C：包含动态分配的成员的结构的范围？

Question

I have a struct and a function that returns an instance of Foo defined as follows:

struct Foo
{
    int a;
    int* b;     
};

struct Foo makeFoo(int a, int bSize)
{
    struct Foo foo;
    foo.a = a;
    foo.b = malloc(sizeof(int) * bSize);
    for (int i = 0; i < bSize; ++i)
        foo.b[i] = i;

    return foo;
}

Initially, I thought foo is a local variable and it'll be gone when makeFoo returns, but from this question Is it safe to return a struct in C or C++? , I know that it's safe to do so.

Now my question is when will memory for foo be collected? Do I have to free its b member first?

Let's say I use makeFoo like this:

void barFunc()
{
    struct Foo foo = makeFoo(3, 10);

    printf("Foo.a = %d;\nFoo.b = [", foo.a);
    for (int i = 0; i < 10; ++i)
        printf("%d, ", foo.b[i]);

    printf("\n");
}

void main(int argc, char* argv)
{
    barFunc();  
}

When barFunc returns and I'm back in main , is memory for foo collected yet? Or do I have to call free(foo.b) at the end of barFunc ?

Answer 1

Initially, I thought foo is a local variable and it'll be gone when makeFoo returns, but from this question Is it safe to return a struct in C or C++?, I know that it's safe to do so.

You do realize the local foo indeed is gone? returning a struct by value as you do here just copies its contents to the instance provided by the caller.(*)

But, of course, the contents include a pointer pointing to some memory you allocated with malloc() . So it has to be free() d later.

(*) This can be a good idea for small structs, depending on your needs, but always keep in mind the whole contents are copied -- it's definitely not what you want for a somehow large struct.

Answer 2

You have to call free(foo.b) by yourself before the foo variable in barFunc goes out of scope. This is not necessary in the makeFoo function because foo and the pointer to the allocated memory is copied to the caller, so it's all right.

Since you have a makeFoo function, it would be good practice to have a deleteFoo function too:

void deleteFoo(struct Foo *foo)
{
  free(foo->b);
}

void barFunc()
{
  struct Foo foo = makeFoo(3, 10);
  ...
  deleteFoo(&foo);
}

Answer 3

Yes it Is it safe to return a struct from a C/C++ function, but struct with large static data as struct fooo{int a[1024*1024*1024];}; can cause program to crash. so it is much safer to return a pointer to an allocated block of memory for similar structs.