# Troubleshooting

The experience of configuring any software build tool to build a large code base usually, at some point, involves trying to figure out why the tool is behaving a certain way, and how to get it to behave the way you want. &SCons; is no different. This appendix contains a number of different ways in which you can get some additional insight into &SCons;' behavior.

Note that we're always interested in trying to improve how you can troubleshoot configuration problems. If you run into a problem that has you scratching your head, and which there just doesn't seem to be a good way to debug, odds are pretty good that someone else will run into the same problem, too. If so, please let the SCons development team know using the contact information at so that we can use your feedback to try to come up with a better way to help you, and others, get the necessary insight into &SCons; behavior to help identify and fix configuration issues.

## Why is That Target Being Rebuilt? the &debug-explain; Option

Let's look at a simple example of a misconfigured build that causes a target to be rebuilt every time &SCons; is run:

# Intentionally misspell the output file name in the # command used to create the file: Command('file.out', 'file.in', 'cp $SOURCE file.oout')
file.in

(Note to Windows users: The POSIX &cp; command copies the first file named on the command line to the second file. In our example, it copies the &file_in; file to the &file_out; file.)

Now if we run &SCons; multiple times on this example, we see that it re-runs the &cp; command every time:

scons -Q
scons -Q
scons -Q

In this example, the underlying cause is obvious: we've intentionally misspelled the output file name in the &cp; command, so the command doesn't actually build the &file_out; file that we've told &SCons; to expect. But if the problem weren't obvious, it would be helpful to specify the &debug-explain; option on the command line to have &SCons; tell us very specifically why it's decided to rebuild the target:

scons -Q --debug=explain

If this had been a more complicated example involving a lot of build output, having &SCons; tell us that it's trying to rebuild the target file because it doesn't exist would be an important clue that something was wrong with the command that we invoked to build it.

Note that you can also use `--warn=target-not-built` which checks whether or not expected targets exist after a build rule is executed.

scons -Q --warn=target-not-built

The &debug-explain; option also comes in handy to help figure out what input file changed. Given a simple configuration that builds a program from three source files, changing one of the source files and rebuilding with the &debug-explain; option shows very specifically why &SCons; rebuilds the files that it does:

Program('prog', ['file1.c', 'file2.c', 'file3.c'])
file1.c
file2.c
file3.c
scons -Q
edit file2.c
scons -Q --debug=explain

This becomes even more helpful in identifying when a file is rebuilt due to a change in an implicit dependency, such as an incuded `.h` file. If the `file1.c` and `file3.c` files in our example both included a &hello_h; file, then changing that included file and re-running &SCons; with the &debug-explain; option will pinpoint that it's the change to the included file that starts the chain of rebuilds:

Program('prog', ['file1.c', 'file2.c', 'file3.c'], CPPPATH='.')
#include <hello.h> file1.c
file2.c
#include <hello.h> file3.c
#define string "world"
scons -Q
edit hello.h
scons -Q --debug=explain

(Note that the &debug-explain; option will only tell you why &SCons; decided to rebuild necessary targets. It does not tell you what files it examined when deciding not to rebuild a target file, which is often a more valuable question to answer.)

## What's in That Construction Environment? the &f-Dump; Method

When you create a &consenv;, &SCons; populates it with &consvars; that are set up for various compilers, linkers and utilities that it finds on your system. Although this is usually helpful and what you want, it might be frustrating if &SCons; doesn't set certain variables that you expect to be set. In situations like this, it's sometimes helpful to use the &consenv; &f-link-Dump; method to print all or some of the &consvars;. Note that the &f-Dump; method returns the representation of the variables in the environment for you to print (or otherwise manipulate):

env = Environment() print(env.Dump())

On a POSIX system with gcc installed, this might generate:

scons

On a Windows system with Visual C++ the output might look like:

scons

The &consenvs; in these examples have actually been restricted to just gcc and Visual C++, respectively. In a real-life situation, the &consenvs; will likely contain a great many more variables. Also note that we've massaged the example output above to make the memory address of all objects a constant 0x700000. In reality, you would see a different hexadecimal number for each object.

To make it easier to see just what you're interested in, the &Dump; method allows you to specify a specific &consvar; that you want to disply. For example, it's not unusual to want to verify the external environment used to execute build commands, to make sure that the PATH and other environment variables are set up the way they should be. You can do this as follows:

env = Environment() print(env.Dump('ENV'))

Which might display the following when executed on a POSIX system:

scons

And the following when executed on a Windows system:

scons

## What Dependencies Does &SCons; Know About? the &tree; Option

Sometimes the best way to try to figure out what &SCons; is doing is simply to take a look at the dependency graph that it constructs based on your &SConscript; files. The `--tree` option will display all or part of the &SCons; dependency graph in an "ASCII art" graphical format that shows the dependency hierarchy.

For example, given the following input &SConstruct; file:

env = Environment(CPPPATH = ['.']) env.Program('prog', ['f1.c', 'f2.c', 'f3.c'])
#include "inc.h"
#include "inc.h"
#include "inc.h"
inc.h

Running &SCons; with the `--tree=all` option yields:

scons -Q --tree=all

The tree will also be printed when the `-n` (no execute) option is used, which allows you to examine the dependency graph for a configuration without actually rebuilding anything in the tree.

By default &SCons; uses "ASCII art" to draw the tree. It is possible to use line-drawing characters (Unicode calls these Box Drawing) to make a nicer display. To do this, add the `linedraw` qualifier:

scons -Q --tree=all,linedraw

The `--tree` option only prints the dependency graph for the specified targets (or the default target(s) if none are specified on the command line). So if you specify a target like `f2.o` on the command line, the `--tree` option will only print the dependency graph for that file:

scons -Q --tree=all f2.o

This is, of course, useful for restricting the output from a very large build configuration to just a portion in which you're interested. Multiple targets are fine, in which case a tree will be printed for each specified target:

scons -Q --tree=all f1.o f3.o

The `status` argument may be used to tell &SCons; to print status information about each file in the dependency graph:

scons -Q --tree=status

Note that `--tree=all,status` is equivalent; the `all` is assumed if only `status` is present. As an alternative to `all`, you can specify `--tree=derived` to have &SCons; only print derived targets in the tree output, skipping source files (like `.c` and `.h` files):

scons -Q --tree=derived

You can use the `status` modifier with `derived` as well:

scons -Q --tree=derived,status

Note that the order of the `--tree=` arguments doesn't matter; `--tree=status,derived` is completely equivalent.

The default behavior of the `--tree` option is to repeat all of the dependencies each time the library dependency (or any other dependency file) is encountered in the tree. If certain target files share other target files, such as two programs that use the same library:

env = Environment(CPPPATH = ['.'], LIBS = ['foo'], LIBPATH = ['.']) env.Library('foo', ['f1.c', 'f2.c', 'f3.c']) env.Program('prog1.c') env.Program('prog2.c')
#include "inc.h"
#include "inc.h"
#include "inc.h"
#include "inc.h"
#include "inc.h"
inc.h

Then there can be a lot of repetition in the `--tree=` output:

scons -Q --tree=all

In a large configuration with many internal libraries and include files, this can very quickly lead to huge output trees. To help make this more manageable, a `prune` modifier may be added to the option list, in which case &SCons; will print the name of a target that has already been visited during the tree-printing in square brackets (`[]`) as an indication that the dependencies of the target file may be found by looking farther up the tree:

scons -Q --tree=prune

Like the `status` keyword, the `prune` argument by itself is equivalent to `--tree=all,prune`.

## How is &SCons; Constructing the Command Lines It Executes? the &debug-presub; Option

Sometimes the command lines that &SCons; executes don't come out looking as you expect. In this case it may be useful to look at the strings before &SCons; performs substitution on them. This can be done with the &debug-presub; option:

env = Environment(CPPPATH = ['.']) env.Program('prog', 'prog.c')
prog.c

    % scons -Q --debug=presub
    Building prog.o with action:
      $CC -o $TARGET -c $CFLAGS $CCFLAGS $_CCOMCOM $SOURCES
    cc -o prog.o -c -I. prog.c
    Building prog with action:
      $SMART_LINKCOM
    cc -o prog prog.o

## Where is &SCons; Searching for Libraries? the &debug-findlibs; Option

To get some insight into what library names &SCons; is searching for, and in which directories it is searching, Use the &debug-findlibs; option. Given the following input &SConstruct; file:

env = Environment(LIBPATH = ['libs1', 'libs2']) env.Program('prog.c', LIBS=['foo', 'bar'])
prog.c
libs1/libfoo.a
libs2/libbar.a

And the libraries `libfoo.a` and `libbar.a` in `libs1` and `libs2`, respectively, use of the &debug-findlibs; option yields:

scons -Q --debug=findlibs

## Where is &SCons; Blowing Up? the &debug-stacktrace; Option

In general, &SCons; tries to keep its error messages short and informative. That means we usually try to avoid showing the stack traces that are familiar to experienced Python programmers, since they usually contain much more information than is useful to most people.

For example, the following &SConstruct; file:

Program('prog.c')

Generates the following error if the `prog.c` file does not exist:

scons -Q

In this case, the error is pretty obvious. But if it weren't, and you wanted to try to get more information about the error, the &debug-stacktrace; option would show you exactly where in the &SCons; source code the problem occurs:

scons -Q --debug=stacktrace

Of course, if you do need to dive into the &SCons; source code, we'd like to know if, or how, the error messages or troubleshooting options could have been improved to avoid that. Not everyone has the necessary time or Python skill to dive into the source code, and we'd like to improve &SCons; for those people as well...

## How is &SCons; Making Its Decisions? the &taskmastertrace; Option

The internal &SCons; subsystem that handles walking the dependency graph and controls the decision-making about what to rebuild is the Taskmaster. &SCons; supports a &taskmastertrace; option that tells the Taskmaster to print information about the children (dependencies) of the various Nodes on its walk down the graph, which specific dependent Nodes are being evaluated, and in what order.

The &taskmastertrace; option takes as an argument the name of a file in which to put the trace output, with `-` (a single hyphen) indicating that the trace messages should be printed to the standard output:

env = Environment(CPPPATH = ['.']) env.Program('prog.c')
#include "inc.h" prog.c
#define STRING "one"
scons -Q --taskmastertrace=- prog

The &taskmastertrace; option doesn't provide information about the actual calculations involved in deciding if a file is up-to-date, but it does show all of the dependencies it knows about for each Node, and the order in which those dependencies are evaluated. This can be useful as an alternate way to determine whether or not your &SCons; configuration, or the implicit dependency scan, has actually identified all the correct dependencies you want it to.

## Watch &SCons; prepare targets for building: the &debug-prepare; Option

Sometimes SCons doesn't build the target you want and it's difficult to figure out why. You can use the &debug-prepare; option to see all the targets &SCons; is considering, and whether they are already up-to-date or not. The message is printed before &SCons; decides whether to build the target.

## Why is a file disappearing? the &debug-duplicate; Option

When using the &Duplicate; option to create variant dirs, sometimes you may find files not getting linked or copied to where you expect (or not at all), or files mysteriously disappearing. These are usually because of a misconfiguration of some kind in the SConscript files, but they can be tricky to debug. The &debug-duplicate; option shows each time a variant file is unlinked and relinked from its source (or copied, depending on settings), and also shows a message for removing "stale" variant-dir files that no longer have a corresponding source file. It also prints a line for each target that's removed just before building, since that can also be mistaken for the same thing.

## Keep it simple

Over the years, many developers have chosen to dive in and make vastly complicated build systems out of &SCons;, which sometimes don't work quite as expected. As a general rule, make sure you need to reach for a complex solution before you do so. &SCons; is mature software and has evolved over time to meet a lot of feature requests, so there is often an easier way to do something if you can just find it. The &SCons; community can be helpful here - the discussion lists and chat channels can be a way to find out if something can be done an easier way before embarking on an implementation.

When something does misbehave, trying to isolate the problem to a simple test case can really help. The work to create a reproducer often helps you spot the issue yourself, and a simple example is much easier for others to look over and possibly spot logical flaws, misuse of the API, or other ways something could have been done. In addition, if it turns out there's actually a real &SCons; bug (we believe it's a high quality piece of software, but all software has some bugs), it's very likely the bug filing will result in a request for a simple reproducer anyway.