"BUS error" or "SEGMENTATION fault" mean
Note: If you are running someone else's program, the messages
Bus error
or
Segmentation fault
mean that there's a bug (error) in the program. If you're not interested in
debugging the program yourself, you can skip the rest of this section. For
Athena-supported software, you can report the bug by contacting a consultant
through olc, or if you know the exact cause, by typing the following at the
athena% prompt:
sendbug
For non-Athena supported software, contact the maintainers of the software
(for example, bugs in software in the sipb and games lockers should be
reported by sending mail to bug-sipb or bug-games, respectively).
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If the program displays this message:
Bus error
or
Segmentation fault
... then the program was trying to access a memory location outside its
address space. The computer detected this problem and sent a signal to your
program, which caused it to abort.
Things that cause bus errors and segmentation violations are typically
out-of-bounds array references and/or references through uninitialized or
mangled pointers. Look very closely in your program for bizarre things like
that. A common example is:
int c;
scanf("%d", c);
instead of the correct version:
int c;
scanf("%d", &c);
There are a number of methods for finding out where the program went out of
bounds. One method is to use printf() statements to determine how far the
program is getting before it crashes, and to print out the contents of
interesting variables. A more sophisticated method is using 'dbx', a source
level symbolic debugger. C programmers can also use 'saber', a new C
interpreter. To learn about 'dbx' or 'saber', you can read the manual pages
by using the 'man' command, as in:
man dbx
or man saber
If you need to debug your program, you may want to enable a core dump.
Usually, those two messages above would also have "(core dumped)" by them,
indicating that the program wrote an image of its current memory into a file
called "core" in that directory. Athena turned this off by default, since for
most people "core" is just a waste of disk space.
If you want to turn it back on again, type 'quota -v' and find out how many
blocks (kilobytes) you have available in your quota. Then you can type, say:
limit coredumpsize 100
.. to limit your core dump size to 100K for your current login. BE CAREFUL
not to let yourself go over quota, as you would then not be able to create or
edit files. You should delete the "core" file when you don't need it any
more.
To allow core dumps and still not have quota worries, you can make a symbolic
link from the name core into the /tmp directory. Core dumps created this way
will not take up quota, and they will automatically vanish when you log out.
The dump will also occur more quickly since it doesn't have to go over the
network to your directory. To set up a core link in a directory:
ln -s /tmp/core.project ./core
unlimit coredumpsize
/tmp/core.project in the first line is the name that the core file is really
called; the "ln -s" command creates a symbolic link from the filename "core"
in the current directory, to a file named "core.project" in the /tmp direcory.
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