The Jam/MR Executable Program
jam [ -a ] [ -n ] [ -v ] [ -d debug ] [ -f rulesfile ... ] [ -j jobs ] [ -s var=value ... ] [ -t target ... ] [ target ... ]
jam, the Jam/MR executable program, recursively builds target files from source files using dependency and build specification rules defined in rulesfile. jam parses rulesfile to identify targets and sources, examines the filesystem to determine which targets need updating, and issues OS commands to update targets.
Normally, rulesfile is compiled into jam, allowing jam to be run as a stand-alone program. A base set of build rules is provided in the file "Jambase", and is described in Using Jamfiles and Jambase and the Jambase Reference. For general information, refer to the Jam/MR Language document.
If target is provided on the command line, jam attempts to build target; otherwise jam attempts to build the target 'all'.
jam may be invoked with the following options:
-a Build all targets anyway, even if they are up-to-date. -d<n> Enable cummulative debugging levels from 1 to <n>. Interesting values are: 1 Show actions (the default) 2 Show "quiet" actions and display all action text 3 Show dependency analysis, and target/source timestamps/paths 4 Show shell arguments 5 Show rule invocations and variable expansions 6 Show directory/header file/archive scans 7 Show variable settings 8 Show variable fetches 9 Show variable manipulation, scanner tokens -d+<n> Enable debugging level <n>. -d0 Turn off all debugging levels. Only errors are not suppressed. -f<rulesfile> Read <rulesfile> instead of Jambase. -j<n> Run up to <n> shell commands concurrently (UNIX only). The default is 1. -k Continue after build error. The default behavior is to exit after the first build failure. -n Don't actually execute the updating actions, but do everything else. This changes the debug level default to -d2. -s<var>=<value> Set the variable <var> to <value>, overriding both internal variables and variables imported from the environment. -t<target> Rebuild <target>, even if it is up-to-date, and/or build dependencies of <target> as if <target> were newer. -v Print the version of jam and exit.
jam has three phases of operation: parsing, binding, and updating.
In the parsing phase, jam reads the rules file(s), evaluates variables, identifies and invokes rules, identifies targets, and builds the dependency graph.
Which rules files get read depends on the site-specific implementation of jam. The normal implementation is this: jam reads the Jambase rules file, the text of which is stored inside the jam executable itself. Jambase may "include" other rules files, and the last rule invoked from Jambase is "include Jamfile", which reads file "Jamfile" from the current directory. Jamfile is expected to specify which targets get built from the source files in the current directory. (All of this is explained in detail in Using Jamfiles and Jambase.) If there's no Jamfile in the current directory, jam emits a message and exits.
Environment variable settings are imported into Jam/MR variables. To pass a value to a variable on the jam command line, overriding the variable's environment value, use the -s option. To see variable assignments made during jam's execution, use the -d+7 option.
Rules are defined in rules files using the Jam/MR language, and are invoked in rules files after they are defined. Targets are identified by rule invocations. At the completion of the parsing phase, all targets are uniquely identified and a dependency graph is constructed.
After parsing, jam recursively descends the dependency graph and binds every file target with a location in the filesystem. The existence and modification times of the bound files are used to determine which targets need updating. If jam detects a circular dependency in the graph, it issues a warning.
A file target is bound with a location as follows:
After binding each target, jam determines whether the target needs updating, and marks the target if necessary for the updating phase. A target is marked for updating for any of these three reasons:
For targets of the built-in rules ALWAYS, LEAVES, NOCARE, TEMPORARY, NOTFILE, and NOUPDATE, jam's updating behavior is slightly different:
- The target is always updated.
- The target is only updated if it is missing or if its leaf sources are newer. Leaf sources are those dependencies of the target that have no dependencies themselves.
- The target is ignored if it is missing and has no updating actions. Normally, jam issues a warning and skips other targets that depend on missing targets without updating actions.
- If the target is missing, then its source's modification time is used when comparing against dependencies.
- The target is only updated if any of its sources are marked for updating.
- The target is only updated if it is missing. Also, if it exists, it will appear eternally old; that is, older than anything that depends on it.
If $(HDRSCAN) is set on a file target, jam scans the file for header file include lines. It scans the file by matching each line against a regexp(3) pattern that has ()'s surrounding the included file name. The pattern is provided by the user through the special variable $(HDRSCAN) (see HDRPATTERN in Jambase for an example). The result of the scan is formed into a rule invocation, with the scanned file as the target and the found included file names as the sources. The rule invoked is named by the special variable $(HDRRULE). jam only scans files if $(HDRSCAN) is set, and $(HDRSCAN) is normally set target-specific.
Between binding and updating, jam announces the number of targets to be updated.
After binding, jam again recursively descends the dependency graph, this time executing the update actions for each target marked for update during the binding phase. If a target's updating actions fail, then all targets which depend on it are skipped.
The -j flag instructs jam to build more than one target at a time. If there are multiple actions on a single target, they are run sequentially.
The special variable $(JAMSHELL) gives jam a command execution shell to be used instead of /bin/sh. This variable's value must be a multi-element list, corresponding to the argument vector for the command shell. An element "%" is replaced with the command string to execute. An element "!" is replaced with the multiprocess slot number, which is (inclusively) between 1 and the maximum number of concurrent jobs specified with the -j flag (default 1). If no element of the list is "%", the command string is tacked on as the last argument. The default value is: "/bin/sh -c %".
In addition to generic error messages, jam 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 does is not a dependency of any other target is being referenced with $(<) or $(>). X removed jam removed a partially built target after being interrupted.
The -j flag can cause jam to get confused when single actions update more than one target at a time. jam may try to execute actions to build those targets' dependencies before the targets themselves have all been built.
With the -j flag, errors from failed commands can get staggeringly mixed up. Also, because targets tend to get built in a quickest-first ordering, dependency information must be quite exact. Finally, beware of parallelizing commands that drop fixed-named files into the current directory, like yacc(1) does.
A poorly set $(JAMSHELL) is likely to result in silent failure.
Jam/MR's author is Christopher Seiwald (email@example.com). Documentation is provided by Perforce Software, Inc.
Copyright 1997 Perforce Software, Inc.
Comments to firstname.lastname@example.org
Last updated: Oct 19, 1997