Msys stands for “system mbufs” and is a set of API built on top of the mbuf code. The basic idea behind msys is the following. The developer can create different size mbuf pools and register them with msys. The application then allocates mbufs using the msys API (as opposed to the mbuf API). The msys code will choose the mbuf pool with the smallest mbufs that can accommodate the requested size.
Let us walk through an example where the user registers three mbuf pools with msys: one with 32 byte mbufs, one with 256 and one with 2048. If the user requests an mbuf with 10 bytes, the 32-byte mbuf pool is used. If the request is for 33 bytes the 256 byte mbuf pool is used. If an mbuf data size is requested that is larger than any of the pools (say, 4000 bytes) the largest pool is used. While this behaviour may not be optimal in all cases that is the currently implemented behaviour. All this means is that the user is not guaranteed that a single mbuf can hold the requested data.
The msys code will not allocate an mbuf from a larger pool if the chosen mbuf pool is empty. Similarly, the msys code will not chain together a number of smaller mbufs to accommodate the requested size. While this behaviour may change in future implementations the current code will simply return NULL. Using the above example, say the user requests 250 bytes. The msys code chooses the appropriate pool (i.e. the 256 byte mbuf pool) and attempts to allocate an mbuf from that pool. If that pool is empty, NULL is returned even though the 32 and 2048 byte pools are not empty.
Note that no added descriptions on how to use the msys API are presented here (other than in the API descriptions themselves) as the msys API is used in exactly the same manner as the mbuf API. The only difference is that mbuf pools are added to msys by calling os_msys_register().
The functions available in msys are:
| Function | Description |
|---|---|
| os_msys_get | Retrieve an mbuf from the system mbuf pools with the given number of bytes available in the mbuf. |
| os_msys_get_pkthdr | Retrieve a packet header mbuf from the system mbuf pools with the given number of bytes available for the user header in the mbuf. |
| os_msys_register | Register an mbuf pool for use as a system mbuf pool. |
| os_msys_reset | Resets msys module. |