Heap Memory in C Programming

Question

What exactly is heap memory?

Whenever a call to malloc is made, memory is assigned from something called as heap. Where exactly is heap. I know that a program in main memory is divided into instruction segment where program statements are presents, Data segment where global data resides and stack segment where local variables and corresponding function parameters are stored. Now, what about heap?

Answer 1

Reading through this, this is actually beyond the realms of C. C doesn't specify that there's a heap behind malloc; it could just as easily be called a linked list; you're just calling it a heap by convention.

What the standard guarantees is that malloc will either return a pointer to an object that has dynamic storage duration, and your heap is just one type of data structure which facilitates the provision of such a storage duration. It's the common choice. Nonetheless, the very developers who wrote your heap have recognised that it might not be a heap, and so you'll see no reference of the term heap in the POSIX malloc manual for example.

Other things that are beyond the realms of standard C include such details of the machine code binary which is no longer C source code following compilation. The layout details, though typical, are all implementation-specific as opposed to C-specific.

The heap, or whichever book-keeping data structure is used to account for allocations, is generated during runtime; as malloc is called, new entries are (presumably) added to it and as free is called, new entries are (again, presumably) removed from it.

As a result, there's generally no need to have a section in the machine code binary for objects allocated using malloc, however there are cases where applications are shipped standalone baked into microprocessors, and in some of these cases you might find that flash or otherwise non-volatile memory might be reserved for that use.

Answer 2

The heap is the diametrical opposite of the stack. The heap is a large pool of memory that can be used dynamically – it is also known as the “free store”. This is memory that is not automatically managed – you have to explicitly allocate (using functions such as malloc), and deallocate (e.g. free) the memory. Failure to free the memory when you are finished with it will result in what is known as a memory leak – memory that is still “being used”, and not available to other processes. Unlike the stack, there are generally no restrictions on the size of the heap (or the variables it creates), other than the physical size of memory in the machine. Variables created on the heap are accessible anywhere in the program.

Oh, and heap memory requires you to use pointers.

A summary of the heap:

• the heap is managed by the programmer, the ability to modify it is somewhat boundless
• in C, variables are allocated and freed using functions like malloc() and free()
• the heap is large, and is usually limited by the physical memory available
• the heap requires pointers to access it

credit to craftofcoding

Answer 3

The heap is part of your process's address space. The heap can be grown or shrunk; you manipulate it by calling brk(2) or sbrk(2). This is in fact what malloc(3) does.

Allocating from the heap is more convenient than allocating memory on the stack because it persists after the calling routine returns; thus, you can call a routine, say funcA(), to allocate a bunch of memory and fill it with something; that memory will still be valid after funcA() returns. If funcA() allocates a local array (on the stack) then when funcA() returns, the on-stack array is gone.

A drawback of using the heap is that if you forget to release heap-allocated memory, you may exhaust it. The failure to release heap-allocated memory (e.g., failing to free() memory gotten from malloc()) is sometimes called a memory leak.

Another nice feature of the heap, vs. just allocating a local array/struct/whatever on the stack, is that you get a return value saying whether your allocation succeeded; if you try to allocate a local array on the stack and you run out, you don't get an error code; typically your thread will simply be aborted.

Answer 4

Basically, after memory is consumed by the needs of programs, what is left is the heap. In C that will be the memory available for the computer, for virtual machines it will be less than that.

But, this is the memory that can be used at run-time as your program needs memory dynamically.

You may want to look at this for more info:

http://computer.howstuffworks.com/c28.htm