1. brk:是系统调用接口：
内核的syscall_table.s 中定义：
ENTRY_SAME(brk)
例如：
在malloc函数实现中有调用brk
malloc 函数调用链中：
malloc-->expand_heap-->__expand_heap
即在__expand_heap调用系统brk函数：
__syscall(SYS_brk, 0);
注意：malloc中，当if (n > MMAP_THRESHOLD) 一般是128K，使用mmap分配内存了，否则使用brk分配内存。
2. sbrk:是glibc封装的一个应用函数，sbrk其实是调用系统接口brk：
glibc的sbrk.c文件中：
void *sbrk(intptr_t inc)
{
if (inc) return (void *)__syscall_ret(-ENOMEM);
return (void *)__syscall(SYS_brk, 0);
}
3. 附上__expand_heap和__simple_malloc简短代码实现：
/* Expand the heap in-place if brk can be used, or otherwise via mmap,
* using an exponential lower bound on growth by mmap to make
* fragmentation asymptotically irrelevant. The size argument is both
* an input and an output, since the caller needs to know the size
* allocated, which will be larger than requested due to page alignment
* and mmap minimum size rules. The caller is responsible for locking
* to prevent concurrent calls. */
static void *__expand_heap(size_t *pn)
{
static uintptr_t brk;
static unsigned mmap_step;
size_t n = *pn;
if (n > SIZE_MAX/2 - PAGE_SIZE) {
errno = ENOMEM;
return 0;
}
n += -n & PAGE_SIZE-1;
if (!brk) {
brk = __syscall(SYS_brk, 0);
brk += -brk & PAGE_SIZE-1;
}
if (n < SIZE_MAX-brk && !traverses_stack_p(brk, brk+n)
&& __syscall(SYS_brk, brk+n)==brk+n) {
*pn = n;
brk += n;
return (void *)(brk-n);
}
size_t min = (size_t)PAGE_SIZE << mmap_step/2;
if (n < min) n = min;
void *area = __mmap(0, n, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
if (area == MAP_FAILED) return 0;
*pn = n;
mmap_step++;
return area;
}
-----------------------------------------------------------------------------
static void *__simple_malloc(size_t n)
{
static uintptr_t brk, cur, end;
static unsigned mmap_step;
size_t align=1;
void *p;
if (n > SIZE_MAX/2) {
errno = ENOMEM;
return 0;
}
if (!n) n++;
while (align<n && align<ALIGN)
align += align;
LOCK(lock);
cur += -cur & align-1;
if (n > end-cur) {
size_t req = n - (end-cur) + PAGE_SIZE-1 & -PAGE_SIZE;
if (!cur) {
brk = __syscall(SYS_brk, 0);
brk += -brk & PAGE_SIZE-1;
cur = end = brk;
}
if (brk == end && req < SIZE_MAX-brk
&& !traverses_stack_p(brk, brk+req)
&& __syscall(SYS_brk, brk+req)==brk+req) {
brk = end += req;
} else {
int new_area = 0;
req = n + PAGE_SIZE-1 & -PAGE_SIZE;
/* Only make a new area rather than individual mmap
* if wasted space would be over 1/8 of the map. */
if (req-n > req/8) {
/* Geometric area size growth up to 64 pages,
* bounding waste by 1/8 of the area. */
size_t min = PAGE_SIZE<<(mmap_step/2);
if (min-n > end-cur) {
if (req < min) {
req = min;
if (mmap_step < 12)
mmap_step++;
}
new_area = 1;
}
}
void *mem = __mmap(0, req, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
if (mem == MAP_FAILED || !new_area) {
UNLOCK(lock);
return mem==MAP_FAILED ? 0 : mem;
}
cur = (uintptr_t)mem;
end = cur + req;
}
}
p = (void *)cur;
cur += n;
UNLOCK(lock);
return p;
}