If you have a struct in C like this:
struct mystruct{
int i;
char c;
};
and you do:
mystruct m;
m.i = 5;
m.c = 'i';
do you create the struct on the heap or on the stack ?
2 Answers
In the stack:
void func()
{
mystruct m;
...
}
// The address of 'm' is within the stack memory space
In the heap:
void func()
{
mystruct* m = malloc(sizeof(mystruct));
...
}
// The value of 'm' is an address within the heap memory space
In the data-section:
mystruct m;
static mystruct m;
void func()
{
static mystruct m;
...
}
// The address of 'm' is within the data-section memory space
In the code-section:
const mystruct m;
const static mystruct m;
void func()
{
const mystruct m;
...
}
void func()
{
const static mystruct m;
...
}
// The address of 'm' is within the code-section memory space
UPDATE:
Although not directly related to your question, please note that the above rule for const is not entirely accurate, as this keyword has in fact two purposes:
- Allocate a variable in the (read-only) code-section of the program.
- Prevent you (the programmer) from writing erroneous code, such as changing the value of a variable that you initially intended to keep constant throughout the execution of your program.
But feature #1 is really up to the compiler in use, which may place it elsewhere, depending on your project configuration. For example, sometimes you might want to declare a constant variable just for the sake of feature #2, while feature #1 is not feasible due to insufficient memory space in the code-section.
answered May 2, 2014 at 23:20
14 Comments
Crowman
const won't generally affect where the variables live. Take something like void func(char * p) { const char * cp = p; }, where cp will be - and would have to be - created on the stack like any other local variable.
barak manos
@Paul Griffiths: Sorry to disappoint you, but although variable
cp will point to an address on the stack, it will be placed in the code-section (i.e., the address of cp will be an address in the code-section).
user3386109
@Arun it still needs a
sizeof, even if you don't like the sizeof(mystruct) style.Crowman
@barak: Since functions are allowed to call themselves, this won't be true. That function might recursively call itself 20 times, in which case you'd need 21 separate copies of
cp, and the compiler cannot possibly know how many times it will be called.barak manos
@user3386109: Thank you for the correction; was not my idea to remove the
sizeof, and I fixed it as soon as I noticed it was edited by someone else. |
struct mystruct m;.