全局声明是在栈中还是堆中？

Question

I need to declare a variable that is not in the stack memory but in the heap, like below

struct mystruct *name;

Should I declare it outside all functions (even outside main()) at the begin of file?

Answer 1

Please note that a variable is never declared "on the heap". Only memory pointed to by a pointer can be allocated on (from) the heap.

In your example, you can declare name indeed outside any function and then it will exist in global memory. You can also delcare the variable inside a function, preceded by the keyword static . This latter will allocate the variable also in global memory, but it will only be visible in the function where you declared it.

To use your pointer variable, you now must alocate memory for it to point to, which you allocate on the heap using malloc .

Answer 2

TL;DR Version

You cannot declare a variable such that the variable itself lives on the heap.

James Michener Version

The C language definition doesn't talk about stacks or heaps; it talks about storage durations .

Objects with auto storage duration (anything declared within a block and without the static keyword) have lifetimes that extend from the beginning of that block and end when the block exits ¹ :

void foo( void )
{
  int a = 0;                        // lifetime of a extends to the end of
                                    // the function
  for ( int i = 0; i < 10; i++ )    // lifetime of i and b extend to the end
  {                                 // of the for loop
    int b = a + i;    
    printf( "b = %d\n", b );     
  }
}

Most implementations allocate storage for auto objects from the hardware stack, because a stack makes that behavior easy to implement ² .

Objects with static storage duration (anything declared outside of a function or with the static keyword) have lifetimes that extend from the time the program is loaded into memory until the program exits:

int a = 0;                   // lifetime of a extends over the lifetime of
                             // the entire program
int main( void )
{
  static int b = 10;         // lifetime of b also extends over the lifetime
                             // of the program, but is only visible within
                             // main
  ...
}

Most implementations will set aside storage for static objects within the body of the executable itself (for an executable using the ELF format, such objects will be stored in the .bss , .data , or .rodata sections of the image).

Objects with allocated storage duration (anything allocated with malloc , calloc , or realloc ) have lifetimes that extend from the time that they are allocated until they are explicitly deallocated with a call to free .

int *foo( size_t size )
{
  int *ptr = malloc( sizeof *ptr * size );  
  return ptr;                               
}

void bar( void )
{
  int *p = foo( 10 );                       
  // do something with p
  free( p );
}

The variables ptr and p only exist for the lifetimes of their respective functions, and they will be typically allocated from the stack. The object both variables point to exists from the time it is allocated with malloc until it is deallocated with free .

Most implementations allocate storage for allocated objects from the heap.

There's really no way for you to declare an object that has allocated storage duration; the only way you can create such an object is via malloc , calloc , or realloc . Whatever object you declare to store the pointer value returned by any of those functions will have either auto or static storage duration.

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<sup>1. In practice, storage for all local objects is allocated at function entry and released at function exit, regardless of whether the object's lifetime is over the entire function or limited to a block within the function. However, you should never rely on that storage being accessible outside of that object's lifetime. For example, the lifetimes of i and b are limited to the for loop; even though the storage for each may have been allocated at function entry, you should not attempt to access that storage outside of the loop body.

2. C was designed on a machine with a stack, after all.</sup>