Errno Libretto

By Hal Stern, Unix Insider |  Operating Systems

Opera is something I do not appreciate fully. The
costumes are exquisite, the music is emotional, but without
understanding Italian the plots are hard to follow. Pavarotti could be
performing a free-form exploration of the UUCP source code and I would
have trouble distinguishing it from Madama Butterfly. Bugs
Bunny making a fruit salad on Elmer Fudd's head is the most
comprehensible opera I've witnessed. "Wait," my cultured friends tell
me, "use the libretto to grasp the story." While it's not quite a set
of Cliff Notes, the libretto (text of the opera) helps you build a
framework for understanding the action on stage.

What does this have to do with the world of system administration?
If the error messages, user questions, system-call errors, and other
cryptic failures you encounter sometimes make as much sense La
Traviata
, then you need a libretto -- a framework for
understanding what the system is trying to tell you. We'll look at the
various ways in which system calls fail, and the symptoms by which
those failures manifest themselves. Starting with general file
permission issues, we'll then dive down into NFS failures, and close
with some comments on the importance of vigilance in enforcing system
programming guidelines. You may not understand Puccini any better than
before, but such help is easier to find.

Trap defense

System calls represent the boundary between user processes and
operating system (kernel) services. When a process executes a system
call, the associated wrapper in the libc.so library is called
to perform some basic argument checking. If the call is syntactically
acceptable, the wrapper executes a privileged instruction to force a
trap into the kernel. From there, the operating system takes over by
copying arguments, performing extensive checking, and completing the
service request. If you dump out the code for a system call in
libc.so, you'll see a "ta 8" instruction to issue trap 0x08,
which is a system call (see /usr/include/sys/trap.h for trap
types):

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