add "extern C" as a compiler option for a symbol?

Question

I'm working with FIPS Capable OpenSSL. The source code is sequestered and cannot be changed.

To link to the static version of the OpenSSL library, all we need to do is:

export FIPS_SIG=`find /usr/local/ssl -iname incore`
export CC=`find /usr/local/ssl -iname fipsld`
export FIPSLS_CC=`find /usr/bin -iname gcc`

Then, simply perform:

$CC $CFLAGS <sources> -o myprogram <openssl libs>

The reasons for the gyration is OpenSSL will insert an additional source file - fips_premain.c - and compile it with the program sources. (Some additional steps occur, but the compilation of fips_premain.c is the relevant step).

However, when using g++, a couple of symbols are undefined because they were compiled with the C compiler when OpenSSL was installed, and g++ cannot find them when invoked as above:

/tmp/fips_premain-20db15.o: In function `FINGERPRINT_premain()':
/usr/local/ssl/fips-2.0/lib/fips_premain.c:103: undefined reference to `FIPS_text_start()'
/usr/local/ssl/fips-2.0/lib/fips_premain.c:116: undefined reference to `FIPS_incore_fingerprint(unsigned char*, unsigned int)'

If I add the --no-demangle linker option, here's what is output:

/tmp/fips_premain-be4611.o: In function `_Z19FINGERPRINT_premainv':
/usr/local/ssl/fips-2.0/lib/fips_premain.c:103: undefined reference to `_Z15FIPS_text_startv'
/usr/local/ssl/fips-2.0/lib/fips_premain.c:116: undefined reference to `_Z23FIPS_incore_fingerprintPhj'

Here are the lines of interest in fips_premain.c (around line 85):

extern const void         *FIPS_text_start(),  *FIPS_text_end();
extern const unsigned char FIPS_rodata_start[], FIPS_rodata_end[];
extern unsigned char       FIPS_signature[20];
extern unsigned int        FIPS_incore_fingerprint(unsigned char *,unsigned int);

Is there a way to mark a symbol as extern "C" from the command line?

Answer 1

Supplying the -x c switch with g++ before the file on the compilation command line will force it to be compiled as a C file instead of the default C++.

From the Fipsld and C++ page:

Open fipsld++ and find occurrences where fips_premain.c is compiled. It is compiled through the variable {PREMAIN_C} ...

Change the lines so that -x c preceeds "${PREMAIN_C}", and -x none follows it.

${CC}  ${CANISTER_O_CMD:+"${CANISTER_O_CMD}"} \
    -x c "${PREMAIN_C}" -x none \
    ${_WL_PREMAIN} "$@"

Forcing C compilation on fips_premain.c applies to all configurations (libraries, shared objects, and executables), so you are safe to apply it to all sections of fipsld++.