diff options
Diffstat (limited to 'src/libsystemd/sd-bus/PORTING-DBUS1')
| -rw-r--r-- | src/libsystemd/sd-bus/PORTING-DBUS1 | 40 |
1 files changed, 0 insertions, 40 deletions
diff --git a/src/libsystemd/sd-bus/PORTING-DBUS1 b/src/libsystemd/sd-bus/PORTING-DBUS1 index 6205e32736..9f0a91d695 100644 --- a/src/libsystemd/sd-bus/PORTING-DBUS1 +++ b/src/libsystemd/sd-bus/PORTING-DBUS1 @@ -362,46 +362,6 @@ ioctl()s are added for a single match strings. MEMFDS -The "memfd" concept is used for zero-copy data transfers (see -above). memfds are file descriptors to memory chunks of arbitrary -sizes. If you have a memfd you can mmap() it to get access to the data -it contains or write to it. They are comparable to file descriptors to -unlinked files on a tmpfs, or to anonymous memory that one may refer -to with an fd. They have one particular property: they can be -"sealed". A memfd that is "sealed" is protected from alteration. Only -memfds that are currently not mapped and to which a single fd refers -may be sealed (they may also be unsealed in that case). - -The concept of "sealing" makes memfds useful for using them as -transport for kdbus messages: only when the receiver knows that the -message it has received cannot change while looking at, it can safely -parse it without having to copy it to a safe memory area. memfds can also -be reused in multiple messages. A sender may send the same memfd to -multiple peers, and since it is sealed, it can be sure that the receiver -will not be able to modify it. "Sealing" hence provides both sides of -a transaction with the guarantee that the data stays constant and is -reusable. - -memfds are a generic concept that can be used outside of the immediate -kdbus usecase. You can send them across AF_UNIX sockets too, sealed or -unsealed. In kdbus themselves, they can be used to send zero-copy -payloads, but may also be sent as normal fds. - -memfds are allocated with the KDBUS_CMD_MEMFD_NEW ioctl. After allocation, -simply memory map them and write to them. To set their size, use -KDBUS_CMD_MEMFD_SIZE_SET. Note that memfds will be increased in size -automatically if you touch previously unallocated pages. However, the -size will only be increased in multiples of the page size in that -case. Thus, in almost all cases, an explicit KDBUS_CMD_MEMFD_SIZE_SET -is necessary, since it allows setting memfd sizes in finer -granularity. To seal a memfd use the KDBUS_CMD_MEMFD_SEAL_SET ioctl -call. It will only succeed if the caller has the only fd reference to -the memfd open, and if the memfd is currently unmapped. - -If memfds are shared, keep in mind that the file pointer used by -write/read/seek is shared too, only pread/pwrite are safe to use -in that case. - memfds may be sent across kdbus via KDBUS_ITEM_PAYLOAD_MEMFD items attached to messages. If this is done, the data included in the memfd is considered part of the payload stream of a message, and are treated |