Memory Allocation
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A process can allocate memory by increasing the size of the heap, a variable-size segment of contiguous virtual memory that begins just after the uninitialized data segment of a process and grows and shrinks as memory is allocated and freed. The current limit of the heap is referred to as the program break.
Resizing the heap (i.e., allocating or deallocating memory) is actually as simple as telling the kernel to adjust its idea of where the process's program break is. Initially, the program break lies just past the end of the .bss segment, as in the following diagram:
brk() and sbrk()Traditionally, the UNIX system has provided two syscalls for manipulating the program break, and these are both available on Linux: brk() and sbrk(). Although these syscalls are seldom used directly in programs, understanding them helps clarify how memory allocation works.
Key Ideas:
The brk() syscall sets the program break to the location specified by addr. Since virtual memory is allocated in units of pages, addr is effectively rounded to the next page boundary.
A call to sbrk() adjusts the program break by adding increament to it. On Linux, sbrk() is a library function implemented on top of brk(). On success, sbrk() returns the previous address of the program break. In other word, if we have increased the program break, the return value points to the start of the newly allocated block of memory.
A call to sbrk(0) returns the current setting of the program break without changing it.
malloc() and free()In general, C programs use the malloc family of functions to allocate and deallocate memory on the heap. These functions offer several advantages over brk() and sbrk(). In particular:
They are standardized as part of the C language
They are easier to use in threaded programs
They provide a simple interface that allows memory to be allocated in small units
They allow us to arbitrarily deallocate blocks of memory, which are maintained on a free list and recycled in future calls to allocate memory.
Key Ideas:
The malloc(size) function allocate size bytes from the heap and returns a pointer to the start of the newly allocated block of memory. The allocated memory is not initialized.
Because malloc() returns void *, we can assign it to any type of C pointers. The block of memory returned by malloc() is always aligned on a byte boundary suitable for efficient access to any type of C data structure. In practice, this means that it is allocated on an 8-byte or 16-byte boundary on most architectures.
malloc(0) returns a pointer to a small piece of memory that can (and should) be freed with free().
The free() function deallocates the block of memory pointed to by its ptr argument, which should be an address previously returned by malloc().
In general, free() doesn't lower the program break, but instead adds the block of memory to a list of free blocks that are recycled by future calls to malloc(). This is done for several reasons:
The block of memory being freed is typically somewhere in the middle of the heap, rather than at the end, so that lowering the program break is not possible.
It minimizes the number of sbrk() calls that the program must perform. Syscalls have a small but significant overhead.
In many cases, lowering the break would not help programs that allocate large amounts of memory, since they typically tend to hold on to allocated memory or repeatedly release and reallocate memory, rather than release it all and then continue to run for an extended period of time.
malloc() and free()Although malloc() and free() provide an interface for allocating memory that is much easier to use than brk() and sbrk(), it is still possible to make various programming errors when using them. Understanding how malloc() and free() are implemented provides us with insights into the causes of these errors and how we can avoid them.
The implementation of malloc() is straightforward. It first scans the list of memory blocks previously released by free() in order to find one whose size is larger than or equal to its requirements. Different strategies may be employed for this scan, depending on the implementation; for example, first-fit or best-fit. If the block is exactly the right size, then it is returned to the caller. If it is larger, then it is split, so that a block of the correct size is returned to the caller and a smaller free block is left on the free list.
If no block on the free list is large enough, then malloc() calls sbrk() to allocate more memory. To reduce the number of calls to sbrk(), rather than allocating exactly the number of bytes required, malloc() increases the program break in larger units (some multiple of the virtual memory page size), putting the excess memory onto the free list.
Looking at the implementation of free(), things start to become more interesting. When free() places a block of memory onto the free list, how does it know what size that block is? This is done via a trick. When malloc() allocates the block, it allocates extra bytes to hold an integer containing the size of the block. This integer is located at the beginning of the block; the address actually returned to the caller points to the location just past this length value, as shown in the following diagram:
When a block is placed on the (doubly linked) free list, free() uses the bytes of the block itself in order to add the block to the list, as shown in the following diagram:
As blocks are deallocated and reallocated over time, the blocks of the free list will become intermingled with blocks of allocated, in-use memory, as shown in the following diagram:
Now consider the fact that C allows us to create pointers to any location in the heap, and modify the locations they point to, including the length, previous free block, and next free block pointers maintained by free() and malloc(). Add this to the preceding description, and we have a fairly combustible mix when it comes to creating obscure programming bugs. For example, if, via a misdirected pointer, we accidentally increase one of the length values preceding an allocated block of memory (metadata corruption), and subsequently deallocate that block, then free() will record the wrong size block of memory on the free list. Subsequently, malloc() may reallocate this block, leading to a scenario where the program has pointers to two blocks of allocated memory that it understands to be distinct, but which actually overlap. Numerous other pictures of what could go wrong can be drawn.
After we allocate a block of memory, we should be careful not to touch any bytes outside the range of that block. This could occur, for example, as a result of faulty pointer arithmetic or off-by-one errors in loops updating the contents of a block.
It is an error to free the same piece of allocated memory more than once. With glibc on Linux, we often get a segmentation violation (SIGSEGV signal). This is good, because it alerts us that we’ve made a programming error. However, more generally, freeing the same memory twice leads to unpredictable behavior (double free).
We should never call free() with a pointer value that wasn’t obtained by a call to one of the functions in the malloc package.
If we are writing a long-running program (e.g., a shell or a network daemon process) that repeatedly allocates memory for various purposes, then we should ensure that we deallocate any memory after we have finished using it. Failure to do so means that the heap will steadily grow until we reach the limits of available virtual memory, at which point further attempts to allocate memory fail. Such a condition is known as a memory leak.
