Direct allocation from typed memory

Directly allocating memory from a memory pool that was reserved by the startup program is safe, secure, and efficient.

You can use the SHMCTL_TYMEM flag for shm_ctl() to directly allocate memory from these pools. The steps are as follows:

1. Use posix_typed_mem_open() to open the memory pool.
2. Use shm_open() to create a shared memory object.
3. Use shm_ctl() to populate the object with memory from the pool.

// Open the graphics pool
int tymem_fd = posix_typed_mem_open("/graphics", O_RDWR,
                                    POSIX_TYPED_MEM_ALLOCATE);

// Create a shared memory object
int shm_fd = shm_open(SHM_ANON, O_RDWR, 0600);

// Populate with 1 MB of mostly-contiguous memory
shm_ctl(shm_fd, SHMCTL_ANON | SHMCTL_TYMEM, tymem_fd, 0x100000);

A few things to note:

• If you specify SHMCTL_TYMEM, you must also specify SHMCTL_ANON. When used without SHMCTL_PHYS, SHMCTL_ANON requests mostly contiguous memory.
• If you need contiguous memory, specify POSIX_TYPED_MEM_ALLOCATE_CONTIG when you call posix_typed_mem_open(). You must then specify SHMCTL_PHYS in addition to SHMCTL_ANON and SHMCTL_TYMEM when you call shm_ctl().
• If you specify SHMCTL_TYMEM, the paddr argument for shm_ctl() must be the the file descriptor that posix_typed_mem_open() gave you for the typed memory pool. Once you've called shm_ctl(), it isn't necessary to keep this file descriptor open.