I have this C function:
fill_array(&data, &size);
void fill_array(int **data, int *size){
printf("Size is:");
scanf("%d", size);
*data = malloc(*size * sizeof(int *));
int i = 0;
for (i = 0; i < size; i++){
(*data)[i] = rand() % 11;
}
}
I want to assign data[i] for example, to random number. How to do such a thing? I have tried many variations, but all of the time my program crashes.
Thanks.
*data = malloc(*size * sizeof(**data));
(*data)[5] = 15;
Refer to cdecl web site.
Do not cast malloc
Edit according to the question edit
the for loop contains typo
for (i = 0; i < size; i++)
it should be
for (i = 0; i < *size; i++)
you can use (*data)[5] = 15; instead of this *data[5] = 15; Because precedence of [] greater than precedence of *..
As others said, you need to put parentheses to get the operator precedence right. If you want to use the "array" a lot, it might make sense to create a temporary variable that is easy to use:
int *p;
...
*data = malloc(*size * sizeof **data);
p = *data;
And then you could use p[5] etc.
Good program design dictates that we should keep memory allocation and the actual algorithm separated. To have a function that takes user input and allocates memory and performs some algorithm is probably not the optimal program design.
So the proper solution is not to patch that function to make it work, but instead to make some new ones:
int get_size_from_user (void)
{
int size;
printf("Size is:");
scanf("%d", &size);
return size;
}
bool alloc_array (int** array, int size)
{
*array = malloc(size * sizeof(int));
return *array != NULL;
}
void fill_array (int* array, int size)
{
// ...whatever you want to do here
data[5] = 15;
}
And look at that, the need for obscure syntax disappeared as soon as we improved the program design! Coincidence?
Related
I've a problem about deallocating memory using free() in C.
My program generates a random genealogic tree using a matrix. This matrix can be very huge depending on the number of family members. The program seemed to work fine until I decided to generate more than one tree. I noticed that generating about 100 trees causes my 8GB RAM to fill! I'm sure I can make a better code to reduce the demand of memory, but my problem remains.
I use free() to deallocate memory and there's no error. I installed Valgrind to se what's happening and it says that about 100 million byte per tree are definitely lost. This means that free() doesn't work fine. I don't now where is the problem. I link some functions that I think are correlated to the problem.
typedef struct{
int f_id;
char f_name[L_NAMES];
int generations;
int n_members;
type_people *members;
int_mtx *mtx;
}type_family;
The struct above is for the family.
typedef struct temp{
int p_id;
char name[L_NAMES];
char f_name[L_NAMES];
int generation;
int n_sons;
struct temp **sons;
int f_id;
int sex;
int age;
}type_people;
This is for the members.
typedef struct{
int i;
int j;
int **val;
}int_mtx;
And the matrix.
In the main i call the function to initialize the tree:
type_family *family_a;
family_a = malloc(sizeof(type_family));
family_a = init_family_n_gen(family_a, 6);
This is the frist part of init_family_n_gen():
type_family *init_family_n_gen(type_family *family, int n){
...
family->members = malloc(max_people * sizeof(type_people));
family->mtx = mtxcalloc(family->mtx, max_people, max_people - 1);
...
This code is for mtxcalloc that initializes the matrix:
int_mtx *mtxcalloc(int_mtx *mtx, int i, int j){
mtx = malloc(sizeof(int_mtx));
mtx->i = i;
mtx->j = j;
mtx->val = malloc(i * sizeof(int *));
for(int a = 0; a < i; a++){
mtx->val[a] = malloc(j * sizeof(int));
for(int b = 0; b < j; b++){
mtx->val[a][b] = 0;
}
}
return mtx;
}
And to conclude the code to deallocate the family:
void free_family(type_family *family){
for(int m = 0; m < family->n_members; m++){
if(family->members[m].n_sons != 0){
free(family->members[m].sons);
}
}
mtxfree(family->mtx);
free(family->members);
}
And the one to deallocate the matrix:
void mtxfree(int_mtx *mtx){
for(int i = 0; i < mtx->i; i++){
free(mtx->val[i]);
}
free(mtx->val);
free(mtx);
}
Screen capture of Valgrind output
So I call the free_family(family_a) every time i need to regenerate the family but the memory still increases. (In the photo above the number of byte become 1 billion if i regenerate the family for 50 times).
Thanks for the support!
EDITED
I made a minimal reproducible example that emulates my original code. The structs and variables are the same but I changed the functions according to Weather Vane: they are all void and I pass them the double **.
The init_family_n_gen becomes:
void init_family(type_family **f){
type_family *family = malloc(sizeof(type_family));
family->members = malloc(100 * sizeof(type_people));
for(int m = 0; m < 100; m++){
family->members[m].n_sons = 0;
}
mtxcalloc(&family->mtx, 100, 99);
family->mtx->val[0][1] = 7;
family->mtx->val[9][8] = 1;
mtxrealloc(&family->mtx, 5, 4);
*f = family;
}
The main is:
type_family *family_a;
init_family(&family_a);
free_family(&family_a);
The only thing I added is this function(Is the code right?):
void mtxrealloc(int_mtx **mtx, int i, int j){
(*mtx)->i = i;
(*mtx)->j = j;
(*mtx)->val = realloc((*mtx)->val, (*mtx)->i * sizeof(int *));
for(int a = 0; a < (*mtx)->i; a++){
(*mtx)->val[a] = realloc((*mtx)->val[a], (*mtx)->j * sizeof(int));
}
}
I noticed that the problem occours when i use the realloc function and i can't figure why. I link the images of Valgrind with and without the function mtxrealloc. (I see that there is aslo a 48 byte leak...).
Valgrind with realloc
Valgrind without realloc
Thanks again for your support!
This:
init_family(&family_a);
Causes this code from mtxcalloc to execute:
mtx->val = malloc(i * sizeof(int *));
for(int a = 0; a < i; a++){
mtx->val[a] = malloc(j * sizeof(int));
for(int b = 0; b < j; b++){
mtx->val[a][b] = 0;
}
}
, with i, j = 100, 99. That is, you allocate space for 100 pointers, and for each one, you allocate space for 99 ints. These are then accessible via family_a->mtx.
Very shortly thereafter, you make this call:
mtxrealloc(&family->mtx, 5, 4);
, which does this, among other things:
(*mtx)->val = realloc((*mtx)->val, (*mtx)->i * sizeof(int *));
That loses all the pointers (*mtx)->val[5] through (*mtx)->val[99], each of which is the sole pointer to allocated space sufficient for 99 ints. Overall, sufficient space for 9405 ints is leaked before you even perform any computations with the object you are preparing.
It is unclear why you overallocate, just to immediately (attempt to) free the excess, but perhaps that's an artifact of your code simplification. It would be much better to come up with a way to determine how much space you need in advance, and then allocate only that much in the first place. But if you do need to reallocate this particular data, then you need to first free each of the (*mtx)->val[x] that will be lost. Of course, if you were going to reallocate larger, then you would need to allocate / reallocate all of the (*mtx)->val[x].
I'm working on a C implementation for Conway's game of life, I have been asked to use the following header:
#ifndef game_of_life_h
#define game_of_life_h
#include <stdio.h>
#include <stdlib.h>
// a structure containing a square board for the game and its size
typedef struct gol{
int **board;
size_t size;
} gol;
// dynamically creates a struct gol of size 20 and returns a pointer to it
gol* create_default_gol();
// creates dynamically a struct gol of a specified size and returns a pointer to it.
gol* create_gol(size_t size);
// destroy gol structures
void destroy_gol(gol* g);
// the board of 'g' is set to 'b'. You do not need to check if 'b' has a proper size and values
void set_pattern(gol* g, int** b);
// using rules of the game of life, the function sets next pattern to the g->board
void next_pattern(gol* g);
/* returns sum of all the neighbours of the cell g->board[i][j]. The function is an auxiliary
function and should be used in the following function. */
int neighbour_sum(gol* g, int i, int j);
// prints the current pattern of the g-board on the screen
void print(gol* g);
#endif
I have added the comments to help out with an explanation of what each bit is.
gol.board is a 2-level integer array, containing x and y coordinates, ie board[x][y], each coordinate can either be a 1 (alive) or 0 (dead).
This was all a bit of background information, I'm trying to write my first function create_default_gol() that will return a pointer to a gol instance, with a 20x20 board.
I then attempt to go through each coordinate through the 20x20 board and set it to 0, I am getting a Segmentation fault (core dumped) when running this program.
The below code is my c file containing the core code, and the main() function:
#include "game_of_life.h"
int main()
{
// Create a 20x20 game
gol* g_temp = create_default_gol();
int x,y;
for (x = 0; x < 20; x++)
{
for (y = 0; y < 20; y++)
{
g_temp->board[x][y] = 0;
}
}
free(g_temp);
}
// return a pointer to a 20x20 game of life
gol* create_default_gol()
{
gol* g_rtn = malloc(sizeof(*g_rtn) + (sizeof(int) * 20 * 20));
return g_rtn;
}
This is the first feature I'd like to implement, being able to generate a 20x20 board with 0's (dead) state for every coordinate.
Please feel free to criticise my code, I'm looking to determine why I'm getting the segmentation fault, and if I'm allocating memory properly in the create_default_gol() function.
Thanks!
The type int **board; means that board must contain an array of pointers, each of which points to the start of each row. Your existing allocation omits this, and just allocates *g_rtn plus the ints in the board.
The canonical way to allocate your board, supposing that you must stick to the type int **board;, is:
gol* g_rtn = malloc(sizeof *g_rtn);
g_rtn->size = size;
g_rtn->board = malloc(size * sizeof *g_rtn->board);
for (int i = 0; i < size; ++i)
g_rtn->board[i] = malloc(size * sizeof **g_rtn->board);
This code involves a lot of small malloc chunks. You could condense the board rows and columns into a single allocation, but then you also need to set up pointers to the start of each row, because board must be an array of pointers to int.
Another issue with this approach is alignment. It's guaranteed that a malloc result is aligned for any type; however it is possible that int has stricter alignment requirements than int *. My following code assumes that it doesn't; if you want to be portable then you could add in some compile-time checks (or run it and see if it aborts!).
The amount of memory required is the sum of the last two mallocs:
g_rtn->board = malloc( size * size * sizeof **g_rtn->board
+ size * sizeof *g_rtn->board );
Then the first row will start after the end of the row-pointers (a cast is necessary because we are converting int ** to int *, and using void * means we don't have to repeat the word int):
g_rtn->board[0] = (void *) (g_rtn->board + size);
And the other rows each have size ints in them:
for (int i = 1; i < size; ++i)
g_rtn->board[i] = g_rtn->board[i-1] + size;
Note that this is a whole lot more complicated than just using a 1-D array and doing arithmetic for the offsets, but it was stipulated that you must have two levels of indirection to access the board.
Also this is more complicated than the "canonical" version. In this version we are trading code complexity for the benefit of having a reduced number of mallocs. If your program typically only allocates one board, or a small number of boards, then perhaps this trade-off is not worth it and the canonical version would give you fewer headaches.
Finally - it would be possible to allocate both *g_rtn and the board in the single malloc, as you attempted to do in your question. However my advice (based on experience) is that it is simpler to keep the board separate. It makes your code clearer, and your object easier to use and make changes to, if the board is a separate allocation to the game object.
create_default_gol() misses to initialise board, so applying the [] operator to it (in main() ) the program accesses "invaid" memory and with ethis provokes undefined behaviour.
Although enough memory is allocated, the code still needs to make board point to the memory by doing
gol->board = ((char*) gol) + sizeof(*gol);
Update
As pointed out by Matt McNabb's comment board points to an array of pointers to int, so initialisation is more complicate:
gol * g_rtn = malloc(sizeof(*g_rtn) + 20 * sizeof(*gol->board));
g_rtn->board = ((char*) gol) + sizeof(*gol);
for (size_t i = 0; i<20; ++i)
{
g_rtn->board[i] = malloc(20 * sizeof(*g_rtn->board[i])
}
Also the code misses to set gol's member size. From what you tell us it is not clear whether it shall hold the nuber of bytes, rows/columns or fields.
Also^2 coding "magic numbers" like 20 is bad habit.
Also^3 create_default_gol does not specify any parameters, which explictily allows any numberm and not none as you might perhaps have expected.
All in all I'd code create_default_gol() like this:
gol * create_default_gol(const size_t rows, const size_t columns)
{
size_t size_rows = rows * sizeof(*g_rtn->board));
size_t size_column = columns * sizeof(**g_rtn->board));
gol * g_rtn = malloc(sizeof(*g_rtn) + size_rows);
g_rtn->board = ((char*) gol) + sizeof(*gol);
if (NULL ! = g_rtn)
{
for (size_t i = 0; i<columns; ++i)
{
g_rtn->board[i] = malloc(size_columns); /* TODO: Add error checking here. */
}
g_rtn->size = size_rows * size_columns; /* Or what ever this attribute is meant for. */
}
return g_rtn;
}
gol* create_default_gol()
{
int **a,i;
a = (int**)malloc(20 * sizeof(int *));
for (i = 0; i < 20; i++)
a[i] = (int*)malloc(20 * sizeof(int));
gol* g_rtn = (gol*)malloc(sizeof(*g_rtn));
g_rtn->board = a;
return g_rtn;
}
int main()
{
// Create a 20x20 game
gol* g_temp = create_default_gol();
int x,y;
for (x = 0; x < 20; x++)
{
for (y = 0; y < 20; y++)
{
g_temp->board[x][y] = 10;
}
}
for(x=0;x<20;x++)
free(g_temp->board[x]);
free(g_temp->board);
free(g_temp);
}
main (void)
{
gol* gameOfLife;
gameOfLife = create_default_gol();
free(gameOfLife);
}
gol* create_default_gol()
{
int size = 20;
gol* g_rtn = malloc(sizeof *g_rtn);
g_rtn = malloc(sizeof g_rtn);
g_rtn->size = size;
g_rtn->board = malloc(size * sizeof *g_rtn->board);
int i, b;
for (i = 0; i < size; ++i){
g_rtn->board[i] = malloc(sizeof (int) * size);
for(b=0;b<size;b++){
g_rtn->board[i][b] = 0;
}
}
return g_rtn;
}
Alternatively, since you also need to add a create_gol(size_t new_size) of custom size, you could also write it as the following.
main (void)
{
gol* gameOfLife;
gameOfLife = create_default_gol();
free(gameOfLife);
}
gol* create_default_gol()
{
size_t size = 20;
return create_gol(size);
}
gol* create_gol(size_t new_size)
{
gol* g_rtn = malloc(sizeof *g_rtn);
g_rtn = malloc(sizeof g_rtn);
g_rtn->size = new_size;
g_rtn->board = malloc(size * sizeof *g_rtn->board);
int i, b;
for (i = 0; i < size; ++i){
g_rtn->board[i] = malloc(sizeof (int) * size);
for(b=0;b<size;b++){
g_rtn->board[i][b] = 0;
}
}
return g_rtn;
}
Doing this just minimizes the amount of code needed.
I need to write a function that creates a double pointer using malloc.
This is how I declared my double pointer as I normally would:
double **G; //Create double pointer to hold 2d matrix
*G = malloc(numNodes * sizeof(double*));
for(i = 0; i < numNodes; i++)
{
G[i] = malloc(numNodes*sizeof(double));
for (j = 0; j < numNodes; j++)
{
G[i][j] = 0;
}
}
Now I tried replacing it with:
double **G;
mallocDoubleArr(G, numNodes);
With the function being:
void mallocDoubleArr(double **arr, int size)
{
int i, j;
*arr = malloc(size * sizeof(double*));
for(i = 0; i < size; i++)
{
arr[i]= malloc(size*sizeof(double));
for (j = 0; j < size; j++)
{
arr[i][j] = 0;
}
}
}
Why doesn't this work?
You need one more "indirection", in other words pass G by reference like a pointer to a pointer to a pointer to float:
void mallocDoubleArr(double ***arr, int size);
And then call it as
mallocDoubleArr(&G, numNodes);
Modify mallocDoubleArr accordingly, like for example
(*arr)[i] = malloc(size*sizeof(double));
For starters, you need to change the line
*G = malloc(numNodes * sizeof(double*));
to
G = malloc(numNodes * sizeof(double*));
(you can't dereference a pointer safely until you've assigned something to it.)
Secondly, your function modifies the pointer passed in, so you need a pointer to it. Your signature should instead by
void mallocDoubleArr(double ***arr, int size)
and you will need to add the relevant indirections in your code to access the pointer that the pointer is pointing to.
A lot of confusion for beginners working with pointers comes from, in my opinion, thinking that they are something different than regular old variables. Pointers, like ints, floats, etc. are just variables that live on the stack: they have addresses, and they are passed to functions the same way. If you want to change a variable (int, float, pointer, etc) in a function, you need to pass a pointer to it. There is no difference in this regard.
C is call-by-value. In
double **G;
mallocDoubleArr(G, numNodes);
you are passing an uninitialized variable to mallocDoubleArr. It may be zero, it may be something else, but it almost certainly isn't something that mallocDoubleArr can assign to.
We can change the code, and the function's definition, to pass in G's address, but then you're dealing with yet another level of pointer. That might make it harder to understand the code. If that really isn't a requirement, I'd propose instead to have mallocDoubleArr return a double**.
double **G;
G = mallocDoubleArr(numNodes);
double **mallocDoubleArr(int size)
{
int i, j;
double **arr;
arr = (double **) malloc(size * sizeof(double *));
/* continue with old code */
return arr;
}
[edit: bstamour's post was made while I was writing mine. Sorry for any overlap.]
I use for matrix operations code like following for allocating and freeing.
int **inputMatrix, i, j;
Grid myGrid = *grid;
inputMatrix = (int *) calloc (myGrid.num_nodes, sizeof(int*));
for(i=0; i < myGrid.num_nodes; i++){
inputMatrix[i] = calloc(myGrid.num_nodes, sizeof(int));
for(j=0;j<myGrid.num_nodes;j++){
inputMatrix[i][j] = 0;
}
};
for(i=0; i < myGrid.num_nodes; i++){
free(inputMatrix[i]);
}
free (inputMatrix);
I want after I do some calculation create an array in C would u please help me?
int len_arr = (sizeof inputs)/(sizeof inputs[0]);
int half_arr = len_arr / 2;
if(len_arr%2 != 0)
half_arr++;
int m[half_arr][half_arr];
I'm assuming you wish to create a matrix with row/column size of half_arr.
int i;
int len_arr = (sizeof inputs)/(sizeof inputs[0]);
int half_arr = len_arr / 2;
if(len_arr%2 != 0)
half_arr++;
int **m = malloc(half_arr * sizeof(int));
for (i = 0; i < half_arr; i++)
*m = malloc(half_arr * sizeof(int));
You should then be able to access m using m[row][column] or m[column][row].
You need to use malloc to allocate your 2-dimensional array.
Look at this LINK for a good explanation.
EDIT:
Starting from the code at the given link, I've written a piece of code that should work.
Some differences from James' code:
declaration of m at the top of the code block
conversion from the return type of malloc (void *) to the correct one
memory deallocation at the end
it's a complete code, that must work :P
#include <stdlib.h>
int main ()
{
// just an example of inputs...
int inputs[3][7];
// your code
int i;
int **m;
int len_arr = (sizeof inputs)/(sizeof inputs[0]);
int half_arr = len_arr / 2;
if(len_arr%2 != 0)
half_arr++;
// allocation of the array
m = (int **)malloc(half_arr * sizeof(int *));
for(i = 0; i < half_arr; i++)
m[i] = (int *)malloc(half_arr * sizeof(int));
// use the array as you like...
// ...
// let's release the allocated memory properly
for(i = 0; i < half_arr; i++)
free(m[i]);
free(m);
}
Your code is valid C99. You might want to use a compiler which supports newer revisions of the C language.
On Windows, I'd go with the MinGW edition of gcc. If you're stuck with MSVC, I suggest using C++ instead: the standard container types will make your life less painful.
I need your advice on this piece of code:
the table fields options[0], options[1] etc... doesn't seem to be freed correctly.
Thanks for your answers
int main()
{
....
char **options;
options = generate_fields(user_input);
for(i = 0; i < sizeof(options) / sizeof(options[0]); i++) {
free(options[i]);
options[i] = NULL;
}
free(options);
}
char ** generate_fields(char *)
{
char ** options = malloc(256*sizeof(char *));
...
return options;
}
The problem is this:
for(i = 0; i < sizeof(options) / sizeof(options[0]); i++)
options is a pointer type, not an array type, so sizeof(options) will always be the same (typically 4 bytes on a 32-bit machine or 8 bytes on a 64-bit machine), so sizeof(options)/sizeof(options[0]) will almost always be 1.
The key is to always free memory in the same manner as you malloc'ed it. So, if you malloc a 2-dimensional array and then malloc a series of 1-dimensional arrays, you need to do the reverse when freeing it:
char ** generate_fields(char *)
{
char ** options = malloc(256*sizeof(char *));
for(int i = 0; i < 256; i++)
options[i] = malloc(some_size);
return options;
}
void free_fields(char ** options)
{
for(int i = 0; i < 256; i++)
free(options[i]);
free(options);
}
Note that if the size (256 in this case) is not a constant, you need to keep track of it yourself, since otherwise you have no way of knowing how many times to loop when freeing.
You should have the same number of frees as you have mallocs.
In your code you allocate the array of pointers, but you don't allocate any memory for the individual elements of the array to point to. But your freeing code is written as if you did.
I'm going to add to Adam's answer here, because this probably won't fit in a comment. Adam is completely right. I suspect your generate_fields function, however, perhaps actually gets input from the user, I'm not sure. In any case, there are two ways to approach this:
char ** generate_fields(char *, int num_fields, int size_of_field)
{
char ** options = malloc(num_fields*sizeof(char *));
for(int i = 0; i < num_fields; i++)
options[i] = malloc(size_of_field);
return options;
}
and a corresponding free function, which I'll leave out for brevity. You can see what's going on - we're passing in the number of fields and the field size. Change this around as you need. The other option is to have generate fields pass back to the routine it's called from the size of the array. I'd do something like this:
int generate_fields(char** options)
{
int num_fields = 0;
// somewhere here we get num_fields
options = malloc(num_fields*sizeof(char *));
for(int i = 0; i < num_fields; i++)
options[i] = malloc(size_of_field);
return num_fields;
}
And you call from main like this:
int main()
{
int sizeofarray = 0;
char** fields;
sizeofarray = generate_fields(fields);
Or if you dislike that notation, you can always stick to what you had:
char** generate_fields(int* size)
As the function prototype (return options this time and do size= somewhere in the code and call from main like this:
int sizeofarray = 0;
char** options;
options = generate_fields(&sizeofarray);
Hope that gives you some more ideas, Adam, feel free to edit any / all of this into your answer as appropriate, it comes from your answer anyway.