How does sbrk() work in C++?

问题:

Where can I read about sbrk() in some detail?

How does it exactly work?

In what situations would I want to use sbrk() instead of the cumbersome malloc() and new()?

btw, what is the expansion for sbrk()?

回答1:

Have a look at the specification for brk/sbrk.

The call basically asks the OS to allocate some more memory for the application by incrementing the previous "break value" by a certain amount. This amount (the first parameter) is the amount of extra memory your application then gets.

Most rudimentary malloc implementations build upon the sbrk system call to get blocks of memory that they split up and track. The mmap function is generally accepted as a better choice (which is why mallocs like dlmalloc support both with an #ifdef).

As for "how it works", an sbrk at its most simplest level could look something like this:

uintptr_t current_break; // Some global variable for your application.
                         // This would probably be properly tracked by the OS for the process
void *sbrk(intptr_t incr)
{
    uintptr_t old_break = current_break;
    current_break += incr;
    return (void*) old_break;
}

Modern operating systems would do far more, such as map pages into the address space and add tracking information for each block of memory allocated.

回答2:

sbrk is pretty much obsolete, these days you'd use mmap to map some pages out of /dev/zero. It certainly isn't something you use instead of malloc and friends, it's more a way of implementing those. Also, of course, it exists only on posix-based operating systems that care about backwards compatibility to ancient code.

If you find Malloc and New too cumbersome, you should look into garbage collection instead... but beware, there is a potential performance cost to that, so you need to understand what you are doing.

回答3:

You never want to use sbrk instead of malloc or free. It is non-portable and is typically used only by implementers of the standard C library or in cases where it's not available. It's described pretty well in its man page:

Description

brk() sets the end of the data segment to the value specified by end_data_segment, when that value is reasonable, the system does have enough memory and the process does not exceed its max data size (see setrlimit(2)).

sbrk() increments the program's data space by increment bytes. sbrk() isn't a system call, it is just a C library wrapper. Calling sbrk() with an increment of 0 can be used to find the current location of the program break.

Return Value

On success, brk() returns zero, and sbrk() returns a pointer to the start of the new area. On error, -1 is returned, and errno is set to ENOMEM.

Finally,malloc and free are not cumbersome - they are the standard way to allocate and release memory in C. Even if you want to implement your own memory allocator, it's best to just use malloc and free as the basis - a common approach is to allocate a large chunk at a time with malloc and provide memory allocation from it (this is what suballocators, or pools, usually implement)

---

Re the origin of the name sbrk (or its cousin brk), it may have something to do with the fact that the end of the heap is marked by a pointer known as the "break". The heap starts right after the BSS segments and typically grows up towards the stack.

回答4:

You've tagged this C++ so why would you use 'cumbersome' malloc() rather than new? I am not sure what is cumbersome about malloc in any case; internally maybe so, but why would you care? And if you did care (for reasons of determinism for example), you could allocate a large pool and implement your own allocator for that pool. In C++ of course you can overload the new operator to do that.

sbrk is used to glue the C library to the underlying system's OS memory management. So make OS calls rather than using sbrk(). As to how it works, that is system dependent. If for example you are using the Newlib C library (commonly used on 'bare-metal' embedded systems with the GNU compiler), you have to implement sbrk yourself, so how it works in those circumstances is up to you so long as it achieves its required behaviour of extending the heap or failing.

As you can see from the link, it does not do much and would be extremely cumbersome to use directly - you'd probably end-up wrapping it in all the functionality that malloc and new provide in any case.

回答5:

This depends on what you mean by malloc being "Cumbersome". sbrk is typically not used directly anymore, unless you're implementing your own memory allocator: IE, operator overriding "new". Even then I'd possibly use malloc to give me my initial memory.

If you'd like to see how to to implement malloc() on top of sbrk(), check out http://web.ics.purdue.edu/~cs354/labs/lab6/ which is an exercise going through that.

On a modern system you shouldn't touch this interface, though. Since you're calling malloc and new cumbersome, I suspect you don't have all the requisite experience to safely and properly use sbrk for your code.