8.7 Variables used when building a program

Occasionally it is useful to know which Makefile variables Automake uses for compilations, and in which order (see Flag Variables Ordering); for instance, you might need to do your own compilation in some special cases.

Some variables are inherited from Autoconf; these are CC, CFLAGS, CPPFLAGS, DEFS, LDFLAGS, and LIBS. There are some additional variables that Automake defines on its own:

AM_CPPFLAGS

The contents of this variable are passed to every compilation that invokes the C preprocessor; it is a list of arguments to the preprocessor. For instance, -I and -D options should be listed here.

Automake already provides some -I options automatically, in a separate variable that is also passed to every compilation that invokes the C preprocessor. In particular it generates ‘-I.’, ‘-I$(srcdir)’, and a -I pointing to the directory holding config.h (if you've used AC_CONFIG_HEADERS or AM_CONFIG_HEADER). You can disable the default -I options using the nostdinc option.

When a file to be included is generated during the build and not part of a distribution tarball, its location is under $(builddir), not under $(srcdir). This matters especially for packages that use header files placed in sub-directories and want to allow builds outside the source tree (see VPATH Builds). In that case we recommend to use a pair of -I options, such as, e.g., ‘-Isome/subdir -I$(srcdir)/some/subdir’ or ‘-I$(top_builddir)/some/subdir -I$(top_srcdir)/some/subdir’. Note that the reference to the build tree should come before the reference to the source tree, so that accidentally leftover generated files in the source directory are ignored.

AM_CPPFLAGS is ignored in preference to a per-executable (or per-library) _CPPFLAGS variable if it is defined.

INCLUDES

This does the same job as AM_CPPFLAGS (or any per-target _CPPFLAGS variable if it is used). It is an older name for the same functionality. This variable is deprecated; we suggest using AM_CPPFLAGS and per-target _CPPFLAGS instead.

AM_CFLAGS

This is the variable the Makefile.am author can use to pass in additional C compiler flags. It is more fully documented elsewhere. In some situations, this is not used, in preference to the per-executable (or per-library) _CFLAGS.

COMPILE

This is the command used to actually compile a C source file. The file name is appended to form the complete command line.

AM_LDFLAGS

This is the variable the Makefile.am author can use to pass in additional linker flags. In some situations, this is not used, in preference to the per-executable (or per-library) _LDFLAGS.

LINK

This is the command used to actually link a C program. It already includes ‘-o $@’ and the usual variable references (for instance, CFLAGS); it takes as “arguments” the names of the object files and libraries to link in. This variable is not used when the linker is overridden with a per-target _LINK variable or per-target flags cause Automake to define such a _LINK variable.