int __pmLookupIPC(__pmIPC ** ipcp );
int __pmFdLookupIPC(int fd , __pmIPC ** ipcp );
void __pmOverrideLastFd(int fd );
void __pmResetIPC(int fd );
Newly created IPC channels must be registered with the hash table using __pmAddIPC , such that the PDU sending and decoding routines can determine whether they need to perform any PDU version translations or not, for backward compatibility with previous the PCP 1.x IPC protocol.
__pmLookupIPC and __pmFdLookupIPC both provide handles to the __pmIPC structure associated with the given file descriptor, as previously established by a call to __pmAddIPC . The difference between the two is that one allows an explicit file descriptor lookup, and the other uses the cached, most-recently-used file descriptor. So __pmLookupIPC actually calls __pmFdLookupIPC using this cached file descriptor as the argument. The justification for having both is that in some places it is not possible to use __pmFdLookupIPC (which is preferred), since at that particular level of the PMAPI a file descriptor is not available (see the __pmDecodeError code for an example).
The __pmOverrideLastFd is an escape mechanism for use in those situations where the last PDU fetch did not go through the usual channels (ie. __pmGetPDU), so as to ensure that the cached file descriptor is the correct file descriptor for the PDU which is currently being processed. This will typically be used for archive PDU processing or where version information is not available for a given file descriptor (eg. immediately prior to a PDU version exchange).
__pmPrintIPC is a useful debugging routine for displaying a table mapping all currently registered file descriptors to their associated PDU version numbers. Unused entries in this table should display the value zero in the version column.
__pmResetIPC resets the version information associated with the given file descriptor to some known (invalid) number. Subsequent lookups on this file descriptor will return an UNKNOWN_VERSION embedded within the __pmIPC structure.