Is passing a static array to a function efficient? - c

#define BUFF_SIZE 100000
unsigned char buffer[BUFF_SIZE];
void myfunc(unsigned char[],int,int);
void myfuncinfunc(unsigned char[],int,int);
int main()
{
int a = 10, b = 10;
myfunc(buffer,a,b);
}
void myfunc(unsigned char array[],int a,int b)
{
int m,n;
//blah blah
myfuncinfunc(array,m,n);
}
void myfuncinfunc(unsigned char array[],int a, int b)
{
//blah blah
}
I wish to know the following:
I have created a static array as seen above the 'main' function. Is this efficient? Would it be better if I used a point and malloc instead?
I know it doesn't use the stack, so when I pass the array into inner functions, would it create a copy of the whole array or just send the location of the first entry?
When working on 'array' in the function 'myfunc', am I working directly with the static defined array or some local copy?
Inside the function 'myfunc', when we pass the array into the function 'myfuncinfunc', would it again, send only the first location or a complete copy of the array into the stack?
Thanks for reading the question and would greatly appreciate any help! I'm new to C and trying to learn it off the internet.

I don't see how it would be more or less efficient than an array on the heap.
It decays into a pointer to the first entry.
Therefore it's not a local copy, it's the array itself.
Ditto.
By the way, if a and b are indexes within the array, consider using the size_t type for them (it's an unsigned int guaranteed big enough for indexing arrays).

I have created a static array as seen above the 'main' function. Is this efficient? Would it be better if I used a point and malloc instead?
Define "efficient". Statically allocated arrays are always faster than dynamic ones, because of the runtime overhead for allocation/deallocation.
In this case, you allocate a huge amount of 100k bytes, which might be very memory-inefficient.
In addition, your process might not have that much static memory available, depending on OS. On desktop systems, it is therefore considered best practice to allocate on the heap whenever you are using large amounts of data.
I know it doesn't use the stack, so when I pass the array into inner functions, would it create a copy of the whole array or just send the location of the first entry?
You can't pass arrays by value in C. So a pointer to the first element of the array will be saved on the stack and passed to the function.
When working on 'array' in the function 'myfunc', am I working directly with the static defined array or some local copy?
Directly on the static array. Again, you can't pass arrays by value.
Inside the function 'myfunc', when we pass the array into the function 'myfuncinfunc', would it again, send only the first location or a complete copy of the array into the stack?
A pointer to the first element.

Related

changing fixed size array to dynamic in C

I have an char array of fixed size in C application. I am passing that array to some function and from there I am passing it to multiple functions. So that the array gets filled in some of the functions based on some condition. Since I am sending a fixed size array I am facing problem when I copy data to it if the size is more than the array size. I know that I have to make that char array dynamic but as I said that array gets filled in multiple functions and size will be different. So do I need to dynamically allocate the array wherever it gets filled? Consider the array gets filled in 30+ different functions. Or is there a way to do a minimal modification?
As your question title says C, IMO the best approach will be to decalre a pointer of that particular variable type in your main() function, pass the address of that pointer [essencially a double-pointer] to other functions and allocate memory dynamically.
You can keep on passing the pointer to all other functions. Inside every called functions, measure the amount of memory required to put the data [from that particular function] and use realloc() to increase the available memory.
As mentioned by UncleO, the required pointer should be the pointer to array [i.e, a double pointer]
EDIT
For the very first time allocating memory to that pointer, you can use malloc() or calloc().
From next time onwards, to extend [resize] the amount of memory, you need to use realloc()
You don't pass an array to a function in C, although it appears that way. What gets passed is a pointer to the first element of the array. The pointer is passed by value. That is, the value of the pointer (the memory location) is copied into a local variable. The contents of the array can be changed using this local variable.
If you use malloc() or realloc() with this local variable, then the pointer that was "passed in" won't be affected. realloc() may resize the memory, but it can also free that memory and allocate some new memory to the local variable.
If you want to change the array pointer, then you should pass in a pointer to the pointer. The thing the pointer points to is what is changed. This is a bit more cumbersome. But this way you can allocate more memory is needed.
#include <stdlib.h>
char* arr;
void changeit(char** arrptr)
{
*arrptr = realloc(*arrptr, 20*sizeof(char));
}
void main (void)
{
arr = malloc(10*sizeof(char));
changeit(&arr);
}
To function that do not alter the array, pass the array address and size.
int foo1(const char *array, size_t size, ...)
To each function that does not change the array size, pass array address and array size
int foo2(char *array, size_t size, ...)
To functions that may alter the array size, pass the address of the address of the array and the address of the size.
int foo3(char **array, size_t *size, ...)
Code could put these two variables together
typedef struct {
size_t size;
char array;
} YetAnotherArrayType;
Chux,
There's a little typo at then end of your post. I think you mean:
typedef struct {
size_t size;
char* array;
} YetAnotherArrayType;
You didn't make array a pointer type.
If the poster is handcrafting a container, the classic solution is to track both a size and a capacity.
In that model you allocate the array to some initial a capacity and set size to 0. You then track its population causing it grow by some chunk size or factor each time it fills up.
Frequent reallocation can be a massive performance drain and by the sounds of the program in question such behaviour seems likely.

passing multidimensional array as argument in C

C newbie here, I need some help: Can anyone explain to (and offer a workaroud) me why this works:
int n=1024;
int32_t data[n];
void synthesize_signal(int32_t *data) {
...//do something with data}
which let me alter data in the function; but this does not?
int n=1024;
int number=1024*16;
int32_t data[n][2][number];
void synthesize_signal(int32_t *data) {
...//do something with data}
The compiler error message is something like it expected int32_t * but got int32_t (*)[2][(sizetype)(number)] instead.
First, passing arrays in C is by reference. So you pass a pointer of some sort, and the function can modify the data in the array. You don't have to worry about passing a pointer to the array. In fact, in C there is no real different between a pointer that happens to be to the being of an array, and the array itself.
In your first version. You making a one-dimensional array data[n], and you are passing it to your function. In the array, you'll using it by saying, something like data[i]. This translates directly to (data + (i sizeof(int32_t)). It is using the size of the elements in the array to find the memory location that is i positions in front of the beginning of your array.
int n=1024;
int number=1024*16;
int32_t data[n][2][number];
void synthesize_signal(int32_t *data)
In the second case, you're setting up a mufti-dimensional array (3D in your case). You setup correctly. The problem is that when you pass it to the function, the only thing that gets passed the address of the being of the array. When it gets used inside the function, you'll do something like
data[i][1][x] = 5;
Internally C is calculating how from the beginning of the array this location is. In order for it to do that, it need to know the dimensions of the array. (Unlike some newer languages, C store any extra data about array lengths or sizes or anything). You just need to change the function signature so it knows the shape/size of array to expect. Because of the way, it calculates array positions, it doesn't need the first dimension.
In this case, change your function signature to look like this:
void synthesize_signal(int32_t data[][2][number]) { ...
Setup the array the same way you are doing the second one above, and just call it you'd expect:
synthesize_signal(data);
This should fix everything for you.
The comments mention some useful information about using more descriptive variable names, and global vs. local variable. All valid comments to keep in mind. I just addressed to code problem you're having in terms of mufti-dimensional arrays.
try
synthesize_signal(int32_t** data)
{
}
Your function also needs to know that data is multi dimensional. You should also consider renaming your data array. I suspect that it is a global variable and using the same name in function can lead to problems.
When you call the function, do it like this:
synthesize_signal(&data[0][0][0]);

Memory management during function call

I was writing a code that involves handling a 2D array of dimensions [101]X[101] in C. However I am constrained in terms of memory being used at a given point of time.
void manipulate(int grid_recv[101][101])
{
//Something
}
void main()
{
int grid[101][101];
manipulate(grid);
}
So lets say I create an array grid[101][101] in my main() and then pass it for manipulation to another function. Now does the function manipulate() copy the entire matrix grid into grid_recv i.e by this sort of passing am I using twice the amount of memory ( i.e twice the size of grid)?
No. In C, arrays cannot be passed as parameters to functions.
What you actually do is creating a pointer pointing the array. So the extra memory you use it only the size of that pointer created.

Is it possible to pass a pointer to and array as a function parameter?

I have two integer arrays created at runtime (size depends on the program input). At some point I need to update the contents of an array with the contents of the other doing some calculations.
First I thought about passing those arrays as parameters to a function because I didn't find a way to return functions in C (don't think it's possible). After realizing that was a bad idea since parameters are not really modifiable as they're copied to the stack I resorted to change to array pointers instead.
While the function is still empty, this is the code I have:
1st take (code compiles, no errors):
// Elements is just to be able to iterate through their contents (same for both):
void do_stuff(int first[], int second[], int elements) {}
// Call to the function:
do_stuff(first, second, elements);
2nd take, attempt to translate to pointers to be able to modify the arrays in place:
void do_stuff(int *first[], int *second[], int elements) {}
// Call to the function:
do_stuff(&first, &second, elements);
This code lead to some rightful compile time errors, because apparently what I thought to be pointer to arrays were arrays of pointers.
3rd take, what I think it'd be the right syntax:
void do_stuff(int (*first)[], int (*second)[], int elements) {}
// Call to the function:
do_stuff(&first, &second, elements);
Still this code produces compile time errors when trying to access the elements of the arrays (e.g. *first[0]):
error: invalid use of array with unspecified bounds
So my question is regarding the possibility of using an array pointer as a parameter of a function, is it possible? If so, how could it be done?
Anyway, if you think of a better way to update the first array after performing calculations involving the contents of the second please comment about it.
An array decays to a pointer to the data allocated for the array. Arrays are not copied to the stack when passing to functions. Thus, you needn't pass a pointer to the array. So, the below should function fine.
// Elements is just to be able to iterate through their contents (same for both):
void do_stuff(int first[], int second[], int elements) {}
// Call to the function:
do_stuff(first, second, elements);
The cause of your errors on your second attempt are because int *first[] (and the others like it) are actually of the type array of pointer to int.
The cause of your third errors are because *first[N] is actually *(first[N]), which cannot be done with ease. Array access is really a facade over pointer arithmetic, *(first + sizeof first[0] * N); however, you have an incomplete element type here -- you need to specify the size of the array, otherwise sizeof first[0] is unknown.
Your first attempt is correct. When passing an array as a parameter in C, a pointer to the first element is actually passed, not a copy of the array. So you can write either
void do_stuff(int first[], int second[], int elements) {}
like you had, or
void do_stuff(int *first, int *second, int elements) {}
In C arrays automatically decay to pointers to data, So, you can just pass the arrays and their lengths and get the desired result.
My suggestion is something like this:
void dostuff(int *first, int firstlen, int *second, int secondlen, int elements)
Function call should be:
do_stuff(first, firstlen, second, secondlen, elements);
I am not very clear from your question, why you need elements. But, you must pass array lengths as arrays automatically decays to pointers when passed to a function, but, in the called function, there is no way to determine their size.

List parameters in recursive calls (C)

I'm designing a recursive algorithm :
int f(int[] a, int[] b){
----changing a here
----changing b here
f(a,b)
----writing a here
----writing b here
}
I know all arrays are pointers so this code should be problematic. It'll write the final values of a and b after all the recursive calls finished. I dont want that to happen.
What should I do to pass that array to recursive calls as regular "pass by value"?
PS: Arrays may be dynamically allocated or resized etc anywhere in recursive calls.
I will assume that int f(int[] a, int[] b) is a typo: it should be int f(int a[], int b[]).
First of all, arrays and pointers are different. Under many circumstances, the name of an array "decays" to a pointer to its first element, but not always. Otherwise, sizeof a, for an array a wouldn't "work".
Secondly, let's ignore recursion for a moment. Let's also make the function prototype simpler:
int g(int *a);
(I changed int a[] to int *a, because in this context, the name of an array is equivalent to a pointer.)
Now, you say that you may "dynamically allocate or resize" the array in the function call. Since everything is passed by value in C, a dynamic allocation or resizing of a inside g() cannot be seen by the caller: the caller would still have the old value of a. But more importantly, if the "original" type of a was an array, i.e., the caller had code like:
int data[SIZE];
g(data);
then trying to resize data in g() is bad, because the parameter specified in the call wasn't dynamically allocated to begin with. You can only dynamically resize something that was the result of malloc(), calloc(), or realloc().
So, there are two problems even with this simple definition:
If g() has to dynamically resize the memory referred to by the address it is given, the value has to come from a dynamic allocation, not an array,
After fixing that, since g() wants to be able to signal the change to the caller, you need to pass a pointer to what needs to be changed. So, the prototype of g() becomes: int g(int **a);.
Hopefully the above will help you get started. If you tell us more about your algorithm, in particular, what you mean by: "changing" and "writing", you will get better responses.
Edit: to answer your question in the comment:
So when I passed an array to a function it decays to a pointer and this pointer is passed by value. If that pointer is pointing a place I allocated before that call...
If you allocated something before the call, it never was an array to begin with. It can be indexed as an array, but that's not the point. So, maybe you are getting confused by the terminology here?
...when my new function changes the pointed value then that value is changed at caller, too.
The pointed-to value is changed, yes.
I dont want it to be like this, so I need a copy of the pointed value in the new function so that my original pointer's pointed value would not change. am I clear now?
It's clearer now, but then that raises more questions:
If you are going to dynamically allocate or resize the data in each call to the function, how are you going to return those new pointers? You can't. And that means you have got yourself a memory leak. Unless you free() the data in the (recursively called) function itself.
How would you resize the pointer? You may not be able to know the size of the data pointed to, unless you use a sentinel value.
Are you using the function to iteratively solve a puzzle or a problem? Are you free()ing your data in each invocation of the function? If you can tell us, exactly what are you trying to do?
All arrays are not pointers.
See these:
http://c-faq.com/aryptr/practdiff.html
http://c-faq.com/aryptr/aryptrequiv.html
http://c-faq.com/aryptr/ptrkindofary.html - specifically this one
Assuming you want the order of the output to be the same as in your original, if you don't want the outer call to see changes to your arrays made in the recursive call, you need to copy them. The easiest way is probably to allocate them on the stack then memcopy from the argument, though that will cause a SO if it gets too big. The second easiest is to malloc and free them. You will probably need to pass the array size around too. You can't pass arrays by value on the stack in C.
Just print them before passing them to the next level of recursion.
Your question isn't terribly clear but what I'm reading is this:
You have a recursive algorithm that operates on two heap-allocated arrays. It is possible that one of the recursive calls will have to reallocate the arrays, and so when it returns to the next level up, the old pointers won't be valid anymore. You want to know how to "pass the arrays by value" in order to avoid this.
First, you cannot pass an array by value in C. Period. Arrays are always passed as pointers to the first element, and there's no way around that.
Second, the answer is always another level of indirection. :)
In order to solve the problem of having to reallocate the arrays during the recursive call, what you want to do is have the function take two pointer-to-pointers (int**), where *a gives the address of the pointer to the first element of the A array, and (*a)[n] gives the nth element of the a array. That way, you can reallocate the array to your heart's content, (changing the value of *a) but the value of a itself always remains the same. By doing this, instances of the function further up the call stack will automatically inherit the reallocations made by the recursive calls because the pointed-to value (*a) of the pointer (a) that they passed to the recursive call was modified to reflect the new location of the actual data.
e.g.
int f(int** a, int** b)
{
/* Do stuff on arrays using (*a)[n] and (*b)[n] */
if (some_condition) {
/* Time to reallocate */
*a = realloc(*a, new_size);
*b = realloc(*b, new_size);
}
/* Do stuff on arrays using (*a)[n] and (*b)[n] */
f(a, b); /* Recursive call */
/* Do more stuff using (*a)[n] and (*b)[n] */
/* a and b are still valid pointers, even though *a and *b might point somewhere new */
return foo;
}
void use_f(void)
{
int* a = malloc(starting_size);
int* b = malloc(starting_size);
f(&a, &b);
}
Given the requirements:
int f(int[] a, int[] b){
----changing a here
----changing b here
f(a,b)
----writing a here
----writing b here
}
What should I do to pass that array to recursive calls as regular "pass by value"?
PS: Arrays may be dynamically allocated or resized etc anywhere in recursive calls.
The code in f() is either authorized to make changes to the arrays (as now), or it is not authorized to make changes. If it is authorized to make changes, then there is nothing much you need to do, except worry about whether you are going to leak memory if you are using dynamic allocation.
If the code is not authorized to change the arrays, then it will have to make copies of the arrays. You can prevent the code from casually modifying them by including appropriate const qualifiers:
int f(const int *a, const int *b) { ... }
Note that you cannot pass arrays by value in C. You could have the caller pass a modifiable copy - or you can have the receiver (callee?) make the copy; one or the other will have to o so if the receiver is going to make modifications when it shouldn't.
The easiest and safest option is to pass a pointer and a size. Say you are working on something like quick-sort:
void sort_range( int* ptr, size_t count )
{
size_t pivot;
assert( ptr ); /* make sure we have memory */
if ( count < 2 ) return; /* terminate recursion */
pivot = partition( count ); /* select a pivot */
assert( pivot < count ); /* make sure we don't overrun the buffer */
sort_range( ptr, pivot ); /* sort left part */
sort_range( ptr + pivot, count - pivot ); /* sort right part */
merge( ptr, count, pivot ); /* merge ranges */
}
Always be conscious about size of the memory chunk you are working with. Unfortunately C doesn't make it easy, so you have to develop a habit of checking your memory ranges.

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