(dadospecas *a[]){
a[1]->vida = 5;
printf("%i", a[1]->vida);
}
int main() {
dadospecas stats[2];
stats[1].nome = 'W';
stats[1].vida = 3;
stats[1].dano = 1;
stats[1].x = 4;
stats[1].y = 1;
stats[2].nome = 'F';
stats[2].vida = 33;
stats[2].dano = 11;
stats[2].x = 44;
stats[2].y = 14;
changeValues(&stats);
return 0;
}
In addition to the great answer by #Stuart, you seem to be a little confused on how to handle sending values to your function to be changed, (here you are changing the vida member only). As noted in the other answer, on access, an array is converted to a pointer to its first element. C11 Standard - 6.3.2.1 Other Operands - Lvalues, arrays, and function designators(p3).
To make your function useful (as it is it hardcodes the entire operation), you should provide parameters for a pointer to dadospecas, the index to change and the new value to assign to the vida member. The caller is responsible for ensuring the index to change is within bounds (you can pass the number of elements as an additional index if desired)
If you put those pieces together and change the name of the function to reflect the fact you are only changing the vida member, you could do:
/* pass a pointer to the array as your parameter (inlcude index and value) */
void changeVida (dadospecas *a, int index, int newvida)
{
a[index].vida = newvida;
}
A short example showing the use, and fixing your indexing problem, could be written as:
#include <stdio.h>
#define NSTRUCT 2 /* if you need a constant, #define one (or more) */
typedef struct {
char nome;
int vida,
dano,
x,
y;
} dadospecas;
/* pass a pointer to the array as your parameter (inlcude index and value) */
void changeVida (dadospecas *a, int index, int newvida)
{
a[index].vida = newvida;
}
int main (void)
{
dadospecas stats[NSTRUCT] = {{'W', 3, 1, 4, 1}, {'F', 33, 11, 44, 14}};
for (int i = 0; i < NSTRUCT; i++) { /* loop over each struct */
changeVida (stats, i, stats[i].vida + 5); /* increment vida by 5 */
printf ("stats[%d].vida = %d\n", i, stats[i].vida); /* output new vida */
}
}
Example Use/Output
Where the example simply adds 5 to the existing value of the vida member:
$ ./bin/dadospecas
stats[0].vida = 8
stats[1].vida = 38
If You Pass The Address of stats
While there is no need to pass the address of stats, there is nothing to prevent you from doing it -- it is perfectly fine -- just unnecessary. For sake of argument, let's say you did. Continuing from my comment, in main stats is an array of type dadospecas [2], so when you take the address your type is pointer to array of dadospecas [2]. The formal type is dadospecas (*)[2]
So passing the pointer your function parameter would become: dadospecas (*a)[2].
Within your function to operate on your array, you would first need to dereference the parameter to allow you to operate on the elements of the array, e.g. (*a)[index] and finally to change the vida member you would have:
(*a)[index].vida = newvida;
The changes to the example above to pass the address of stats would be:
/* pass a pointer to array[NSTRUCT] as your parameter (inlcude index and value) */
void changeVida (dadospecas (*a)[NSTRUCT], int index, int newvida)
{
(*a)[index].vida = newvida;
}
int main (void)
{
dadospecas stats[NSTRUCT] = {{'W', 3, 1, 4, 1}, {'F', 33, 11, 44, 14}};
for (int i = 0; i < NSTRUCT; i++) { /* loop over each struct */
changeVida (&stats, i, stats[i].vida + 5); /* increment vida by 5 */
printf ("stats[%d].vida = %d\n", i, stats[i].vida); /* output new vida */
}
}
(same output)
It is simply a matter of keeping the levels of pointer indirection straight and observing C operator precedence.
Look things over and let me know if you have further questions.
I think there are two things you don't understand about arrays in C.
Arrays are indexed starting from 0 and not 1.
The first element of your stats array is stats[0] and the second is stats[1]. You need to change the assignments in your main function, and in your changeValues function if you want it to change the first element of the array.
Arrays are const pointers (i.e. pointers that have a fixed value).
The stats array is really a const pointer to dadospecas. It's a const pointer meaning you can't change the value of stats. You can change the values contained by (i.e. pointed to) by stats.
If you want to define a function that will modify the contents of an array, you don't need to pass a pointer to the array, you can just pass the array.
Your changeValues function should be defined like this:
void changeValues(dadospecas *a){
//put code here.
}
or
void changeValues(dadospecas a[]){
//put code here.
}
In either case, your main function would call changeValues like this
changeValues(stats);
By the way, if you want to check that changeValues has modified the vida member of the first element of stats, I would remove the printf call from the changeValues function and replace it with
printf("%i", stats[0].vida);
in your main function (after the call to changeValues of course).
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.