There is a special designated anonymous private memory mapping (traditionally located just beyond the data/bss, but modern Linux will actually adjust the location with ASLR). In principle it's no better than any other mapping you could create with mmap, but Linux has some optimizations that make it possible to expand the end of this mapping (using the brk syscall) upwards with reduced locking cost relative to what mmap or mremap would incur. This makes it attractive for malloc implementations to use when implementing the main heap.

我可以回答你的第二个问题。Malloc将失败并返回一个空指针。这就是为什么在动态分配内存时总是检查空指针。

您可以自己使用brk和sbrk来避免每个人都在抱怨的“malloc开销”。但是您不能轻易地将此方法与malloc结合使用，因此它仅适用于不需要释放任何东西的情况。因为你不能。另外，应该避免任何可能在内部使用malloc的库调用。Ie。斯特伦可能安全，但fopen可能就不安全了。

调用sbrk就像调用malloc一样。它返回一个指向当前断点的指针，并使该断点增加该值。

void *myallocate(int n){
    return sbrk(n);
}

虽然不能释放单个的分配(因为没有malloc开销，记住)，但是可以通过使用第一次调用sbrk返回的值调用brk来释放整个空间，从而使brk返回。

void *memorypool;
void initmemorypool(void){
    memorypool = sbrk(0);
}
void resetmemorypool(void){
    brk(memorypool);
}

您甚至可以堆叠这些区域，通过将换行符倒回到区域的开始位置来丢弃最近的区域。

还有一件事……

SBRK在code golf中也很有用，因为它比malloc短2个字符。

堆放在程序数据段的最后。Brk()用于改变(扩展)堆的大小。当堆不能再增长时，任何malloc调用都将失败。

数据段是内存中保存所有静态数据的部分，在启动时从可执行文件读取，通常为零填充。

There is a special designated anonymous private memory mapping (traditionally located just beyond the data/bss, but modern Linux will actually adjust the location with ASLR). In principle it's no better than any other mapping you could create with mmap, but Linux has some optimizations that make it possible to expand the end of this mapping (using the brk syscall) upwards with reduced locking cost relative to what mmap or mremap would incur. This makes it attractive for malloc implementations to use when implementing the main heap.