In addition to generic error messages, bjam may emit one
of the following:
warning: unknown rule X
A rule was invoked that has not been defined with an "actions"
or "rule" statement.
using N temp target(s)
Targets marked as being temporary (but nonetheless present) have been found.
updating N target(s)
Targets are out-of-date and will be updated.
can't find N target(s)
Source files can't be found and there are no actions to create them.
can't make N target(s)
Due to sources not being found, other targets cannot be made.
warning: X depends on itself
A target depends on itself either directly or through its sources.
don't know how to make X
A target is not present and no actions have been defined to create it.
X skipped for lack of Y
A source failed to build, and thus a target cannot be built.
warning: using independent target X
A target that is not a dependency of any other target is being referenced
with $(<) or $(>).
X removed
BJam removed a partially built target after being interrupted.
The -j flag can cause bjam to get confused
when single actions update more than one target at a time. bjam
may proceed as if the targets were built even though they are still under
construction.
For parallel building to be successful, the dependencies among files must
be properly spelled out, as targets tend to get built in a quickest-first
ordering. Also, beware of un-parallelizable commands that drop fixed-named
files into the current directory, like yacc(1) does.
With the -j flag, errors from failed commands can get
staggeringly mixed up.
A poorly set $(JAMSHELL) is likely to result in silent
failure.
This section is derived from the official Jam documentation and from experience
using it and reading the Jambase rules. We repeat the information here mostly
because it is essential to understanding and using Jam, but is not consolidated
in a single place. Some of it is missing from the official documentation
altogether. We hope it will be useful to anyone wishing to become familiar
with Jam and the Boost build system.
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Jam "rules" are actually simple procedural
entities. Think of them as functions. Arguments are separated by colons.
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A Jam target is an abstract entity identified
by an arbitrary string. The build-in DEPENDS rule creates
a link in the dependency graph between the named targets.
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Note that the documentation for the built-in INCLUDES
rule is incorrect: INCLUDES targets1 : targets2
causes everything that depends on a member of targets1
to depend on all members of targets2. It does this
in an odd way, by tacking targets2 onto a special
tail section in the dependency list of everything in targets1.
It seems to be OK to create circular dependencies this way; in fact, it
appears to be the "right thing to do" when a single build action
produces both targets1 and targets2.
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When a rule is invoked, if there are actions declared
with the same name as the rule, the actions are added to the updating actions
for the target identified by the rule's first argument. It is actually
possible to invoke an undeclared rule if corresponding actions are declared:
the rule is treated as empty.
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Targets (other than NOTFILE targets) are associated
with paths in the file system through a process called binding. Binding
is a process of searching for a file with the same name as the target (sans
grist), based on the settings of the target-specific SEARCH
and LOCATE variables.
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In addition to local and global variables, jam allows you to set a variable
on a target. Target-specific variable values can usually
not be read, and take effect only in the following contexts:
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In updating actions, variable values are first looked up on
the target named by the first argument (the target being updated).
Because Jam builds its entire dependency tree before executing actions,
Jam rules make target-specific variable settings as a way of supplying
parameters to the corresponding actions.
-
Binding is controlled entirely by the target-specific
setting of the SEARCH and LOCATE
variables, as described here.
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In the special rule used for header file scanning, variable values
are first looked up on the target named by the rule's
first argument (the source file being scanned).
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The "bound value" of a variable is the path associated with the
target named by the variable. In build actions, the first two arguments
are automatically replaced with their bound values. Target-specific variables
can be selectively replaced by their bound values using the bind
action modifier.
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Note that the term "binding" as used in the Jam documentation
indicates a phase of processing that includes three sub-phases: binding
(yes!), update determination, and header file scanning. The repetition
of the term "binding" can lead to some confusion. In particular,
the Modifying Binding section in the Jam documentation should probably
be titled "Modifying Update Determination".
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"Grist" is just a string prefix of the form <characters>.
It is used in Jam to create unique target names based on simpler names.
For example, the file name "test.exe" may
be used by targets in separate subprojects, or for the debug and release
variants of the "same" abstract target. Each distinct target
bound to a file called "test.exe" has its own unique grist prefix.
The Boost build system also takes full advantage of Jam's ability to divide
strings on grist boundaries, sometimes concatenating multiple gristed elements
at the beginning of a string. Grist is used instead of identifying targets
with absolute paths for two reasons:
-
The location of targets cannot always be derived solely from what the
user puts in a Jamfile, but sometimes depends also on the binding process.
Some mechanism to distinctly identify targets with the same name is
still needed.
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Grist allows us to use a uniform abstract identifier for each built
target, regardless of target file location (as allowed by setting ALL_LOCATE_TARGET.
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When grist is extracted from a name with $(var:G), the result includes
the leading and trailing angle brackets. When grist is added to a name
with $(var:G=expr), existing grist is first stripped. Then, if expr is
non-empty, leading <s and trailing >s are added if necessary to form
an expression of the form <expr2>; <expr2> is then prepended.
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When Jam is invoked it imports all environment variable settings into corresponding
Jam variables, followed by all command-line (-s...) variable settings.
Variables whose name ends in PATH, Path, or path are split into string
lists on OS-specific path-list separator boundaries (e.g. ":"
for UNIX and ";" for Windows). All other variables are split
on space (" ") boundaries. Boost Jam modifies that behavior by
allowing variables to be quoted.
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A variable whose value is an empty list or which consists entirely of empty
strings has a negative logical value. Thus, for example, code like the
following allows a sensible non-empty default which can easily be overridden
by the user: MESSAGE ?= starting jam... ; if $(MESSAGE) { ECHO
The message is: $(MESSAGE) ; } If the user wants a specific message,
he invokes jam with "-sMESSAGEmessage text". If he wants
no message, he invokes jam with -sMESSAGE\ and nothing at all
is printed.
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The parsing of command line options in Jam can be rather unintuitive, with
regards to how other Unix programs accept options. There are two variants
accepted as valid for an option: