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Define SIZEOF_type-or-expr (see Standard Symbols) to be
the size in bytes of type-or-expr, which may be either a type or
an expression returning a value that has a size. If the expression
‘sizeof (type-or-expr)’ is invalid, the result is 0.
includes is a series of include directives, defaulting to
AC_INCLUDES_DEFAULT (see Default Includes), which are used
prior to the expression under test.
This macro now works even when cross-compiling. The unused argument was used when cross-compiling.
For example, the call
AC_CHECK_SIZEOF([int *])
defines SIZEOF_INT_P to be 8 on DEC Alpha AXP systems.
This macro caches its result in the ac_cv_sizeof_type-or-expr
variable, with ‘*’ mapped to ‘p’ and other characters not
suitable for a variable name mapped to underscores.
Define ALIGNOF_type (see Standard Symbols) to be the
alignment in bytes of type. ‘type y;’ must be valid as
a structure member declaration. If ‘type’ is unknown, the result
is 0. If no includes are specified, the default includes are used
(see Default Includes).
This macro caches its result in the ac_cv_alignof_type-or-expr
variable, with ‘*’ mapped to ‘p’ and other characters not
suitable for a variable name mapped to underscores.
Store into the shell variable var the value of the integer
expression. The
value should fit in an initializer in a C variable of type signed
long. To support cross compilation (in which case, the macro only works on
hosts that use twos-complement arithmetic), it should be possible to evaluate
the expression at compile-time. If no includes are specified, the
default includes are used (see Default Includes).
Execute action-if-fails if the value cannot be determined correctly.
Normally Autoconf ignores warnings generated by the compiler, linker, and
preprocessor. If this macro is used, warnings count as fatal
errors for the current language. This macro is useful when the
results of configuration are used where warnings are unacceptable; for
instance, if parts of a program are built with the GCC
-Werror
option. If the whole program is built using -Werror it is
often simpler to put -Werror in the compiler flags (CFLAGS,
etc.).
OpenMP specifies extensions of C, C++, and Fortran that simplify optimization of shared memory parallelism, which is a common problem on multi-core CPUs.
If the current language is C, the macro AC_OPENMP sets the
variable OPENMP_CFLAGS to the C compiler flags needed for
supporting OpenMP. OPENMP_CFLAGS is set to empty if the
compiler already supports OpenMP, if it has no way to activate OpenMP
support, or if the user rejects OpenMP support by invoking
‘configure’ with the ‘--disable-openmp’ option.
OPENMP_CFLAGS needs to be used when compiling programs, when
preprocessing program source, and when linking programs. Therefore you
need to add $(OPENMP_CFLAGS) to the CFLAGS of C programs
that use OpenMP. If you preprocess OpenMP-specific C code, you also
need to add $(OPENMP_CFLAGS) to CPPFLAGS. The presence of
OpenMP support is revealed at compile time by the preprocessor macro
_OPENMP.
Linking a program with OPENMP_CFLAGS typically adds one more
shared library to the program’s dependencies, so its use is recommended
only on programs that actually require OpenMP.
If the current language is C++, AC_OPENMP sets the variable
OPENMP_CXXFLAGS, suitably for the C++ compiler. The same remarks
hold as for C.
If the current language is Fortran 77 or Fortran, AC_OPENMP sets
the variable OPENMP_FFLAGS or OPENMP_FCFLAGS,
respectively. Similar remarks as for C hold, except that
CPPFLAGS is not used for Fortran, and no preprocessor macro
signals OpenMP support.
For portability, it is best to avoid spaces between ‘#’ and ‘pragma omp’. That is, write ‘#pragma omp’, not ‘# pragma omp’. The Sun WorkShop 6.2 C compiler chokes on the latter.
This macro caches its result in the ac_cv_prog_c_openmp,
ac_cv_prog_cxx_openmp, ac_cv_prog_f77_openmp, or
ac_cv_prog_fc_openmp variable, depending on the current language.
Caution: Some of the compiler options that AC_OPENMP
tests, mean “enable OpenMP” to one compiler, but “write output to a
file named mp or penmp” to other compilers. We cannot
guarantee that the implementation of AC_OPENMP will not overwrite
an existing file with either of these names.
Therefore, as a defensive measure, a configure script that
uses AC_OPENMP will issue an error and stop (before doing any of
the operations that might overwrite these files) upon encountering
either of these files in its working directory.
autoconf will also issue an error if it finds either of
these files in the same directory as a configure.ac that
uses AC_OPENMP.
If you have files with either of these names at the top level of your
source tree, and you need to use AC_OPENMP, we recommend you
either change their names or move them into a subdirectory.
Next: C Compiler Characteristics, Previous: Specific Compiler Characteristics, Up: Compilers and Preprocessors [Contents][Index]