Jam build tool (转载)

Posted on September 7, 2014 by jeffreyzksun

Overview

Jam is a small open-source build tool that can be used as a replacement for Make. Its original Jam author is Perforce.

The main differences between Jam and Make are as follows.

Jam uses ‘Jamfiles’ instead of ‘Makefiles’.
Jamfiles do not normally contain toolset-specific rules or actions. They are thus portable among distinct compilers.
Jamfiles are a lot simpler than Makefiles to write and understand, while providing the same functionality, and much, much more.

Language

Jam requires whitespace (blanks, tabs, or newlines) to surround all tokens, including the colon : and semicolon ; tokens. like rulename field1 : field2 : … : fieldN ;

Rule

It defines the dependency graph for targets.
Rule语法定义了构建目标的依赖关系。

It doesn’t specify the execution commands like actions does.
Rule语法不像actions语法，它不需要具体的执行指令。

There would be or not be an updating actions defined for it.
Rule语法可能有，也可能没有对应的updating actions语法。

updating actions

Updating actions are the OS shell commands to execute when updating the built targets of the rule. [1]
~~When you call a rule, the commands of the corresponding actions block are added to every element of the rule’s first argument. At the updating stage, the commands are executed to update the target.~~
When an rule with updating actions is invoked, those actions are added to those associated with its built targets ($(1)) before the rule's procedure is run.
Each target can record a set of commands to be sent to the shell. This is generally used to ‘build’ file targets like object files or executables.
actions [ modifiers ] rulename

Example [3]

rule Compile
{
Depends $(1) : $(2) ;
}
actions Compile
{
gcc -o $(1) $(2)
}
Compile hello : hello.c ;

This script can compile the source file hello.c into the executable “hello”.

When the rule Compile statement is parsed, Jam records the definition of the Compile rule, but doesn’t execute it.
When the actions Compile statement is parsed, Jam records the commands inside the curly braces as a single string, and associate them to the Compile rule. It doesn’t expand or execute them however.
When the Compile hello : hello.c ; statement is parsed, Jam does two things:
first, and before anything else, it creates a hello target in the dependency graph, then associates the action commands of the Compile rule to it, using the parameter values hello and hello.c for $(1) and $(2), respectively.
then it executes the Compile rule, whose Depends statement will create a hello.c target, then add a dependency from hello to hello.c