automake-1.16: Program Variables
8.7 Variables used when building a program
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Occasionally it is useful to know which ‘Makefile’ variables Automake
uses for compilations, and in which order (⇒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$(srcdir)’, and a ‘-I’ pointing to the directory holding
‘config.h’ (if you’ve used ‘AC_CONFIG_HEADERS’). You can disable
the default ‘-I’ options using the ‘nostdinc’ option.
When a file to be included is generated during the build and not
part of a distribution tarball, its location is under
‘$(builddir)’, not under ‘$(srcdir)’. This matters especially for
packages that use header files placed in sub-directories and want
to allow builds outside the source tree (⇒VPATH Builds). In
that case we recommend using a pair of ‘-I’ options, such as, e.g.,
‘-Isome/subdir -I$(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. In some situations, this is not used,
in preference to the per-executable (or per-library) ‘_CFLAGS’.
‘COMPILE’
This is the command used to 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 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.