Pre-defined preprocessor definitions (GCC)

The predef project is extremely convenient for looking up architecture-dependent, os-dependent, compiler-dependent, or standard C/C++ compiler macros, but it is not available when working offline.

To get around this, the preprocessor definitions defined automatically by GCC can be viewed with this command line:

bash$ gcc -dM < /dev/null

#define __DBL_MIN_EXP__ (-1021)
#define __UINT_LEAST16_MAX__ 65535
#define __FLT_MIN__ 1.17549435082228750797
...


This will output the #define commands as they would be executed by the preprocessor. To get a list of the non-expanded macros, use -dN :

bash$ gcc -dN < /dev/null
# 1 ""
# 1 ""
#define __STDC__
#define __STDC_HOSTED__
#define __GNUC__
#define __GNUC_MINOR__
...

This is a bit more noisy, as undefined macros (e.g. "# 1") are included, but may be more suitable for parsing.

Linking static libraries into a shared library

The ability to link static libraries into a shared library -- e.g.

gcc -shared -o libstuff.so lib1.a lib2.a lib3.a
-- disappeared in one of the 3.x series. This broke my tendency to build applications as static libraries, linked into a shared library, linked to by an executable consisting of little more than a main().

Turns out all that was missing was a GNU ld flag to prevent ld from optimizing the unused object code out of the shared library.

The solution is to wrap the .a files in --whole-archive flags, e.g.

gcc -shared -o libstuff.so -Wl,--whole-archive lib1.a lib2.a lib3.a -Wl,--no-whole-archive