I have written a straightforward C code that uses an engine to run two different algorithms depending on user input. It uses function pointers to the algorithm methods and objects. There is a nasty memory bug somewhere that I can not track down, so maybe I am allocating memory in the wrong way. What is going wrong?
Below is (the relevant parts of) a minimal working example of the code.
main.c
#include "engine.h"
int main()
{
char *id = "one";
Engine_t eng;
Engine_init(&eng);
Engine_select_algorithm(eng, id);
Engine_run(eng);
}
engine.h
typedef struct _Engine *Engine_t;
engine.c
#include "engine.h"
#include "algorithm_one.h"
#include "algorithm_two.h"
typedef struct _Engine
{
void *p_algorithm;
void (*init)(Engine_t);
void (*run)(Engine_t);
} Engine;
void Engine_init(Engine_t *eng)
{
*eng = malloc(sizeof(Engine));
(*eng)->p_algorithm = NULL;
}
void Engine_select_algorithm(Engine_t eng, char *id)
{
if ( strcmp(id, "one") == 0 )
{
eng->init = Algorithm_one_init;
eng->run = Algorithm_one_run;
}
else if ( strcmp(id, "two") == 0 )
{
eng->init = Algorithm_two_init;
eng->run = Algorithm_two_run;
}
else
{
printf("Unknown engine %s.\n", id); exit(0);
}
eng->init(eng);
}
void Engine_run(Engine_t eng)
{
eng->run(eng);
}
void Engine_set_algorithm(Engine_t eng, void *p)
{
eng->p_algorithm = p;
}
void Engine_get_algorithm(Engine_t eng, void *p)
{
p = eng->p_algorithm;
}
algorithm_one.h
typedef struct _A_one *A_one_t;
algorithm_one.c
#include "engine.h"
#include "algorithm_one.h"
typedef struct _A_one
{
float value;
} A_one;
void Algorithm_one_init(Engine_t eng)
{
A_one_t aone;
aone = malloc(sizeof(A_one));
aone->value = 13.0;
//int var = 10;
Engine_set_algorithm(eng, &aone);
}
void Algorithm_one_run(Engine_t eng)
{
A_one_t aone;
Engine_get_algorithm(eng, &aone);
printf("I am running algorithm one with value %f.\n", aone->value);
// The code for algorithm one goes here.
}
The code for algorithm_two.h and algorithm_two.c are identical to the algorithm one files.
There must be a memory bug involved, because the code runs as given, but if I uncomment the
//int var = 10;
line in algoritm_one.c the code crashes with a segmentation fault.
You pass the wrong thing to Engine_set_algorithm. You are passing the address of a local variable rather than the address of the algorithm. You need to write:
Engine_set_algorithm(eng, aone);
And also Engine_get_algorithm is wrong. You are passed a pointer by value and modify that pointer. So the caller cannot see that modification. You need it to be:
void Engine_get_algorithm(Engine_t eng, void **p)
{
*p = eng->p_algorithm;
}
I think your code would be easier if you defined a type to represent an algorithm. That type would be just a void*, but it would make the code much easier to read. What's more, I would make Engine_get_algorithm return the algorithm.
algorithm Engine_get_algorithm(Engine_t eng)
{
return eng->p_algorithm;
}
void Engine_set_algorithm(Engine_t eng, algorithm alg)
{
eng->p_algorithm = alg;
}
Related
TL;DR What should the type of x be if x = x(); is valid in C?
The whole story:
I am working on a simple game with multiple scenes. At first my code looked like this:
enum {kSCENE_A, kSCENE_B} ecene = kSCENE_A;
int main() {
while(1) {
switch(scene) {
case kSCENE_A:
// Renders scene a
// And possibly modifies `scene`
break;
case kSCENE_B:
// Renders scene b
// And possibly modifies `scene`
break;
}
}
}
However the main function is too verbose. I extracted the rendering part into different functions, but the switch still makes the code ugly. I defined a scene_map for this:
typedef enum {
kSCENE_A,
kSCENE_B,
kN_SCENES
} Scene;
Scene RenderSceneA();
Scene RenderSceneB();
int main() {
Scene scene = kSCENE_A;
Scene (*scene_map[kN_SCENES])();
scene_map[kSCENE_A] = SceneA;
scene_map[kSCENE_B] = SceneB;
while(1) scene = scene_map[scene]();
}
But I wonder if it would be possible in C that I write the code in this, or some similar way:
SomeType RenderSceneA();
SomeType RenderSceneB();
int main() {
SomeType scene = RenderSceneA;
while(1) scene = scene();
}
So what type should SomeType be, or can I only use void * for it? If the latter is true, how can I write this code in a clear manner than demonstrated in the second code block?
Here's a solution that will solve the problem. Note that it uses void (*)(void) as a generic function pointer type, as opposed to void * which isn't guaranteed to work for function pointers:
#include <stdio.h>
void (*f(void))(void);
void (*g(void))(void);
void (*f(void))(void)
{
printf("This is f.\n");
return (void (*)(void)) g;
}
void (*g(void))(void)
{
printf("This is g.\n");
return (void (*)(void)) f;
}
#define SRCALL(p) ((void (*(*)(void))(void)) p())
int main(void)
{
void (*(*p)(void))(void);
p = f;
p = SRCALL(p);
p = SRCALL(p);
p = SRCALL(p);
return 0;
}
The function pointer casts are ugly, so I encapsulated them in the macro SRCALL (self-ref-call). The output is:
This is f.
This is g.
This is f.
Closed. This question needs debugging details. It is not currently accepting answers.
Edit the question to include desired behavior, a specific problem or error, and the shortest code necessary to reproduce the problem. This will help others answer the question.
Closed 4 years ago.
Improve this question
I've been trying to work with structures, pointers and memory in C.
I have created this structure
typedef struct {
int id;
char *name;
} Object;
here is constructor
void object_ctor(Object *o, int id, char *name)
{
o->id = id;
o->name = malloc(sizeof(name));
if(sizeof(o->name)!=sizeof(name))
{
o->name=NULL;
}
else
{
strcpy(o->name, name);
}
}
here is decleration of o1
char tmp_name[] = "Hello 1";
Object o1;
object_ctor(&o1, 1, tmp_name);
here is destructor
void object_dtor(Object *o)
{
if(o->name != NULL)
{
free(o->name);
o->name = NULL;
}
}
printing object
void print_object(Object *o)
{
printf("ID: %d, NAME: %s\n", o->id, o->name);
}
calling copy
Object copy;
print_object(object_cpy(©, &o1));
and I´m trying create a copy of one structure to another (I have already constructed them).
Object *object_cpy(Object *dst, Object *src)
{
if(src!=NULL)
{
const size_t len_str=strlen(src->name)+1;
dst->name = malloc(10000000);
dst->id = src->id;
strncpy (dst->name, src->name,len_str);
}
if (strcmp(dst->name,src->name)!=0)
{
dst->name = NULL;
}
return dst;
}
But then when I'm trying to free both copy and original src I get a segmentation fault. I've been trying to run it through gdb and it said that I'm freeing same memory twice so I assume that the code for copying is wrong, but I don't know where.
And here is code that gives me segmentation fault
printf("\nCOPY EMPTY\n");
object_dtor(©);
o1.id = -1;
free(o1.name);
o1.name = NULL;
object_cpy(©, &o1);
print_object(©);
print_object(&o1);
I´m including these libraries
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
I'm using the std=c99 flag for to compile.
There is at least a problem here:
void object_ctor(Object *o, int id, char *name)
{
o->id = id;
o->name = malloc(sizeof(name));
if (sizeof(o->name) != sizeof(name))
{
o->name = NULL;
}
else
{
strcpy(o->name, name);
}
}
sizeof(name) is not the length of the string pointed by name. You need strlen(name) + 1 (+1 for the NUL terminator).
And your test if (sizeof(o->name) != sizeof(name)) is pointless, and I'm not sure what you're trying to achieve here.
You probably want this:
void object_ctor(Object *o, int id, char *name)
{
o->id = id;
o->name = malloc(strlen(name) + 1);
if (o->name != NULL)
strcpy(o->name, name);
}
There are similar problems in object_cpy:
pointless use of strncpy
pointless allocation of a 10Mb buffer
pointless test strcmp(dst->name, src->name)
You probably want this:
Object *object_cpy(Object *dst, Object *src)
{
if (src != NULL)
{
const size_t len_str = strlen(src->name) + 1;
dst->name = malloc(len_str);
if (dst->name != NULL)
{
dst->id = src->id;
strcpy(dst->name, src->name);
}
}
return dst;
}
With these corrections following code works fine:
int main()
{
char tmp_name[] = "Hello 1";
Object o1, copy;
object_ctor(&o1, 1, tmp_name);
object_cpy(©, &o1);
print_object(©);
print_object(&o1);
object_dtor(&o1);
object_dtor(©);
}
Event if this is not directly an answer to your problem, I'll give you how I organize my code in order to avoid memory problem like yours.
First, it all resolve around a structure.
To each structure, if needed, I do a "Constructor" and a "Destructor".
The purpose of the constructor is simply to set the structure in a coherent state. It can't never fail (implying that any code that could fail, like malloc, should not be in the constructor).
The purpose of the destructor is to clean the structure.
One little trick that I like to use is to put the constructor in a macro, allowing me to do something like 'Object var = OBJET_CONSTRUCTOR'.
Of course, it's not alway possible, it's up to you to be carreful.
For your code, it could be :
typedef struct {
int id;
char *name;
} Object;
#define OBJECT_CONSTRUCTOR {.id = -1,\ \\ Assuming -1 is relevant in your case, like an error code or a bad id value. Otherwise, it's useless.
.name = NULL}
void Object_Constructor(Object *self)
{
Object clean = OBJECT_CONSTRUCTOR;
*self = clean;
}
void Object_Destructor(Object *self)
{
free(self->name);
}
Here we go.
How to use it is simple : You always begin by the constructor, and you alway end by the destructor. That's why it's useless to set the char pointer "name" to NULL in the destructor, because it should not be used after by any other function that the constructor.
Now, you can have "initialisation" function. You can do a plain initialisation (it is your constructor function), or a copy initialisation, etc etc
Just keep in mind that the structure have been called into the constructor. If not, it's the developer fault and you do not have to take that in count.
A behavior that can be nice is, in case of error, to not modify the structure.
Either the structure is entierly modified in succes, or not at all.
For complex structure that can fail at many point, you can do that by "swapping" the result at the end.
void Object_Swap(Object *first, Object *second)
{
Object tmp = OBJECT_CONSTRUCTOR;
tmp = *fisrt;
*first = *second;
*second = tmp;
}
bool Object_InitByPlainList(Object *self, int id, consr char *name)
{
Object newly = OBJECT_CONSTRUCTOR;
bool returnFunction = false;
newly.id = id;
if (!(newly.name = strdup(name))) {
printf("error : %s : strdup(name) : name='%s', errno='%s'.\n", __func__, name, strerror(errno));
goto END_FUNCTION;
}
// Success !
Object_Swap(self, &newly);
returnFunction = true;
/* GOTO */END_FUNCTION:
Object_Destructor(&newly);
return (returnFunction);
}
It may be seem overcomplicated at the first glance, but that organization allow you to add more futur step "that can fail" cleanly.
Now, you can even do something this simply :
bool Object_InitByCopy(Object *dst, Object *src)
{
return (Object_InitByPlainList(dst, src->id, src->name));
}
All you have to do is to say in the documentation :
The first function to be called have to be "Object_Constructor"
After the "Object_Constructor", only the "Object_Init*" function can be called.
The last function to be call have to be "Object_Destructor"
That's all. You can add any "Object_*" function that you whant, like :
void Object_Print(const Object *self)
{
printf("ID: %d, NAME: %s\n", self->id, self->name);
}
Hope this organization will solve your memory problem.
An example :
int main(void)
{
Object test = OBJECT_CONSTRUCTOR;
Object copy = OBJECT_CONSTRUCTOR;
if (!Object_InitByPlainList(&test, 1, "Hello World !")) {
// The function itself has logged why it has fail, so no need to add error printf here
return (1);
}
Object_Print(&test);
if (!Object_Copy(©, &test)) {
return (1);
}
Object_Destructor(&test);
Object_Destructor(©);
return (0);
}
I just started programming in C for school. I am being asked to do a program that uses a FIFO struct to resolve math problems. I got the folowing code on the internet for a FIFO, I just don't know how to use it. I tried a lot of things and I can't find anything useful on the internet or maybe that I just don't know the right thing to research but could you please help me? Thanks!
#include <stdio.h>
#include <stdlib.h>
typedef struct pile
{
int donnee;
struct pile *precedent;
} Pile;
void pile_push(Pile **p_pile, int donnee)
{
Pile *p_nouveau = malloc(sizeof *p_nouveau);
if (p_nouveau != NULL)
{
p_nouveau->donnee = donnee;
p_nouveau->precedent = *p_pile;
*p_pile = p_nouveau;
}
}
int pile_pop(Pile **p_pile)
{
int ret = -1;
if (p_pile != NULL)
{
Pile *temporaire = (*p_pile)->precedent;
ret = (*p_pile)->donnee;
free(*p_pile), *p_pile = NULL;
*p_pile = temporaire;
}
return ret;
}
void pile_clear(Pile **p_pile)
{
while (*p_pile != NULL)
{
pile_pop(p_pile);
}
}
I tried doing this:
int main()
{
int return_val;
Pile pile;
pile_push(Pile, 5);
return_val = pile_pop(Pile);
printf(return_val);
}
and got this error:
expected expression before 'Pile'
too few arguments to function 'pile_push'
You have mixed up Pile and pile which is the issue with the first warning. The functions expect a pointer to a pointer to a Pile. That is: They update the value of a pointer, so they need to be passed a reference to a pointer. Your use of printf is also wrong.
int main()
{
int return_val;
Pile *pile = NULL;
pile_push(&pile,5);
return_val = pile_pop(&pile);
printf("return_val is: %d\n",return_val);
}
I'm trying to make a game that requires dynamically sized arrays in C but my code isn't working even though identical code works in another one of my programs.
Here are my #includes
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "SwinGame.h" //API for graphics, physics etc
#include <math.h>
Here are my typedefs for the relevant structs used:
typedef struct position_data
{
double x;
double y;
} position_data;
typedef enum enemy_type_data {CIRCLE, TRIANGLE, SQUARE} enemy_type_data;
typedef struct enemy_data
{
position_data location;
enemy_type_data type;
bitmap bmp;
double health;
double speed;
int path_to;
} enemy_data;
typedef struct enemy_data_array
{
int size;
enemy_data *data;
} enemy_data_array;
Here is the function to add an element to the array:
void add_enemy(enemy_data_array *enemies)
{
enemy_data *new_array;
enemies->size++;
new_array = (enemy_data *)realloc(enemies->data, sizeof(enemy_data) * enemies->size);
if (new_array) //if realloc fails (ie out of memory) it will return null
{
enemies->data = new_array;
// enemies->data[enemies->size - 1] = read_enemy_data();
printf("Enemy added successfully!\n");
}
else
{
printf("FAILED. Out of Memory!\n");
enemies->size--;
}
}
And here is my function call and variable declaration in the main procedure:
int main()
{
path_data my_path[41];
enemy_data_array enemies;
enemies.size = 0;
add_enemy(&enemies);
}
Why isn't this working?
You invoked undefined behavior by passing indeterminate value enemies->data in uninitialized variable having automatic storage duration. Initialize it before using add_enemy().
int main()
{
path_data my_path[41];
enemy_data_array enemies;
enemies.size = 0;
enemies.data = 0; /* add this line */
add_enemy(&enemies);
}
0 is a null pointer constant and can safely be converted to pointer NULL. Unlike NULL, 0 will work without including any headers. Of course you can use enemies.data = NULL; with proper header included.
#2501's explanation is completely correct. Another solution is to change your implementation of add_enemy() to something like this:
void add_enemy(enemy_data_array *enemies)
{
enemy_data *new_array;
// check if size was non-zero
if (enemies->size++)
{
new_array = (enemy_data *)realloc(enemies->data, sizeof(enemy_data) * enemies->size);
}
// start new allocation
else
{
new_array = (enemy_data *)alloc(sizeof(enemy_data) * enemies->size);
}
if (new_array) //if (re)alloc fails (ie out of memory) it will return null
{
enemies->data = new_array;
// enemies->data[enemies->size - 1] = read_enemy_data();
printf("Enemy added successfully!\n");
}
else
{
printf("FAILED. Out of Memory!\n");
enemies->size--;
}
}
If fails because you haven't cleared the content of "enemies". Since it is a stack variable, it will contain whatever garbage data is on the stack.
set enemies.data to NULL in the main function and try it again.
I define a structure in a header file like so:
typedef struct {
void *data;
} point;
I want to keep other people from accessing *data directly, so I thought I'd declare the structure in the .c file instead and use something like extern typedef struct point; in the header file. That doesn't work however.
What's the best way to achieve this?
In your (public) header file:
typedef struct point point;
In your .c file:
struct point
{
void *data;
};
Note that users of your code will no longer be able to create a point on the stack, as the compiler doesn't know how big it is. You may have to provide a point_create() function which allocates memory and returns its address to the caller.
Use C++
Since jokes seem not be allowed here is the pure C version.
As another commenter pointed out if you really want to protect your internals from users of your Api you have seen and used plenty of such Apis. This Apis are e.g. the Windows or Linux user mode Apis. There you create kernel objects to which you never shall have access to. The Apis to deal with kernel objects use a synthetic construct called handle which is not simply a pointer to your own object but instead it is an index to an array where the kernel has stored the relevant meta data for your object.
You can use the same idea for your Apis as well.
Here for example is a C-Style public Api:
// Public.h
#include <stdlib.h>
typedef enum
{
None = 0,
PointType = 1
} Types;
typedef int Handle;
Handle CreateType(Types type);
int DeleteType(Handle object);
void IncrementX(Handle point);
void PrintPoint(Handle point);
As you can see you have generic methods which create and delete your objects which are defined here in an enum. Your methods which use the object will then need to lookup the integer handle to get the meta data object where the real data is stored.
This design is not very efficient if the objects you manage are small since for every object a second object is need which stores the object type, handle value and the pointer to the real data.
But you get much stronger safety guarantees such as
Type safety
Invalid handles are easy to find
Double free is impossible since you can manage the free state in the meta object
A typical usage of your Api might look like this:
Handle h = CreateType(PointType);
IncrementX(h);
IncrementX(h);
PrintPoint(h);
DeleteType(h);
And there is the super secret implementation in private.cpp where the Handle lookup array and some helper methods exist:
// Private.C
#include "stdafx.h"
#include <stdlib.h>
#include <Windows.h> // for ZeroMemory
#include "Public.h"
typedef struct
{
LPVOID pData;
Types type;
Handle handle;
} HandleInfo;
typedef struct
{
int x;
int y;
} Point;
HandleInfo *pAllocated;
int HandleBuffer = 0xffff;
unsigned char bInit = 0;
HandleInfo *GetFreeHandle()
{
int i;
if( !bInit )
{
pAllocated = (HandleInfo *) malloc(sizeof(HandleInfo)*HandleBuffer);
bInit = 1;
ZeroMemory(pAllocated, sizeof(HandleInfo)*HandleBuffer);
}
for(i=0; i<HandleBuffer; i++)
{
HandleInfo *pInfo = (pAllocated+i);
if( 0 == pInfo->handle )
{
pInfo->handle = i+1;
return pInfo;
}
}
return NULL;
}
HandleInfo * GetHandleInfo(Handle h)
{
if( h <= 0 || h >= HandleBuffer-1)
{
return NULL;
}
return (pAllocated+h-1);
}
Handle CreateType(Types typeId)
{
HandleInfo *pInfo;
pInfo = GetFreeHandle();
if( NULL == pInfo )
{
return -1;
}
pInfo->type = typeId;
switch(typeId)
{
case PointType:
pInfo->pData = malloc(sizeof(Point));
ZeroMemory(pInfo->pData, sizeof(Point));
break;
}
return pInfo->handle;
}
int DeleteType(Handle object)
{
HandleInfo *pInfo = GetHandleInfo(object);
if( NULL == pInfo )
{
return -1;
}
if( pInfo->handle != 0 )
{
free(pInfo->pData);
pInfo->pData = NULL;
pInfo->handle = 0;
return 1;
}
else
{
return 0; // Handle was already closed
}
}
void *GetObjectOfCorrectType(Handle object, Types type)
{
HandleInfo *p = GetHandleInfo(object);
if( p == NULL )
{
return NULL;
}
if( p->type != type)
{
return NULL; // handle has wrong object type
}
return p->pData;
}
void IncrementX(Handle point)
{
Point *pPoint = (Point *) GetObjectOfCorrectType(point, PointType);
if( pPoint == NULL )
{
return;
}
pPoint->x++;
}
void PrintPoint(Handle point)
{
Point *pPoint = (Point *) GetObjectOfCorrectType(point, PointType);
if( pPoint == NULL )
{
return;
}
printf("Point has x: %d y: %d", pPoint->x, pPoint->y);
}
Yours,
Alois Kraus
This is the pointer to implementation or pimpl idiom. See http://en.wikibooks.org/wiki/C++_Programming/Idioms#Pointer_To_Implementation_.28pImpl.29 for a tutorial for C++, but the idea should work in C as well.
typedef struct {
/* private members; don't access directly */
void *data;
} point;
You can have separate public header and private header files. Some libraries have conventions for this:
Xt (X11) -> header.h and headerP.h, e.g: X11/Vendor.h vs X11/VendorP.h
Qt -> header.h vs private/header_p.h, e.g: qapplication.h vs private/qapplication_p.h
If you do not want to use the declaration method (because you want the library user to access other members of your struct, for example) it is convention to prepend private member with an underscore, like this:
typedef struct {
void * _data;
} point;
Of course people could still access _data if they would really want to (just like people can access private data in C++ by adding a #define private public before their includes) but that is their own responsibility; at least you have indicated that they shouldn't do that if they want your library to behave as it should.
I use this approach in order to let client alloc the module instance in his STACK.
struct module_private {
int data;
}
typedef uint8_t module_t [sizeof (struct module_private) ];
Client will be able to see private struct content, but not access it without doing a cast that he shouldn't.
Use the following workaround:
#include <stdio.h>
#define C_PRIVATE(T) struct T##private {
#define C_PRIVATE_END } private;
#define C_PRIV(x) ((x).private)
#define C_PRIV_REF(x) (&(x)->private)
struct T {
int a;
C_PRIVATE(T)
int x;
C_PRIVATE_END
};
int main()
{
struct T t;
struct T *tref = &t;
t.a = 1;
C_PRIV(t).x = 2;
printf("t.a = %d\nt.x = %d\n", t.a, C_PRIV(t).x);
tref->a = 3;
C_PRIV_REF(tref)->x = 4;
printf("tref->a = %d\ntref->x = %d\n", tref->a, C_PRIV_REF(tref)->x);
return 0;
}
Result is:
t.a = 1
t.x = 2
tref->a = 3
tref->x = 4