This section contains a prose description of the interface system's functionality and presents the basic syntax of Abuild.Interface without providing all of the details. This material provides the basis for understanding how the interface functionality works. In the next section, we go over the details.

The Abuild.interface file has a fairly simple syntax that supports variable declarations, variable assignments, and conditionals. Interface files are rigorously validated. Any errors detected in an interface file are considered build failures which, as such, will prevent abuild from attempting to build the item with the incorrect interface and any items that depend on it. Most Abuild.interface files will just set existing variables to provide specific information about that item's include and library information, classpath information, or whatever other standard information may be needed depending upon the type of item it is. For casual users, a full understanding of this material is not essential, but for anyone trying to debug interface issues or create support within abuild for more complex cases, it will be important to understand how abuild reads Abuild.interface files.

The basic purpose of Abuild.interface is to set variables that are ultimately used by a build item to access its dependencies. The basic model is that an item effectively reads the Abuild.interface files of all its dependencies in dependency order. (This is not exactly what happens. For the full story, see Section 29.7, “Implementation of the Abuild Interface System”.) As each file is read, it adds information to the lists of include paths, libraries, library directories, compiler flags, classpath, etc. All variables referenced by Abuild.interface are global variables, even if they are declared inside the body of a conditional, much as is the case with shell scripts or makefiles. Although this is not literally what happens, the best way to think about how abuild reads interface files is to imagine that, for each build item, all of the interface files for its dependencies along with its own interface file are concatenated in dependency order and that the results of that concatenation are processed from top to bottom, skipping over any blocks inside of false conditional statements.

Once abuild parses the Abuild.interface files of all of a build item's dependencies and that of the build item itself, the names and values of the resulting variables are passed to the backends by writing them to the abuild dynamic output file, which is called .ab-dynamic.mk for make-based builds and .ab-dynamic-ant.properties for ant-based builds. The dynamic output file is created in the output directory. Although users running abuild don't even have to know this file exists, peeking at it is a useful way to see the results of parsing all the Abuild.interface files in a build item's dependency chain.

The Abuild.interface file contains the following items:

Similar to make or shell script syntax, each statement is terminated by the end of the line. Whitespace characters (spaces or tabs) are used to separate words. A backslash (\) as the last character of the line may be used to continue long statements onto the next line of the file, in which case the newline is treated as a word delimiter like any other whitespace. ^[24] Any line that starts with a # character optionally preceded by whitespace is ignored entirely. Comment lines have no effect on line continuation. In other words, if line one ends with a continuation character and line two is a comment, line one is continued on line three. This makes it possible to embed comments in multiline lists of values. In this example, the value of ODDS would be one three:

ODDS = \
  one \
# odd numbers only, please
  # two \
  three

Characters that have special meanings (space, comma, equal, etc.) may be quoted by preceding them by a backslash. For consistency, a backslash followed by any character is treated as that character. This way, the semantics of backslash quoting won't change if additional special characters are added in the future.

All variables must be declared, though most Abuild.interface files will be assigning to variables that have already been declared in other interface files. There are no variable scoping rules: all variables are global, even if declared inside a conditional block. Variable names may contain alphanumeric characters, dash, underscore, and period. By convention, make-based rules use all uppercase letters in variable names. This convention also has the advantage of avoiding potential conflict with reserved statements. Java-based rules typically use lower-case period-separated properties. Ultimately abuild interface variables become make variables or ant properties, which is the basis for these conventions. Note, however, that variables of both naming styles may be used by either backend, and some of abuild's predefined interface variables that are available to both make and ant are of the all upper-case variety.

Once declared, a variable may be assigned to or referenced. A variable is referenced by enclosing its name with parentheses and preceding it by a dollar sign (as in $(VARIABLE)), much like with standard make syntax, except that there is no special case for single-character variable names. Other than using the backslash character to quote single characters, there is no quoting syntax: the single and double quote characters are treated as ordinary characters with no special meanings.

Environment variables may be referenced using the syntax $(ENV:VARIABLE). Unlike many other systems which treat undefined environment variables as the empty string, abuild will trigger an error condition if the environment variable does not exist. This can be useful for having abuild interface files initialize interface variables from the environment. Use this feature sparingly as it is possible to make a build become overly dependent on the environment in this way. (Even without this feature, there are other ways to fall into this trap that are even worse.) Note that environment variables are not abuild variables. They are expanded as strings and can be used in the interface file wherever ordinary strings can be used.

Variables may contain single scalar values or they may contain lists of values of one of the three supported types: boolean, string, or filename.

Boolean variables are simple true/false values. The values 1 and true are interpreted interchangeably as true, and the values 0 and false are interpreted interchangeably as false. Regardless of whether the word or numeric value is used to assign to boolean variables, the normalized values of 0 and 1 are passed to the backend build system. String variables just contain arbitrary text. It is possible to embed spaces in string variables by quoting them with a backslash, but keep in mind that not all backends handle spaces in single-word variable values cleanly. For example, dealing with embedded spaces in variable names in GNU Make is impractical since it uses space as a word delimiter and offers no specific quoting mechanisms. The values of filename variables are interpreted to be path names. Path names may be specified with either forward slashes or backslashes on any platform. Relative paths (those that do not start with a path separator character or, on Windows, also a drive letter) are interpreted as relative to the file in which they are assigned, not the file in which they are referenced as is the case with make. This means that build items can export information about their local files using relative paths without having to use any special variables that point to their own local directories. Although this is different from how make works, it is the only sensible semantic for files that are referenced from multiple locations.

List variables may contain multiple space-separated words. Assignments to list variables may span multiple lines by using a trailing backslash to indicate continuation to the next line. Each element of a list must be the same type. Lists can be made of any of the supported scalar types. (Lists of boolean values are supported, though they are essentially useless.) List variables must be declared as either append or prepend, depending upon whether successive assignments are appended or prepended to the value of the list. This is described in more depth when we discuss variable assignment below.

Scalar variables may be assigned to in one of three ways: normal, override, and fallback. A normal assignment to a scalar variable fails if the variable already has a value. An override assignment initializes a previously uninitialized variable and replaces any previously assigned value. A fallback assignment sets the value of the variable only if it has not previously been initialized. Uninitialized variables are passed to the backend as empty strings. It is legal to initialize a string variable to the empty string, and doing this is distinct from not initializing it.

List variables work differently from anything you're likely to have encountered in other environments, but they offer functionality that is particularly useful when building software. List variables may be assigned to multiple times. The value in each individual assignment may contain zero or more words. Depending on whether the variable was declared as append or prepend, the values are appended to or prepended to the list in the order in which they appear in the specific assignment. An example is provided below.

Scalar and list variables can both be reset using the reset statement. This resets the variable back to its initial state, which is uninitialized for scalars and empty for lists.

Any variable assignment statement can be made conditional upon the presence of a given interface flag. Interface flags are introduced in Chapter 20, Interface Flags, and the details of how to use them in interface files are discussed later in this chapter.

Abuild supports nested conditionals, each of which may contain an if clause, zero or more elseif clauses, and an optional else clause. The abuild interface syntax supports no relational operators: all conditionals are expressed in terms of function calls, the details of which are provided below.

In addition to supporting variables and conditionals, it is possible to specify that certain variables are relevant only to build items of a specific target type. A target type restriction applies until the next target-type directive or until the end of the current file and all the files it loads as after-build files. By default, declarations in an Abuild.interface file apply to all target types. The vast majority of interface files will not have to include any target type restrictions.

It is possible for a build item to contain interface information that is intended for items that depend on it but not intended for the item itself. Typical uses cases would include when some of this information is a product of the build or when a build item needs to modify interface information provided by a dependency after it has finished using the information itself. To support this, an Abuild.interface file may specify additional interface files that are not to be read until after the item is built. The values in any such files are not available to the build item itself, but they are available to any items that depend on the build item that exports this interface. Such files may be dynamically generated (such as with autoconf; see Section 15.3, “Autoconf Example”), or they may be hand-generated files that are just intended not to apply to the build of the current build item (see Section 25.1, “Opaque Wrapper Example”).