sendmail, 4th Edition

XDEBUG

Support sanity checks Debug with confENVDEF

In past releases of sendmail, changes in file descriptors and other key variables have sometimes occurred for reasons that remain a mystery to this day. Small “sanity checks” have been included in the code to discover such anomalies, should they happen again. To exclude these checks, redefine XDEBUG to 0:

APPENDDEF(`confENVDEF', `-DXDEBUG=0')

Generally, however, XDEBUG should always remain enabled. It adds only a microscopic amount of overhead to sendmail and helps to certify sendmail’s rational behavior.

If sendmail’s notion of who it is (as defined by the $j defined macro, $j on page 830) gets trashed by losing all its dots, sendmail will log the following at LOG_ALERT if XDEBUG is defined, dump its state (SIGUSR1 on page 510), and abort(3):

daemon process $j lost dot; see syslog

At startup, the value in the $j defined macro ($j on page 830) is added to the class w ($=w on page 876). If sendmail is compiled with XDEBUG, it periodically checks to make sure that $j is still listed in class w. If $j should vanish, sendmail will log the following at LOG_ALERT, dump its state (SIGUSR1 on page 510), and abort(3):

daemon process doesn't have $j in $=w; see syslog

With XDEBUG defined, sendmail periodically checks to see whether its standard I/O file descriptors have gotten clobbered. If so, it logs the following and tries to recover by connecting it to /dev/null:

where: fd which not open

Here, where will reflect the internal subroutine name and ...