5.1.1.3 Other Automake variables

For each program or library that the build system can build, there is a collection of variables that affects how this happens. The names of these variables are derived from the values of variables described in the previous section. For example, if ‘foo’ is listed in bin_PROGRAMS, the variable for listing the source files of the foo program would be called ‘foo_SOURCES’. For a list of these variables, see Program and Library Variables.

The name of the program (or library) is used as a base for naming these variables. The text of these names is canonicalized, so that program names and the like do not have to follow Makefile variable naming rules. This entails all characters in the name except for letters, numbers, the at sign (‘@’) and the underscore (‘_’) being turned into underscores.

The at sign is an addition, to make the use of Autoconf substitutions in variable names less obfuscating.

For example, a program named sniff-glue, would be canonicalized as ‘sniff_glue’. An example of a derived variable name is ‘sniff_glue_SOURCES’ (not ‘sniff-glue_SOURCES’.) Similarly, the sources for a library named libmumble++.a would be listed in the ‘libmumble___a_SOURCES’ variable.

As well as the variables that the writer of Makefile.am uses to give information about how to build particular programs, there are variables of that take effect more generally. For example, there are variables specifying how programs and libraries are to be built (see Program Variables).

Some of these general variables correspond to Makefile variables reserved by the GNU Coding Standards for the use of the “user”—the person building the package. For instance, CFLAGS is one such variable (known as a user variable). A package maintainer should never set a user variable (by placing a definition for it in Makefile.am), so that the person building the package is always able to override any of these variables at build time.

If the package maintainer needs to specify any options, they should place them in a so-called shadow variable in Makefile.in. The shadow variable is named by prefixing the user variable’s name. with ‘AM_’: for example, ‘CFLAGS’ becomes ‘AM_CFLAGS’. Shadow variables are only available where they would make sense.

See Flag Variables Ordering for more discussion about these variables and how they interact with per-target variables.