I'm writing an API that has structs such as
struct datast{
int a;
int *items;
size_t numitems;
};
I'm providing functions which free the contents of such structs (in a similar fashion to what C++ destructors do). Constructors are not provided because I'm mandating zero initialization for them (the .items field is required to be a NULL pointer on initialization, which makes it suitable for later realloc() and free()).
I'm providing however, an additem() function, which realloc()s .items and increases .numitems accordingly.
However, because these structs are small, I'd like to encourage the use of designated initializers and compound literals, so that users can conveniently create these objects with one-liners when possible, without having to manually invoke additem().
But then, if you initialize structs like these with designated initializers (or assign to them from a compound literal), the .items field will have automatic storage rather than allocated storage. And so, if you pass this struct to the "freeing" function/destructor later, you'll be calling free() with an illegal pointer (pointing to automatic storage).
Yes, I know the wording could be "don't call the destructor for objects for which you didn't call additem()"... but this looks really clumsy, and seems like bad design.
Somehow, it's like I had to decide if all these objects should have either automatic or allocated storage, without giving both possibilities to the user.
Have you ever been in a scenario like this? Is there any kind of design I could use that could provide a clean and elegant interface for both automatic and allocated storage?
Add a boolean member items_allocated. The zero initialisation that you mandate will make that false. Then additem() will set it true:
struct datast
{
int a;
int *items;
bool items_allocated ;
size_t numitems;
} ;
Then your destructor can have something like:
if( d->items_allocated )
{
free( d->items ) ;
d->items = NULL ;
}
d->numitems = 0 ;
...
I am new to C. I have these two files set up in this way.
I do not fully understand how I am able to assign values in the Item array without dynamically allocating memory.
The line Collection c; places all fields on the stack, so is that why I can directly set array members?
//collection.c
typedef struct {
uint32 price;
uint32 itemId;
} Item;
typedef struct {
Item item[MAX_SIZE];
uint32 name;
} Collection;
void function(Collection * ptr)
{
int i;
uint32 id = 0;
for(i = 0; i < MAX_SIZE; i++)
{
ptr->item[i].price = 10;
ptr->item[i].itemId = id;
id++;
}
}
//collection_main.c
Collection c; //global struct variable
//calls function in collection.c
function(&c);
I do not fully understand how I am able to assign values in the Item array without dynamically allocating memory.
First, as you are new to C, be aware of a potential issue with passing C functions pointers (which is quite reasonable, BTW). Unless you can guarantee that your calling code will always pass a valid pointer you need to check that pointer value in the function as best you can. That will typically amount to checking for a non-null pointer like this :
if ( ptr == NULL )
return <whatever to signal an error> ;
In this case you did allocate memory, because you created a Collection variable and that contains allocated space for the required fields.
The line Collection c; places all fields on the stack,
If it's in a function it will (typically) allocate space on that function's stack frame, which you should logically view as a separate area that the calling code cannot access. Make no assumptions about the layout of the stack. A very typical bug is to try and return a pointer to an item declared inside a function, and even supposedly experienced programmers have been known to do it.
Another potential bug in passing a pointer to a function is trying to access beyond the limits of the space allocated and pointed to. This can do things like corrupt other variables or even crash code. Your own code is correctly using the declared constant size of the array, so no problem.
If you do this outside of a function (which is possible), you would be using space reserved by the OS for these type of variables. That may not be on the stack but elsewhere. The OS gets that information from the compiled code file.
so is that why I can directly set array members ?
C code (and the executable binary that's produced by the compiler) does not care or check whether the pointers you pass are valid or not. So it's possible to pass a bad pointer to a C function and cause chaos.
In this case you did allocate all the required valid memory when you declared the variable and you passed a pointer to that variable. So no problem.
Dynamic memory allocation
It is more usual to consider explicit allocation using the malloc() family of functions as dynamic allocation. Allocations for local and global variables may be dynamic in the sense that they can happen at runtime but the allocation and deallocation are not the responsibility of the programmer to explicitly control so you do not generally need to think about these as part of dynamic memory allocation.
A minor point to close :
uint32 name ;
I'd consider this a bad choice of field name. Using "name" implies a string, whereas you probably mean a string id from e.g. an array. So try something like :
uint32 nameid ;
instead.
You'd be surprised how many coding problems crop up in a production environment simply because of a poor choice of variable name. Make them informative if possible and practical.
This is just a good coding habit to get into, IMO.
Is it allowed to dynamically allocate memory for static variable like this:
#include <stdio.h>
#include <stdlib.h>
struct person
{
int age;
int number;
};
static struct person* person_p = NULL;
int main()
{
person_p = (struct person*)malloc(10 * sizeof(struct person));
}
The above code built, but is it really allowed to dynamically allocate memory for static variable?
Yes, it's valid and allowed. (Unless you're using the pointer as a placeholder) You can (and need to) dynamically allocate and free() memory to and from the pointer before and after using it.
Rather, please make a note, you do not cast the return value of malloc() and family in C.
I don't see why not. Even though static means there can only be one instance of the object, you still need space for that object. Keep in mind however that anything that is malloc'd needs to be free'd, so you will want to do that at the end of your main() function.
Memory isn't "owned" by pointers to it. You can do the following things:
Dynamically allocate memory
Make a pointer point to that memory
It doesn't really make sense to say "dynamically allocate memory for a pointer".
Any pointer can point to any object (subject to alignment and aliasing restrictions), it makes no difference what the storage duration of the pointer is.
Note that it is the pointer that is static, not the memory it points to.
static means two unrelated things:
give static memory allocation (memory allocated at start of program, and only released at end of program)
give internal linkage (do not allow other compilation units - modules - to access the identifier. Here a variable, could be a function)
In your code, 1. static memory allocation is irrelevant, since the variable is global anyway and as such already has it.
Then, 2. internal linkage does not matter either, because what you are trying to do is inside the module anyway.
In other words, person_p is exactly as a usual global variable within your module, and you can do whatever you want to it.
It's only the pointer that is defined by this line of code, so you can dynamically allocate memory elsewhere, and assign the memory address to person_p if you wish.
I've always heard that in C you have to really watch how you manage memory. And I'm still beginning to learn C, but thus far, I have not had to do any memory managing related activities at all.. I always imagined having to release variables and do all sorts of ugly things. But this doesn't seem to be the case.
Can someone show me (with code examples) an example of when you would have to do some "memory management" ?
There are two places where variables can be put in memory. When you create a variable like this:
int a;
char c;
char d[16];
The variables are created in the "stack". Stack variables are automatically freed when they go out of scope (that is, when the code can't reach them anymore). You might hear them called "automatic" variables, but that has fallen out of fashion.
Many beginner examples will use only stack variables.
The stack is nice because it's automatic, but it also has two drawbacks: (1) The compiler needs to know in advance how big the variables are, and (2) the stack space is somewhat limited. For example: in Windows, under default settings for the Microsoft linker, the stack is set to 1 MB, and not all of it is available for your variables.
If you don't know at compile time how big your array is, or if you need a big array or struct, you need "plan B".
Plan B is called the "heap". You can usually create variables as big as the Operating System will let you, but you have to do it yourself. Earlier postings showed you one way you can do it, although there are other ways:
int size;
// ...
// Set size to some value, based on information available at run-time. Then:
// ...
char *p = (char *)malloc(size);
(Note that variables in the heap are not manipulated directly, but via pointers)
Once you create a heap variable, the problem is that the compiler can't tell when you're done with it, so you lose the automatic releasing. That's where the "manual releasing" you were referring to comes in. Your code is now responsible to decide when the variable is not needed anymore, and release it so the memory can be taken for other purposes. For the case above, with:
free(p);
What makes this second option "nasty business" is that it's not always easy to know when the variable is not needed anymore. Forgetting to release a variable when you don't need it will cause your program to consume more memory that it needs to. This situation is called a "leak". The "leaked" memory cannot be used for anything until your program ends and the OS recovers all of its resources. Even nastier problems are possible if you release a heap variable by mistake before you are actually done with it.
In C and C++, you are responsible to clean up your heap variables like shown above. However, there are languages and environments such as Java and .NET languages like C# that use a different approach, where the heap gets cleaned up on its own. This second method, called "garbage collection", is much easier on the developer but you pay a penalty in overhead and performance. It's a balance.
(I have glossed over many details to give a simpler, but hopefully more leveled answer)
Here's an example. Suppose you have a strdup() function that duplicates a string:
char *strdup(char *src)
{
char * dest;
dest = malloc(strlen(src) + 1);
if (dest == NULL)
abort();
strcpy(dest, src);
return dest;
}
And you call it like this:
main()
{
char *s;
s = strdup("hello");
printf("%s\n", s);
s = strdup("world");
printf("%s\n", s);
}
You can see that the program works, but you have allocated memory (via malloc) without freeing it up. You have lost your pointer to the first memory block when you called strdup the second time.
This is no big deal for this small amount of memory, but consider the case:
for (i = 0; i < 1000000000; ++i) /* billion times */
s = strdup("hello world"); /* 11 bytes */
You have now used up 11 gig of memory (possibly more, depending on your memory manager) and if you have not crashed your process is probably running pretty slowly.
To fix, you need to call free() for everything that is obtained with malloc() after you finish using it:
s = strdup("hello");
free(s); /* now not leaking memory! */
s = strdup("world");
...
Hope this example helps!
You have to do "memory management" when you want to use memory on the heap rather than the stack. If you don't know how large to make an array until runtime, then you have to use the heap. For example, you might want to store something in a string, but don't know how large its contents will be until the program is run. In that case you'd write something like this:
char *string = malloc(stringlength); // stringlength is the number of bytes to allocate
// Do something with the string...
free(string); // Free the allocated memory
I think the most concise way to answer the question in to consider the role of the pointer in C. The pointer is a lightweight yet powerful mechanism that gives you immense freedom at the cost of immense capacity to shoot yourself in the foot.
In C the responsibility of ensuring your pointers point to memory you own is yours and yours alone. This requires an organized and disciplined approach, unless you forsake pointers, which makes it hard to write effective C.
The posted answers to date concentrate on automatic (stack) and heap variable allocations. Using stack allocation does make for automatically managed and convenient memory, but in some circumstances (large buffers, recursive algorithms) it can lead to the horrendous problem of stack overflow. Knowing exactly how much memory you can allocate on the stack is very dependent on the system. In some embedded scenarios a few dozen bytes might be your limit, in some desktop scenarios you can safely use megabytes.
Heap allocation is less inherent to the language. It is basically a set of library calls that grants you ownership of a block of memory of given size until you are ready to return ('free') it. It sounds simple, but is associated with untold programmer grief. The problems are simple (freeing the same memory twice, or not at all [memory leaks], not allocating enough memory [buffer overflow], etc) but difficult to avoid and debug. A hightly disciplined approach is absolutely mandatory in practive but of course the language doesn't actually mandate it.
I'd like to mention another type of memory allocation that's been ignored by other posts. It's possible to statically allocate variables by declaring them outside any function. I think in general this type of allocation gets a bad rap because it's used by global variables. However there's nothing that says the only way to use memory allocated this way is as an undisciplined global variable in a mess of spaghetti code. The static allocation method can be used simply to avoid some of the pitfalls of the heap and automatic allocation methods. Some C programmers are surprised to learn that large and sophisticated C embedded and games programs have been constructed with no use of heap allocation at all.
There are some great answers here about how to allocate and free memory, and in my opinion the more challenging side of using C is ensuring that the only memory you use is memory you've allocated - if this isn't done correctly what you end up with is the cousin of this site - a buffer overflow - and you may be overwriting memory that's being used by another application, with very unpredictable results.
An example:
int main() {
char* myString = (char*)malloc(5*sizeof(char));
myString = "abcd";
}
At this point you've allocated 5 bytes for myString and filled it with "abcd\0" (strings end in a null - \0).
If your string allocation was
myString = "abcde";
You would be assigning "abcde" in the 5 bytes you've had allocated to your program, and the trailing null character would be put at the end of this - a part of memory that hasn't been allocated for your use and could be free, but could equally be being used by another application - This is the critical part of memory management, where a mistake will have unpredictable (and sometimes unrepeatable) consequences.
A thing to remember is to always initialize your pointers to NULL, since an uninitialized pointer may contain a pseudorandom valid memory address which can make pointer errors go ahead silently. By enforcing a pointer to be initialized with NULL, you can always catch if you are using this pointer without initializing it. The reason is that operating systems "wire" the virtual address 0x00000000 to general protection exceptions to trap null pointer usage.
Also you might want to use dynamic memory allocation when you need to define a huge array, say int[10000]. You can't just put it in stack because then, hm... you'll get a stack overflow.
Another good example would be an implementation of a data structure, say linked list or binary tree. I don't have a sample code to paste here but you can google it easily.
(I'm writing because I feel the answers so far aren't quite on the mark.)
The reason you have to memory management worth mentioning is when you have a problem / solution that requires you to create complex structures. (If your programs crash if you allocate to much space on the stack at once, that's a bug.) Typically, the first data structure you'll need to learn is some kind of list. Here's a single linked one, off the top of my head:
typedef struct listelem { struct listelem *next; void *data;} listelem;
listelem * create(void * data)
{
listelem *p = calloc(1, sizeof(listelem));
if(p) p->data = data;
return p;
}
listelem * delete(listelem * p)
{
listelem next = p->next;
free(p);
return next;
}
void deleteall(listelem * p)
{
while(p) p = delete(p);
}
void foreach(listelem * p, void (*fun)(void *data) )
{
for( ; p != NULL; p = p->next) fun(p->data);
}
listelem * merge(listelem *p, listelem *q)
{
while(p != NULL && p->next != NULL) p = p->next;
if(p) {
p->next = q;
return p;
} else
return q;
}
Naturally, you'd like a few other functions, but basically, this is what you need memory management for. I should point out that there are a number tricks that are possible with "manual" memory management, e.g.,
Using the fact that malloc is guaranteed (by the language standard) to return a pointer divisible by 4,
allocating extra space for some sinister purpose of your own,
creating memory pools..
Get a good debugger... Good luck!
#Euro Micelli
One negative to add is that pointers to the stack are no longer valid when the function returns, so you cannot return a pointer to a stack variable from a function. This is a common error and a major reason why you can't get by with just stack variables. If your function needs to return a pointer, then you have to malloc and deal with memory management.
#Ted Percival:
...you don't need to cast malloc()'s return value.
You are correct, of course. I believe that has always been true, although I don't have a copy of K&R to check.
I don't like a lot of the implicit conversions in C, so I tend to use casts to make "magic" more visible. Sometimes it helps readability, sometimes it doesn't, and sometimes it causes a silent bug to be caught by the compiler. Still, I don't have a strong opinion about this, one way or another.
This is especially likely if your compiler understands C++-style comments.
Yeah... you caught me there. I spend a lot more time in C++ than C. Thanks for noticing that.
In C, you actually have two different choices. One, you can let the system manage the memory for you. Alternatively, you can do that by yourself. Generally, you would want to stick to the former as long as possible. However, auto-managed memory in C is extremely limited and you will need to manually manage the memory in many cases, such as:
a. You want the variable to outlive the functions, and you don't want to have global variable. ex:
struct pair{
int val;
struct pair *next;
}
struct pair* new_pair(int val){
struct pair* np = malloc(sizeof(struct pair));
np->val = val;
np->next = NULL;
return np;
}
b. you want to have dynamically allocated memory. Most common example is array without fixed length:
int *my_special_array;
my_special_array = malloc(sizeof(int) * number_of_element);
for(i=0; i
c. You want to do something REALLY dirty. For example, I would want a struct to represent many kind of data and I don't like union (union looks soooo messy):
struct data{
int data_type;
long data_in_mem;
};
struct animal{/*something*/};
struct person{/*some other thing*/};
struct animal* read_animal();
struct person* read_person();
/*In main*/
struct data sample;
sampe.data_type = input_type;
switch(input_type){
case DATA_PERSON:
sample.data_in_mem = read_person();
break;
case DATA_ANIMAL:
sample.data_in_mem = read_animal();
default:
printf("Oh hoh! I warn you, that again and I will seg fault your OS");
}
See, a long value is enough to hold ANYTHING. Just remember to free it, or you WILL regret. This is among my favorite tricks to have fun in C :D.
However, generally, you would want to stay away from your favorite tricks (T___T). You WILL break your OS, sooner or later, if you use them too often. As long as you don't use *alloc and free, it is safe to say that you are still virgin, and that the code still looks nice.
Sure. If you create an object that exists outside of the scope you use it in. Here is a contrived example (bear in mind my syntax will be off; my C is rusty, but this example will still illustrate the concept):
class MyClass
{
SomeOtherClass *myObject;
public MyClass()
{
//The object is created when the class is constructed
myObject = (SomeOtherClass*)malloc(sizeof(myObject));
}
public ~MyClass()
{
//The class is destructed
//If you don't free the object here, you leak memory
free(myObject);
}
public void SomeMemberFunction()
{
//Some use of the object
myObject->SomeOperation();
}
};
In this example, I'm using an object of type SomeOtherClass during the lifetime of MyClass. The SomeOtherClass object is used in several functions, so I've dynamically allocated the memory: the SomeOtherClass object is created when MyClass is created, used several times over the life of the object, and then freed once MyClass is freed.
Obviously if this were real code, there would be no reason (aside from possibly stack memory consumption) to create myObject in this way, but this type of object creation/destruction becomes useful when you have a lot of objects, and want to finely control when they are created and destroyed (so that your application doesn't suck up 1GB of RAM for its entire lifetime, for example), and in a Windowed environment, this is pretty much mandatory, as objects that you create (buttons, say), need to exist well outside of any particular function's (or even class') scope.