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<?xml version='1.0'?>
<!DOCTYPE sconsdoc [
<!ENTITY % scons SYSTEM "../scons.mod">
%scons;
<!ENTITY % builders-mod SYSTEM "../generated/builders.mod">
%builders-mod;
<!ENTITY % functions-mod SYSTEM "../generated/functions.mod">
%functions-mod;
<!ENTITY % tools-mod SYSTEM "../generated/tools.mod">
%tools-mod;
<!ENTITY % variables-mod SYSTEM "../generated/variables.mod">
%variables-mod;
]>
<chapter id="chap-output"
xmlns="http://www.scons.org/dbxsd/v1.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.scons.org/dbxsd/v1.0 scons.xsd">
<title>Controlling Build Output</title>
<!--
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LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
-->
<para>
A key aspect of creating a usable build configuration
is providing good output from the build
so its users can readily understand
what the build is doing
and get information about how to control the build.
&SCons; provides several ways of
controlling output from the build configuration
to help make the build
more useful and understandable.
</para>
<section>
<title>Providing Build Help: the &Help; Function</title>
<para>
It's often very useful to be able to give
users some help that describes the
specific targets, build options, etc.,
that can be used for your build.
&SCons; provides the &Help; function
to allow you to specify this help text:
</para>
<scons_example name="output_ex1">
<file name="SConstruct" printme="1">
Help("""
Type: 'scons program' to build the production program,
'scons debug' to build the debug version.
""")
</file>
</scons_example>
<para>
(Note the above use of the Python triple-quote syntax,
which comes in very handy for
specifying multi-line strings like help text.)
</para>
<para>
When the &SConstruct; or &SConscript; files
contain such a call to the &Help; function,
the specified help text will be displayed in response to
the &SCons; <literal>-h</literal> option:
</para>
<scons_output example="output_ex1" suffix="1">
<scons_output_command>scons -h</scons_output_command>
</scons_output>
<para>
The &SConscript; files may contain
multiple calls to the &Help; function,
in which case the specified text(s)
will be concatenated when displayed.
This allows you to split up the
help text across multiple &SConscript; files.
In this situation, the order in
which the &SConscript; files are called
will determine the order in which the &Help; functions are called,
which will determine the order in which
the various bits of text will get concatenated.
</para>
<para>
Another use would be to make the help text conditional
on some variable.
For example, suppose you only want to display
a line about building a Windows-only
version of a program when actually
run on Windows.
The following &SConstruct; file:
</para>
<scons_example name="output_ex2">
<file name="SConstruct" printme="1">
env = Environment()
Help("\nType: 'scons program' to build the production program.\n")
if env['PLATFORM'] == 'win32':
Help("\nType: 'scons windebug' to build the Windows debug version.\n")
</file>
</scons_example>
<para>
Will display the complete help text on Windows:
</para>
<scons_output example="output_ex2" os="win32" suffix="1">
<scons_output_command>scons -h</scons_output_command>
</scons_output>
<para>
But only show the relevant option on a Linux or UNIX system:
</para>
<scons_output example="output_ex2" os="posix" suffix="2">
<scons_output_command>scons -h</scons_output_command>
</scons_output>
<para>
If there is no &Help; text in the &SConstruct; or
&SConscript; files,
&SCons; will revert to displaying its
standard list that describes the &SCons; command-line
options.
This list is also always displayed whenever
the <literal>-H</literal> option is used.
</para>
</section>
<section>
<title>Controlling How &SCons; Prints Build Commands: the <envar>$*COMSTR</envar> Variables</title>
<para>
Sometimes the commands executed
to compile object files or link programs
(or build other targets)
can get very long,
long enough to make it difficult for users
to distinguish error messages or
other important build output
from the commands themselves.
All of the default <envar>$*COM</envar> variables
that specify the command lines
used to build various types of target files
have a corresponding <envar>$*COMSTR</envar> variable
that can be set to an alternative
string that will be displayed
when the target is built.
</para>
<para>
For example, suppose you want to
have &SCons; display a
<literal>"Compiling"</literal>
message whenever it's compiling an object file,
and a
<literal>"Linking"</literal>
when it's linking an executable.
You could write a &SConstruct; file
that looks like:
</para>
<scons_example name="output_COMSTR">
<file name="SConstruct" printme="1">
env = Environment(CCCOMSTR = "Compiling $TARGET",
LINKCOMSTR = "Linking $TARGET")
env.Program('foo.c')
</file>
<file name="foo.c">
foo.c
</file>
</scons_example>
<para>
Which would then yield the output:
</para>
<!--
<scons_output example="output_COMSTR" os="posix" suffix="1">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
-->
<screen>
% <userinput>scons -Q</userinput>
Compiling foo.o
Linking foo
</screen>
<para>
&SCons; performs complete variable substitution
on <envar>$*COMSTR</envar> variables,
so they have access to all of the
standard variables like &cv-TARGET; &cv-SOURCES;, etc.,
as well as any construction variables
that happen to be configured in
the construction environment
used to build a specific target.
</para>
<para>
Of course, sometimes it's still important to
be able to see the exact command
that &SCons; will execute to build a target.
For example, you may simply need to verify
that &SCons; is configured to supply
the right options to the compiler,
or a developer may want to
cut-and-paste a compile command
to add a few options
for a custom test.
</para>
<para>
One common way to give users
control over whether or not
&SCons; should print the actual command line
or a short, configured summary
is to add support for a
<varname>VERBOSE</varname>
command-line variable to your &SConstruct; file.
A simple configuration for this might look like:
</para>
<scons_example name="output_COMSTR-VERBOSE">
<file name="SConstruct" printme="1">
env = Environment()
if ARGUMENTS.get('VERBOSE') != "1':
env['CCCOMSTR'] = "Compiling $TARGET"
env['LINKCOMSTR'] = "Linking $TARGET"
env.Program('foo.c')
</file>
<file name="foo.c">
foo.c
</file>
</scons_example>
<para>
By only setting the appropriate
<envar>$*COMSTR</envar> variables
if the user specifies
<literal>VERBOSE=1</literal>
on the command line,
the user has control
over how &SCons;
displays these particular command lines:
</para>
<!--
<scons_output example="output_COMSTR-VERBOSE" os="posix" suffix="1">
<scons_output_command>scons -Q</scons_output_command>
<scons_output_command>scons -Q -c</scons_output_command>
<scons_output_command>scons -Q VERBOSE=1</scons_output_command>
</scons_output>
-->
<screen>
% <userinput>scons -Q</userinput>
Compiling foo.o
Linking foo
% <userinput>scons -Q -c</userinput>
Removed foo.o
Removed foo
% <userinput>scons -Q VERBOSE=1</userinput>
cc -o foo.o -c foo.c
cc -o foo foo.o
</screen>
</section>
<section>
<title>Providing Build Progress Output: the &Progress; Function</title>
<para>
Another aspect of providing good build output
is to give the user feedback
about what &SCons; is doing
even when nothing is being built at the moment.
This can be especially true for large builds
when most of the targets are already up-to-date.
Because &SCons; can take a long time
making absolutely sure that every
target is, in fact, up-to-date
with respect to a lot of dependency files,
it can be easy for users to mistakenly
conclude that &SCons; is hung
or that there is some other problem with the build.
</para>
<para>
One way to deal with this perception
is to configure &SCons; to print something to
let the user know what it's "thinking about."
The &Progress; function
allows you to specify a string
that will be printed for every file
that &SCons; is "considering"
while it is traversing the dependency graph
to decide what targets are or are not up-to-date.
</para>
<scons_example name="output_Progress-TARGET">
<file name="SConstruct" printme="1">
Progress('Evaluating $TARGET\n')
Program('f1.c')
Program('f2.c')
</file>
<file name="f1.c">
f1.c
</file>
<file name="f2.c">
f2.c
</file>
</scons_example>
<para>
Note that the &Progress; function does not
arrange for a newline to be printed automatically
at the end of the string (as does the Python
<literal>print</literal> statement),
and we must specify the
<literal>\n</literal>
that we want printed at the end of the configured string.
This configuration, then,
will have &SCons;
print that it is <literal>Evaluating</literal>
each file that it encounters
in turn as it traverses the dependency graph:
</para>
<scons_output example="output_Progress-TARGET" os="posix" suffix="1">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
<para>
Of course, normally you don't want to add
all of these additional lines to your build output,
as that can make it difficult for the user
to find errors or other important messages.
A more useful way to display
this progress might be
to have the file names printed
directly to the user's screen,
not to the same standard output
stream where build output is printed,
and to use a carriage return character
(<literal>\r</literal>)
so that each file name gets re-printed on the same line.
Such a configuration would look like:
</para>
<sconstruct>
Progress('$TARGET\r',
file=open('/dev/tty', 'w'),
overwrite=True)
Program('f1.c')
Program('f2.c')
</sconstruct>
<para>
Note that we also specified the
<literal>overwrite=True</literal> argument
to the &Progress; function,
which causes &SCons; to
"wipe out" the previous string with space characters
before printing the next &Progress; string.
Without the
<literal>overwrite=True</literal> argument,
a shorter file name would not overwrite
all of the charactes in a longer file name that
precedes it,
making it difficult to tell what the
actual file name is on the output.
Also note that we opened up the
<filename>/dev/tty</filename> file
for direct access (on POSIX) to
the user's screen.
On Windows, the equivalent would be to open
the <filename>con:</filename> file name.
</para>
<para>
Also, it's important to know that although you can use
<literal>$TARGET</literal> to substitute the name of
the node in the string,
the &Progress; function does <emphasis>not</emphasis>
perform general variable substitution
(because there's not necessarily a construction
environment involved in evaluating a node
like a source file, for example).
</para>
<para>
You can also specify a list of strings
to the &Progress; function,
in which case &SCons; will
display each string in turn.
This can be used to implement a "spinner"
by having &SCons; cycle through a
sequence of strings:
</para>
<sconstruct>
Progress(['-\r', '\\\r', '|\r', '/\r'], interval=5)
Program('f1.c')
Program('f2.c')
</sconstruct>
<para>
Note that here we have also used the
<literal>interval=</literal>
keyword argument to have &SCons;
only print a new "spinner" string
once every five evaluated nodes.
Using an <literal>interval=</literal> count,
even with strings that use <literal>$TARGET</literal> like
our examples above,
can be a good way to lessen the
work that &SCons; expends printing &Progress; strings,
while still giving the user feedback
that indicates &SCons; is still
working on evaluating the build.
</para>
<para>
Lastly, you can have direct control
over how to print each evaluated node
by passing a Python function
(or other Python callable)
to the &Progress; function.
Your function will be called
for each evaluated node,
allowing you to
implement more sophisticated logic
like adding a counter:
</para>
<scons_example name="output_Progress-callable">
<file name="SConstruct" printme="1">
screen = open('/dev/tty', 'w')
count = 0
def progress_function(node)
count += 1
screen.write('Node %4d: %s\r' % (count, node))
Progress(progress_function)
</file>
</scons_example>
<para>
Of course, if you choose,
you could completely ignore the
<varname>node</varname> argument to the function,
and just print a count,
or anything else you wish.
</para>
<para>
(Note that there's an obvious follow-on question here:
how would you find the total number of nodes
that <emphasis>will be</emphasis>
evaluated so you can tell the user how
close the build is to finishing?
Unfortunately, in the general case,
there isn't a good way to do that,
short of having &SCons; evaluate its
dependency graph twice,
first to count the total and
the second time to actually build the targets.
This would be necessary because
you can't know in advance which
target(s) the user actually requested
to be built.
The entire build may consist of thousands of Nodes,
for example,
but maybe the user specifically requested
that only a single object file be built.)
</para>
</section>
<section>
<title>Printing Detailed Build Status: the &GetBuildFailures; Function</title>
<para>
SCons, like most build tools, returns zero status to
the shell on success and nonzero status on failure.
Sometimes it's useful to give more information about
the build status at the end of the run, for instance
to print an informative message, send an email, or
page the poor slob who broke the build.
</para>
<para>
SCons provides a &GetBuildFailures; method that
you can use in a python <function>atexit</function> function
to get a list of objects describing the actions that failed
while attempting to build targets. There can be more
than one if you're using <literal>-j</literal>. Here's a
simple example:
</para>
<scons_example name="output_gbf1">
<file name="SConstruct" printme="1">
import atexit
def print_build_failures():
from SCons.Script import GetBuildFailures
for bf in GetBuildFailures():
print "%s failed: %s" % (bf.node, bf.errstr)
atexit.register(print_build_failures)
</file>
</scons_example>
<para>
The <function>atexit.register</function> call
registers <function>print_build_failures</function>
as an <function>atexit</function> callback, to be called
before &SCons; exits. When that function is called,
it calls &GetBuildFailures; to fetch the list of failed objects.
See the man page
for the detailed contents of the returned objects;
some of the more useful attributes are
<literal>.node</literal>,
<literal>.errstr</literal>,
<literal>.filename</literal>, and
<literal>.command</literal>.
The <literal>filename</literal> is not necessarily
the same file as the <literal>node</literal>; the
<literal>node</literal> is the target that was
being built when the error occurred, while the
<literal>filename</literal>is the file or dir that
actually caused the error.
Note: only call &GetBuildFailures; at the end of the
build; calling it at any other time is undefined.
</para>
<para>
Here is a more complete example showing how to
turn each element of &GetBuildFailures; into a string:
</para>
<scons_example name="output_gbf2">
<file name="SConstruct" printme="1">
# Make the build fail if we pass fail=1 on the command line
if ARGUMENTS.get('fail', 0):
Command('target', 'source', ['/bin/false'])
def bf_to_str(bf):
"""Convert an element of GetBuildFailures() to a string
in a useful way."""
import SCons.Errors
if bf is None: # unknown targets product None in list
return '(unknown tgt)'
elif isinstance(bf, SCons.Errors.StopError):
return str(bf)
elif bf.node:
return str(bf.node) + ': ' + bf.errstr
elif bf.filename:
return bf.filename + ': ' + bf.errstr
return 'unknown failure: ' + bf.errstr
import atexit
def build_status():
"""Convert the build status to a 2-tuple, (status, msg)."""
from SCons.Script import GetBuildFailures
bf = GetBuildFailures()
if bf:
# bf is normally a list of build failures; if an element is None,
# it's because of a target that scons doesn't know anything about.
status = 'failed'
failures_message = "\n".join(["Failed building %s" % bf_to_str(x)
for x in bf if x is not None])
else:
# if bf is None, the build completed successfully.
status = 'ok'
failures_message = ''
return (status, failures_message)
def display_build_status():
"""Display the build status. Called by atexit.
Here you could do all kinds of complicated things."""
status, failures_message = build_status()
if status == 'failed':
print "FAILED!!!!" # could display alert, ring bell, etc.
elif status == 'ok':
print "Build succeeded."
print failures_message
atexit.register(display_build_status)
</file>
</scons_example>
<para>
When this runs, you'll see the appropriate output:
</para>
<scons_output example="output_gbf2" suffix="1">
<scons_output_command>scons -Q</scons_output_command>
<scons_output_command>scons -Q fail=1</scons_output_command>
</scons_output>
</section>
</chapter>
|