I have a struct defined like this
typedef struct {
char* Value;
unsigned int Length;
} MY_STRUCT;
I'm creating an array of these structs using calloc:
MY_STRUCT* arr = (MY_STRUCT*)calloc(50, sizeof(MY_STRUCT));
Then, in a loop, I'm accessing each struct and trying to allocate and assign a value to the Value field using calloc and memcpy:
int i;
for(i = 0; i < 50; i++)
{
MY_STRUCT myStruct = arr[i];
int valueLength = get_value_length(i);//for sake of example, we can assume that this function returns any value [1-99]
myStruct.Length = valueLength;
myStruct.Value = (char*) calloc(valueLength, sizeof(char));
memcpy(myStruct.Value, get_value(i), valueLength); //assume get_value(i) returns char* pointing to start of desired value
}
This code block crashes on the calloc line with Visual Studio indicating heap corruption. It doesn't fail the first time through the loop. Instead, it fails on the second pass when I'm trying to allocate a length 20 char array (first pass is length 5). I've tried using malloc as well, and I've tried using recommendations in:
Heap Corruption with malloc, struct and char *
Do I cast the result of malloc?
Nothing seems to mitigate the problem. I am originally a managed code programmer so my knowledge of memory allocation and management is not always the best. I'm sure I'm doing something boneheaded, but I'm not sure what. Any help would be greatly appreciated. Thank you!
I'm currently working on dynamically allocating my array of structures and I'm unsure how to continue. This is my structure:
struct Word_setup
{
char word[M];
int count;
} phrase[N];
I know malloc returns a pointer to a block of memory, but I'm not sure how this works when it comes to an array of structures.
If anyone could please clarify that would be much appreciated!
Probably you meant:
struct Word_setup {
char word[M];
int count;
};
It's a good idea to avoid defining variables in the same line as a struct definition anyway, to help with code readability.
Then you can allocate an array of these:
int main()
{
struct Word_setup *phrase = malloc(N * sizeof *phrase);
// use phrases[x] where 0 <= x < N
phrase = realloc(phrase, (N+5) * sizeof *phrase);
// now can go up to phrases[N+4]
free(phrase);
}
Of course you should check for failure and abort the program if malloc or realloc returns NULL.
If you also want to dynamically allocate each string inside the word then there are a few options; the simplest one to understand is to change char word[M] to char *word; and each time you allocate a phrase, write the_phrase.word = malloc(some_number); . If you allocate an array of words you'll need to loop through doing that for each word.
I suppose that N and M is a compile-time known constants. Then just use sizeof, .e.g.
struct Word_setup*ptr = malloc(sizeof(struct Word_setup)*N);
Maybe you want a flexible array member. Then, it should always be the last member of your struct, e.g.
struct Word_setup {
int count;
unsigned size;
char word[]; // of size+1 dimension
};
Of course it is meaningless to have an array of flexibly sized structures -you need an array of pointers to them.
I've allocated an "array" of mystruct of size n like this:
if (NULL == (p = calloc(sizeof(struct mystruct) * n,1))) {
/* handle error */
}
Later on, I only have access to p, and no longer have n. Is there a way to determine the length of the array given just the pointer p?
I figure it must be possible, since free(p) does just that. I know malloc() keeps track of how much memory it has allocated, and that's why it knows the length; perhaps there is a way to query for this information? Something like...
int length = askMallocLibraryHowMuchMemoryWasAlloced(p) / sizeof(mystruct)
I know I should just rework the code so that I know n, but I'd rather not if possible. Any ideas?
No, there is no way to get this information without depending strongly on the implementation details of malloc. In particular, malloc may allocate more bytes than you request (e.g. for efficiency in a particular memory architecture). It would be much better to redesign your code so that you keep track of n explicitly. The alternative is at least as much redesign and a much more dangerous approach (given that it's non-standard, abuses the semantics of pointers, and will be a maintenance nightmare for those that come after you): store the lengthn at the malloc'd address, followed by the array. Allocation would then be:
void *p = calloc(sizeof(struct mystruct) * n + sizeof(unsigned long int),1));
*((unsigned long int*)p) = n;
n is now stored at *((unsigned long int*)p) and the start of your array is now
void *arr = p+sizeof(unsigned long int);
Edit: Just to play devil's advocate... I know that these "solutions" all require redesigns, but let's play it out.
Of course, the solution presented above is just a hacky implementation of a (well-packed) struct. You might as well define:
typedef struct {
unsigned int n;
void *arr;
} arrInfo;
and pass around arrInfos rather than raw pointers.
Now we're cooking. But as long as you're redesigning, why stop here? What you really want is an abstract data type (ADT). Any introductory text for an algorithms and data structures class would do it. An ADT defines the public interface of a data type but hides the implementation of that data type. Thus, publicly an ADT for an array might look like
typedef void* arrayInfo;
(arrayInfo)newArrayInfo(unsignd int n, unsigned int itemSize);
(void)deleteArrayInfo(arrayInfo);
(unsigned int)arrayLength(arrayInfo);
(void*)arrayPtr(arrayInfo);
...
In other words, an ADT is a form of data and behavior encapsulation... in other words, it's about as close as you can get to Object-Oriented Programming using straight C. Unless you're stuck on a platform that doesn't have a C++ compiler, you might as well go whole hog and just use an STL std::vector.
There, we've taken a simple question about C and ended up at C++. God help us all.
keep track of the array size yourself; free uses the malloc chain to free the block that was allocated, which does not necessarily have the same size as the array you requested
Just to confirm the previous answers: There is no way to know, just by studying a pointer, how much memory was allocated by a malloc which returned this pointer.
What if it worked?
One example of why this is not possible. Let's imagine the code with an hypothetic function called get_size(void *) which returns the memory allocated for a pointer:
typedef struct MyStructTag
{ /* etc. */ } MyStruct ;
void doSomething(MyStruct * p)
{
/* well... extract the memory allocated? */
size_t i = get_size(p) ;
initializeMyStructArray(p, i) ;
}
void doSomethingElse()
{
MyStruct * s = malloc(sizeof(MyStruct) * 10) ; /* Allocate 10 items */
doSomething(s) ;
}
Why even if it worked, it would not work anyway?
But the problem of this approach is that, in C, you can play with pointer arithmetics. Let's rewrite doSomethingElse():
void doSomethingElse()
{
MyStruct * s = malloc(sizeof(MyStruct) * 10) ; /* Allocate 10 items */
MyStruct * s2 = s + 5 ; /* s2 points to the 5th item */
doSomething(s2) ; /* Oops */
}
How get_size is supposed to work, as you sent the function a valid pointer, but not the one returned by malloc. And even if get_size went through all the trouble to find the size (i.e. in an inefficient way), it would return, in this case, a value that would be wrong in your context.
Conclusion
There are always ways to avoid this problem, and in C, you can always write your own allocator, but again, it is perhaps too much trouble when all you need is to remember how much memory was allocated.
Some compilers provide msize() or similar functions (_msize() etc), that let you do exactly that
May I recommend a terrible way to do it?
Allocate all your arrays as follows:
void *blockOfMem = malloc(sizeof(mystruct)*n + sizeof(int));
((int *)blockofMem)[0] = n;
mystruct *structs = (mystruct *)(((int *)blockOfMem) + 1);
Then you can always cast your arrays to int * and access the -1st element.
Be sure to free that pointer, and not the array pointer itself!
Also, this will likely cause terrible bugs that will leave you tearing your hair out. Maybe you can wrap the alloc funcs in API calls or something.
malloc will return a block of memory at least as big as you requested, but possibly bigger. So even if you could query the block size, this would not reliably give you your array size. So you'll just have to modify your code to keep track of it yourself.
For an array of pointers you can use a NULL-terminated array. The length can then determinate like it is done with strings. In your example you can maybe use an structure attribute to mark then end. Of course that depends if there is a member that cannot be NULL. So lets say you have an attribute name, that needs to be set for every struct in your array you can then query the size by:
int size;
struct mystruct *cur;
for (cur = myarray; cur->name != NULL; cur++)
;
size = cur - myarray;
Btw it should be calloc(n, sizeof(struct mystruct)) in your example.
Other have discussed the limits of plain c pointers and the stdlib.h implementations of malloc(). Some implementations provide extensions which return the allocated block size which may be larger than the requested size.
If you must have this behavior you can use or write a specialized memory allocator. This simplest thing to do would be implementing a wrapper around the stdlib.h functions. Some thing like:
void* my_malloc(size_t s); /* Calls malloc(s), and if successful stores
(p,s) in a list of handled blocks */
void my_free(void* p); /* Removes list entry and calls free(p) */
size_t my_block_size(void* p); /* Looks up p, and returns the stored size */
...
really your question is - "can I find out the size of a malloc'd (or calloc'd) data block". And as others have said: no, not in a standard way.
However there are custom malloc implementations that do it - for example http://dmalloc.com/
I'm not aware of a way, but I would imagine it would deal with mucking around in malloc's internals which is generally a very, very bad idea.
Why is it that you can't store the size of memory you allocated?
EDIT: If you know that you should rework the code so you know n, well, do it. Yes it might be quick and easy to try to poll malloc but knowing n for sure would minimize confusion and strengthen the design.
One of the reasons that you can't ask the malloc library how big a block is, is that the allocator will usually round up the size of your request to meet some minimum granularity requirement (for example, 16 bytes). So if you ask for 5 bytes, you'll get a block of size 16 back. If you were to take 16 and divide by 5, you would get three elements when you really only allocated one. It would take extra space for the malloc library to keep track of how many bytes you asked for in the first place, so it's best for you to keep track of that yourself.
This is a test of my sort routine. It sets up 7 variables to hold float values, then assigns them to an array, which is used to find the max value.
The magic is in the call to myMax:
float mmax = myMax((float *)&arr,(int) sizeof(arr)/sizeof(arr[0]));
And that was magical, wasn't it?
myMax expects a float array pointer (float *) so I use &arr to get the address of the array, and cast it as a float pointer.
myMax also expects the number of elements in the array as an int. I get that value by using sizeof() to give me byte sizes of the array and the first element of the array, then divide the total bytes by the number of bytes in each element. (we should not guess or hard code the size of an int because it's 2 bytes on some system and 4 on some like my OS X Mac, and could be something else on others).
NOTE:All this is important when your data may have a varying number of samples.
Here's the test code:
#include <stdio.h>
float a, b, c, d, e, f, g;
float myMax(float *apa,int soa){
int i;
float max = apa[0];
for(i=0; i< soa; i++){
if (apa[i]>max){max=apa[i];}
printf("on i=%d val is %0.2f max is %0.2f, soa=%d\n",i,apa[i],max,soa);
}
return max;
}
int main(void)
{
a = 2.0;
b = 1.0;
c = 4.0;
d = 3.0;
e = 7.0;
f = 9.0;
g = 5.0;
float arr[] = {a,b,c,d,e,f,g};
float mmax = myMax((float *)&arr,(int) sizeof(arr)/sizeof(arr[0]));
printf("mmax = %0.2f\n",mmax);
return 0;
}
In uClibc, there is a MALLOC_SIZE macro in malloc.h:
/* The size of a malloc allocation is stored in a size_t word
MALLOC_HEADER_SIZE bytes prior to the start address of the allocation:
+--------+---------+-------------------+
| SIZE |(unused) | allocation ... |
+--------+---------+-------------------+
^ BASE ^ ADDR
^ ADDR - MALLOC_HEADER_SIZE
*/
/* The amount of extra space used by the malloc header. */
#define MALLOC_HEADER_SIZE \
(MALLOC_ALIGNMENT < sizeof (size_t) \
? sizeof (size_t) \
: MALLOC_ALIGNMENT)
/* Set up the malloc header, and return the user address of a malloc block. */
#define MALLOC_SETUP(base, size) \
(MALLOC_SET_SIZE (base, size), (void *)((char *)base + MALLOC_HEADER_SIZE))
/* Set the size of a malloc allocation, given the base address. */
#define MALLOC_SET_SIZE(base, size) (*(size_t *)(base) = (size))
/* Return base-address of a malloc allocation, given the user address. */
#define MALLOC_BASE(addr) ((void *)((char *)addr - MALLOC_HEADER_SIZE))
/* Return the size of a malloc allocation, given the user address. */
#define MALLOC_SIZE(addr) (*(size_t *)MALLOC_BASE(addr))
malloc() stores metadata regarding space allocation before 8 bytes from space actually allocated. This could be used to determine space of buffer. And on my x86-64 this always return multiple of 16. So if allocated space is multiple of 16 (which is in most cases) then this could be used:
Code
#include <stdio.h>
#include <malloc.h>
int size_of_buff(void *buff) {
return ( *( ( int * ) buff - 2 ) - 17 ); // 32 bit system: ( *( ( int * ) buff - 1 ) - 17 )
}
void main() {
char *buff = malloc(1024);
printf("Size of Buffer: %d\n", size_of_buff(buff));
}
Output
Size of Buffer: 1024
This is my approach:
#include <stdio.h>
#include <stdlib.h>
typedef struct _int_array
{
int *number;
int size;
} int_array;
int int_array_append(int_array *a, int n)
{
static char c = 0;
if(!c)
{
a->number = NULL;
a->size = 0;
c++;
}
int *more_numbers = NULL;
a->size++;
more_numbers = (int *)realloc(a->number, a->size * sizeof(int));
if(more_numbers != NULL)
{
a->number = more_numbers;
a->number[a->size - 1] = n;
}
else
{
free(a->number);
printf("Error (re)allocating memory.\n");
return 1;
}
return 0;
}
int main()
{
int_array a;
int_array_append(&a, 10);
int_array_append(&a, 20);
int_array_append(&a, 30);
int_array_append(&a, 40);
int i;
for(i = 0; i < a.size; i++)
printf("%d\n", a.number[i]);
printf("\nLen: %d\nSize: %d\n", a.size, a.size * sizeof(int));
free(a.number);
return 0;
}
Output:
10
20
30
40
Len: 4
Size: 16
If your compiler supports VLA (variable length array), you can embed the array length into the pointer type.
int n = 10;
int (*p)[n] = malloc(n * sizeof(int));
n = 3;
printf("%d\n", sizeof(*p)/sizeof(**p));
The output is 10.
You could also choose to embed the information into the allocated memory yourself with a structure including a flexible array member.
struct myarray {
int n;
struct mystruct a[];
};
struct myarray *ma =
malloc(sizeof(*ma) + n * sizeof(struct mystruct));
ma->n = n;
struct mystruct *p = ma->a;
Then to recover the size, you would subtract the offset of the flexible member.
int get_size (struct mystruct *p) {
struct myarray *ma;
char *x = (char *)p;
ma = (void *)(x - offsetof(struct myarray, a));
return ma->n;
}
The problem with trying to peek into heap structures is that the layout might change from platform to platform or from release to release, and so the information may not be reliably obtainable.
Even if you knew exactly how to peek into the meta information maintained by your allocator, the information stored there may have nothing to do with the size of the array. The allocator simply returned memory that could be used to fit the requested size, but the actual size of the memory may be larger (perhaps even much larger) than the requested amount.
The only reliable way to know the information is to find a way to track it yourself.
I'm trying to declare and allocate memory for an array of structures defined as follows:
typedef struct y{
int count;
char *word;
} hstruct
What I have right now is:
hstruct *final_list;
final_list = calloc (MAX_STR, sizeof(hstruct));
MAX_STRbeing the max size of the char word selector.
I plan on being able to refer to the it as:
final_list[i].count, which would be an integer and
final_list[i].word, which would be a string.
ibeing an integer variable.
However, such expressions always return (null). I know I'm doing something wrong, but I don't know what. Any help would be appreciated. Thanks.
A struct that contains a pointer doesn't directly holds the data, but holds a pointer to the data. The memory for the pointer itself is correctly allocated through your calloc but it is just an address.
This means that is your duty to allocate it:
hstruct *final_list;
final_list = calloc(LIST_LENGTH, sizeof(hstruct));
for (int i = 0; i < LIST_LENGTH; ++i)
final_list[i].word = calloc(MAX_STR, sizeof(char));
This requires also to free the memory pointed by final_list[i].word before releasing the array of struct itself.
This question is a continuation of Malloc call crashing, but works elsewhere
I tried the following program and I found it working (i.e. not crashing - and this was mentioned in the above mentioned link too). I May be lucky to have it working but I'm looking for a reasonable explanation from the SO experts on why this is working?!
Here are some basic understanding on allocation of memory using malloc() w.r.t structures and pointers
malloc(sizeof(struct a) * n) allocates n number of type struct a elements. And, this memory location can be stored and accessed using a pointer-to-type-"struct a". Basically a struct a *.
malloc(sizeof(struct a *) * n) allocates n number of type struct a * elements. Each element can then point to elements of type struct a. Basically malloc(sizeof(struct a *) * n) allocates an array(n-elements)-of-pointers-to-type-"struct a". And, the allocated memory location can be stored and accessed using a pointer-to-(pointer-to-"struct a"). Basically a struct a **.
So when we create an array(n-elements)-of-pointers-to-type-"struct a", is it
valid to assign that to struct a * instead of struct a ** ?
valid to access/de-reference the allocated array(n-elements)-of-pointers-to-type-"struct a" using pointer-to-"struct a" ?
data * array = NULL;
if ((array = (data *)malloc(sizeof(data *) * n)) == NULL) {
printf("unable to allocate memory \n");
return -1;
}
The code snippet is as follows:
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
typedef struct {
int value1;
int value2;
}data;
int n = 1000;
int i;
int val=0;
data * array = NULL;
if ((array = (data *)malloc(sizeof(data *) * n)) == NULL) {
printf("unable to allocate memory \n");
return -1;
}
printf("allocation successful\n");
for (i=0 ; i<n ; i++) {
array[i].value1 = val++;
array[i].value2 = val++;
}
for (i=0 ; i<n ; i++) {
printf("%3d %3d %3d\n", i, array[i].value1, array[i].value2);
}
free(array);
printf("freeing successful\n");
return 0;
}
EDIT:
OK say if I do the following by mistake
data * array = NULL;
if ((array = (data *)malloc(sizeof(data *) * n)) == NULL) {
Is there a way to capture (during compile-time using any GCC flags) these kind of unintended programming typo's which could work at times and might blow out anytime! I compiled this using -Wall and found no warnings!
There seems to be a fundamental misunderstanding.
malloc(sizeof(struct a) * n) allocates n number of type struct a elements.
No, that's just what one usually does use it as after such a call. malloc(size) allocates a memory region of size bytes. What you do with that region is entirely up to you. The only thing that matters is that you don't overstep the limits of the allocated memory. Assuming 4 byte float and int and 8 byte double, after a successful malloc(100*sizeof(float));, you can use the first 120 of the 400 bytes as an array of 15 doubles, the next 120 as an array of 30 floats, then place an array of 20 chars right behind that and fill up the remaining 140 bytes with 35 ints if you wish. That's perfectly harmless defined behaviour.
malloc returns a void*, which can be implicitly cast to a pointer of any type, so
some_type **array = malloc(100 * sizeof(data *)); // intentionally unrelated types
is perfectly fine, it might just not be the amount of memory you wanted. In this case it very likely is, because pointers tend to have the same size regardless of what they're pointing to.
More likely to give you the wrong amount of memory is
data *array = malloc(n * sizeof(data*));
as you had it. If you use the allocated piece of memory as an array of n elements of type data, there are three possibilities
sizeof(data) < sizeof(data*). Then your only problem is that you're wasting some space.
sizeof(data) == sizeof(data*). Everything's fine, no space wasted, as if you had no typo at all.
sizeof(data) > sizeof(data*). Then you'll access memory you shouldn't have accessed when touching later array elements, which is undefined behaviour. Depending on various things, that could consistently work as if your code was correct, immediately crash with a segfault or anything in between (technically it could behave in a manner that cannot meaningfully be placed between those two, but that would be unusual).
If you intentionally do that, knowing point 1. or 2. applies, it's bad practice, but not an error. If you do it unintentionally, it is an error regardless of which point applies, harmless but hard to find while 1. or 2. applies, harmful but normally easier to detect in case of 3.
In your examples. data was 4 resp. 8 bytes (probably), which on a 64-bit system puts them into 1. resp. 2. with high probability, on a 32-bit system into 2 resp. 3.
The recommended way to avoid such errors is to
type *pointer = malloc(num_elems * sizeof(*pointer));
No.
sizeof(struct a*) is the size of a pointer.
sizeof(struct a) is the size of the entire struct.
This array = (data *)malloc(sizeof(data *) * n) allocates a sizeof(data*) (pointer) to struct data, if you want to do that, you need a your array to be a data** array.
In your case you want your pointer to point to sizeof(data), a structure in memory, not to another pointer. That would require a data** (pointer to pointer).
is it valid to assign that to struct a * instead of struct a ** ?
Well, technically speaking, it is valid to assign like that, but it is wrong (UB) to dereference such pointer. You don't want to do this.
valid to access/de-reference the allocated array(n-elements)-of-pointers-to-type-"struct a" using pointer-to-"struct a" ?
No, undefined behavior.