Listing SystemV IPC queues

24 May 2024

The correct way to list the queues on the Linux machine is by using the ipcs -q command. However, I have not seen anything in the system calls that would provide such a capability. But Today I Learned…

The ipcs utility has two ways of retrieving the list of queues. The first and main one is to read and parse /proc/sysvipc/msg. If the file is not present, it does a fallback by using a strange Linux-specific msgctl call with MSG_INFO command. Which among other things has this property:

successful IPC_INFO or MSG_INFO operation returns the index of the highest used entry in the kernel’s internal array recording information about all message queues

And then it’s just a loop from 0 to that max ID and checking all possible queue identifiers.