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.\"	aegis - project change supervisor
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.\" MANIFEST: document describing symlink farms
.\"
.SH SYMBOLIC LINKS
Many dependency maintenance tools,
and indeed some compilers,
have little or no support for include file search paths,
and thus for the concept of the two-level directory hierarchy employed by
.IR \*(n) .
(It becomes multi-level when \*(n)' branching functionality is used.)
To allow these tools to be used,
.I \*(n)
provides the ability to maintain a set of symbolic links between the
development directory of a change and the baseline of a project,
so it appears to these tools that all of the project's files are present
in the development directory.
.SS Project Configuration
Two boolean fields of the project
.I config
file control the behavior of this feature.
They are the
.I create_\%symlinks_\%before_\%build
and
.I remove_\%symlinks_\%after_\%build
fields.
See
.IR aepconf (5)
for more information.
.PP
If both fields are set to
.IR true ,
the user never sees the symbolic links,
because they are added purely for the benefit of
the dependency maintenance tool
during the execution of the
.IR aeb (1)
command.
.PP
If only the
.I create_\%symlinks_\%before_\%build
field is set to
.I true
(the other will default to false)
the symbolic links will be created at develop begin time (see
.IR aedb (1)
for more information)
and also maintained by each
.IR aeb (1)
invocation.
Note that the symbolic links are only maintained at these times,
so project integrations during the course of development
may leave the symbolic links in an inconsistent state
until the next build.
.PP
When files are copied from the baseline into a change,
using the
.IR aecp (1)
command,
the symbolic link pointing into the baseline,
if any,
will be removed before the file is copied.
.PP
.B Note:
Using this functionality in either form
has implications for how the rules file of the
dependency maintenance tool is written.
Rules must
.I remove
their targets before creating them.
(Usually with an
.I "rm -f"
command.)
This is to avoid attempting to write the result on the symbolic link,
which will point at a read-only file in the project baseline.
This is similar to the same requirement for using the
.I link_\%integration_\%directory
field of the project
.I config
file.
.SS User Configuration
There is a \fIsymbolic_\%link_\%preference\fP field in the user
configuration file (see \fIaeuconf\fP(5) for more information).  This
controls whether \fIaeb\fP(1) will verify the symbolic links before
the build (default) or whether it will assume they are up-to-date.
(This field is only relevant if \fIcreate_\%symlinks_\%before_\%build\fP
is true.)
.PP
For medium-to-large projects, verifying the symbolic links can take
as long as the build itself.  Assuming the symbolic links are up-to-date
can be a large time-saving for these projects.
It may be advisable to review your choice of DMT in such a situation.
.PP
The \fIaedb\fP(1) command \fBdoes not\fP consult this preference.
Thus, in most situations, the symbolic links will be up-to-date when
the build is performed.  The only Aegis function which may result in the
symbolic links becoming out-of-date is the integration of another change,
as this may alter the presence or absence of files in the baseline.
In this situation, the default \fIaeb\fP(1) action is to ignore the user
preference and the verify symbolic links.
.PP
There are two command line options which modify \fIaeb\fP(1) behavior
further: the \fB--Verify_\%Symbolic_\%Links\fP option says to verify the
symbolic links; and the \fB--Assume_\%Symbolic_\%Links\fP option says
to assume the symbolic links are up-to-date.
In each case the option over-rides the default and the user preference.