sd_bus_message_get_cookie() returns the transaction cookie of a
message. The cookie uniquely identifies a message within each bus
peer, but is not globally unique. It is assigned when a message is
sd_bus_message_get_reply_cookie() returns the transaction cookie of
the message the specified message is a response to. When a reply
message is generated for a method call message, its cookie is copied
over into this field. Note that while every message that is
transferred is identified by a cookie, only response messages carry a
reply cookie field.
Both functions take a message object as first parameter and a place
to store the 64-bit cookie in.
On success, these calls return 0 or a positive integer. On failure,
these calls return a negative errno-style error code.
On success, the cookie/reply cookie is returned in the specified
64-bit unsigned integer variable.
Returned errors may indicate the following problems:
A specified parameter is invalid.
No cookie has been assigned to this message. This either
indicates that the message has not been sent yet and hence has no
cookie assigned, or that the message is not a method response
message and hence carries a reply cookie field.
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systemd 233 SD_BUS_MESSAGE_GET_COOKIE(3)