In standard malloc implementations, when you use the malloc function to allocate memory, the memory manager typically allocates a large block of memory from the operating system and then subdivides it into smaller chunks to fulfill the program's memory requests. When you use the free function to deallocate memory, these memory blocks are usually marked as free and can be reused by subsequent malloc calls, though they are not always immediately returned to the operating system.
malloc commonly employs memory allocation algorithms (such as segregated fit, first-fit, and best-fit) to manage memory. These algorithms help minimize memory fragmentation and enhance memory utilization efficiency. However, they typically operate within their internal memory pool rather than frequently returning memory to the operating system.
In certain cases, specific malloc implementations may support returning large unused memory blocks back to the operating system. This is often achieved through system calls like sbrk, mmap/munmap, or similar mechanisms. A common optimization technique involves returning a large contiguous memory region to the operating system when it becomes available, thereby reducing the program's overall memory footprint.
For instance, glibc's malloc (using ptmalloc) sometimes utilizes mmap to allocate large memory regions and returns them to the operating system via munmap when no longer needed. However, this typically applies only to relatively large allocations, while smaller allocations remain in the process's memory pool for reuse.
In summary, standard malloc implementations do not necessarily return free memory back to the system; this depends on the memory allocation strategy and implementation details. Returning memory is generally reserved for large allocations, whereas smaller memory blocks typically stay within the process's memory pool for future use.