C++ – How to set a define through “./configure” with Autoconf

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I have one project that can generate two diferent applications based on one define.

libfoo_la_CXXFLAGS = -DMYDEFINE

I have to modify the Makefile.am to set this define, so it is not automatic.

Can I set this define somehow through the configure command?
Is there any other way to set one define using autotools?

Best Answer

You have to edit the file configure.ac, and before AC_OUTPUT (which is the last thing in the file) add a call to AC_DEFINE.

In a simple case like yours, it should be enough with:

AC_DEFINE(MYDEFINE)

If you want to set a value, you use:

AC_DEFINE(MYDEFINE, 123)

This last will add -DMYDEFINE=123 to the flags (DEFS = in Makefile), and #define MYDEFINE 123 in the generated autoconf header if you use that.

I recommend you read the documentation from the beginning, and work through their examples and tutorials. Also check other projects' configure files to see how they use different features.

Edit: If you want to pass flags on the command line to the make command, then you do something like this:

libfoo_la_CXXFLAGS = $(MYFLAGS)

Then you call make like this:

$ make MYFLAGS="-DMYDEFINE"

If you don't set MYFLAGS on the command line, it will be undefined and empty in the makefile.

You can also set target-specific CPPFLAGS in Makefile.am, in which case the source files will be recompiled, once for each set of flags:

lib_LTLIBRARIES = libfoo.la libbar.la
libfoo_la_SOURCES = foo.c
libfoo_la_CPPFLAGS = -DFOO
libbar_la_SOURCES = foo.c
libbar_la_CPPFLAGS = -DBAR

Setting a bit

Use the bitwise OR operator (|) to set a bit.

number |= 1UL << n;

That will set the nth bit of number. n should be zero, if you want to set the 1st bit and so on upto n-1, if you want to set the nth bit.

Use 1ULL if number is wider than unsigned long; promotion of 1UL << n doesn't happen until after evaluating 1UL << n where it's undefined behaviour to shift by more than the width of a long. The same applies to all the rest of the examples.

Clearing a bit

Use the bitwise AND operator (&) to clear a bit.

number &= ~(1UL << n);

That will clear the nth bit of number. You must invert the bit string with the bitwise NOT operator (~), then AND it.

Toggling a bit

The XOR operator (^) can be used to toggle a bit.

number ^= 1UL << n;

That will toggle the nth bit of number.

Checking a bit

You didn't ask for this, but I might as well add it.

To check a bit, shift the number n to the right, then bitwise AND it:

bit = (number >> n) & 1U;

That will put the value of the nth bit of number into the variable bit.

Changing the nth bit to x

Setting the nth bit to either 1 or 0 can be achieved with the following on a 2's complement C++ implementation:

number ^= (-x ^ number) & (1UL << n);

Bit n will be set if x is 1, and cleared if x is 0. If x has some other value, you get garbage. x = !!x will booleanize it to 0 or 1.

To make this independent of 2's complement negation behaviour (where -1 has all bits set, unlike on a 1's complement or sign/magnitude C++ implementation), use unsigned negation.

number ^= (-(unsigned long)x ^ number) & (1UL << n);

unsigned long newbit = !!x;    // Also booleanize to force 0 or 1
number ^= (-newbit ^ number) & (1UL << n);

It's generally a good idea to use unsigned types for portable bit manipulation.

number = (number & ~(1UL << n)) | (x << n);

(number & ~(1UL << n)) will clear the nth bit and (x << n) will set the nth bit to x.

It's also generally a good idea to not to copy/paste code in general and so many people use preprocessor macros (like the community wiki answer further down) or some sort of encapsulation.

Hello World Library using autotools

Makefile.am:

lib_LIBRARIES = libhello.a
libhello_a_SOURCES = hello.c

configure.ac:

AC_INIT([libhello], [1.0], [bug@libhello.org])
AM_INIT_AUTOMAKE([-Wall -Werror foreign])
AC_PROG_CC
AC_PROG_RANLIB
AC_CONFIG_FILES([Makefile])
AC_OUTPUT

The documentation for building libraries with Automake is here.