记得好早之前刚刚接触 Linux 下 C/C++ 开发的时候就有个疑问：在程序内 free 掉的空间没有立刻交换给操作系统，程序进程的内存没有减少。后来一度在开发各种长连接程序时 GOOGLE 各种资料，最近又偶尔遇到，借此机会把相关的说法整理如下。

其中两个重点内容如下：

1. Freeing Memory Allocated with malloc： 注意这里的 “Occasionally” 用词；

   Occasionally, free can actually return memory to the operating system and make the process smaller. Usually, all it can do is allow a later call to malloc to reuse the space. In the meantime, the space remains in your program as part of a free-list used internally by malloc.

2. Linux Programmer’s Manual - mallopt： 注意默认配置 128*1024 数值；

   M_TRIM_THRESHOLD
   When the amount of contiguous free memory at the top of the heap grows sufficiently large, free(3) employs sbrk(2) to release this memory back to the system. (This can be useful in programs that continue to execute for a long period after freeing a significant amount of memory.) The M_TRIM_THRESHOLD parameter specifies the minimum size (in bytes) that this block of memory must reach before sbrk(2) is used to trim the heap.

   The default value for this parameter is 128*1024. Setting M_TRIM_THRESHOLD to -1 disables trimming completely.

   Modifying M_TRIM_THRESHOLD is a trade-off between increasing the number of system calls (when the parameter is set low) and wasting unused memory at the top of the heap (when the parameter is set high).

根据上面的资料，总结一下：

内存会在 free 后被后续的 malloc 复用，当 free 的栈顶连续内存达到 M_TRIM_THRESHOLD 描述的值（默认 128kB）后归还系统;