Will malloc implementations return free-ed memory back to the system?

Most implementations don't bother identifying those (relatively rare) cases where entire "blocks" (of whatever size suits the OS) have been freed and could be returned, but there are of course exceptions. For example, and I quote from the wikipedia page, in OpenBSD:

On a call to free, memory is released and unmapped from the process address space using munmap. This system is designed to improve security by taking advantage of the address space layout randomization and gap page features implemented as part of OpenBSD's mmap system call, and to detect use-after-free bugs—as a large memory allocation is completely unmapped after it is freed, further use causes a segmentation fault and termination of the program.

Most systems are not as security-focused as OpenBSD, though.

Knowing this, when I'm coding a long-running system that has a known-to-be-transitory requirement for a large amount of memory, I always try to fork the process: the parent then just waits for results from the child [[typically on a pipe]], the child does the computation (including memory allocation), returns the results [[on said pipe]], then terminates. This way, my long-running process won't be uselessly hogging memory during the long times between occasional "spikes" in its demand for memory. Other alternative strategies include switching to a custom memory allocator for such special requirements (C++ makes it reasonably easy, though languages with virtual machines underneath such as Java and Python typically don't).

The following analysis applies only to glibc (based on the ptmalloc2 algorithm). There are certain options that seem helpful to return the freed memory back to the system:

1. mallopt() (defined in malloc.h) does provide an option to set the trim threshold value using one of the parameter option M_TRIM_THRESHOLD, this indicates the minimum amount of free memory (in bytes) allowed at the top of the data segment. If the amount falls below this threshold, glibc invokes brk() to give back memory to the kernel.

   The default value of M_TRIM_THRESHOLD in Linux is set to 128K, setting a smaller value might save space.

   The same behavior could be achieved by setting trim threshold value in the environment variable MALLOC_TRIM_THRESHOLD_, with no source changes absolutely.

   However, preliminary test programs run using M_TRIM_THRESHOLD has shown that even though the memory allocated by malloc does return to the system, the remaining portion of the actual chunk of memory (the arena) initially requested via brk() tends to be retained.

2. It is possible to trim the memory arena and give any unused memory back to the system by calling malloc_trim(pad) (defined in malloc.h). This function resizes the data segment, leaving at least pad bytes at the end of it and failing if less than one page worth of bytes can be freed. Segment size is always a multiple of one page, which is 4,096 bytes on i386.

   The implementation of this modified behavior of free() using malloc_trim could be done using the malloc hook functionality. This would not require any source code changes to the core glibc library.

3. Using madvise() system call inside the free implementation of glibc.