Comment by jvanderbot
2 days ago
I'll narrow my scope more explicitly:
close(x) is not memory management - not at the user level. This should be done.
free(p) has no O/S side effects like this in C - this can be not-done if you don't malloc all your memory.
You can get away with not de-allocating program memory, but (as mentioned), that has nothing to do with freeing Os/ kernel / networking resources in C.
Most kernel resources are fairly well behaved, as they will automatically decrement their refcount when a process exits. Even mutexes have a "robust" flag for this exact reason. Programs which rely on destructors or any other form or orderly exit are always brittle and should be rewritten to use atomic operations.
Which kernel, on which specific OS?
This is a very non portable assumption, even we constrain it to only across UNIX/POSIX flavours.
As far as assumptions go, it's actually one of the most portable ones and for a good reason, considering it is a basic part of building a reliable system. Quoting POSIX:
Consequences of Process Termination
Process termination caused by any reason shall have the following consequences:
[..] All of the file descriptors, directory streams, conversion descriptors, and message catalog descriptors open in the calling process shall be closed.
[..] Each attached shared-memory segment is detached and the value of shm_nattch (see shmget()) in the data structure associated with its shared memory ID shall be decremented by 1.
For each semaphore for which the calling process has set a semadj value (see semop()), that value shall be added to the semval of the specified semaphore.
[..] If the process is a controlling process, the controlling terminal associated with the session shall be disassociated from the session, allowing it to be acquired by a new controlling process.
[..] All open named semaphores in the calling process shall be closed as if by appropriate calls to sem_close().
Any memory locks established by the process via calls to mlockall() or mlock() shall be removed. If locked pages in the address space of the calling process are also mapped into the address spaces of other processes and are locked by those processes, the locks established by the other processes shall be unaffected by the call by this process to _Exit() or _exit().
Memory mappings that were created in the process shall be unmapped before the process is destroyed.
Any blocks of typed memory that were mapped in the calling process shall be unmapped, as if munmap() was implicitly called to unmap them.
All open message queue descriptors in the calling process shall be closed as if by appropriate calls to mq_close().