sendmail, 4th Edition

SM_CONF_SHM

Use shared memory (V8.12 and above) Port with confENVDEF

Beginning with V8.12, sendmail includes limited support for the use of shared memory. Shared memory is a region of memory maintained by the operating system so that an arbitrary number of programs can have common access to that memory.

The sendmail program forks a copy of itself every time it processes a queue. Because V8.9 and above sendmail support multiple queues, it is likely that a separate sendmail invocation will be processing each queue. Each queue processor knows the contents of each queue—specifically, the number of messages that are in its queue at any given time. A convenient place to store that information is in shared memory.

When you run V8.12 and above sendmail with the -bP command-line switch (Print the Number of Messages in the Queue on page 425), sendmail reads shared memory to gather a count of the number of messages in each queue.

Shared memory is turned on by default for some operating systems and off for others. If you run sendmail with the -bP command-line switch and get the following error, you might need to define this SM_CONF_SHM compile-time macro:

Data unavailable without shared memory support

If you need to enable shared memory, you can do so by placing a line such as the following in your Build m4 file:

APPENDDEF(`conf_sendmail_ENVDEF', `-DSM_CONF_SHM=1')
                                                 ↑
                                                                   to turn on shared
memory support

Note that just turning on SM_CONF_SHM is not enough. To actually use that shared memory you also need ...