g++ cannot find symbols when linking - gcc

Question

OK, this is probably very basic, but I can't find the problem. I'm trying to link my C++ program with a C shared library (YAJL to name it) with -lyajl ; the linker seems to find it (it does not complain for that) but cannot find the symbols I use in my code :
$> g++ test.c -lyajl
Undefined symbols for architecture x86_64:
"yajl_tree_parse(char const*, char*, unsigned long)", referenced from:
_main in ccEqsAaJ.o
"yajl_tree_get(yajl_val_s*, char const**, yajl_type)", referenced from:
_main in ccEqsAaJ.o
"yajl_tree_free(yajl_val_s*)", referenced from:
_main in ccEqsAaJ.o
ld: symbol(s) not found for architecture x86_64
collect2: ld returned 1 exit status
$>
I made a minimalist test.c file to isolate the behavior. Well, what's very strange is that gcc seems perfectly OK with the code (as, incidentally, test.c is just C code, no C++ feature here):
$> gcc test.c -lyajl
$>
What is gcc doing that g++ doesn't seem to be able to do right ?

Answer 1

Could it be that the symbols in libyajl.so are C linkage, but your code (or the header file you are using) does not declare them extern "C"? In that case, compiling your code as C++ would result in name-mangled symbols in your object file, which would not match the (C linkage, unmangled) symbols in the library.
The solution would be to write, in the header file, something like this:
#ifdef __cplusplus
// "__cplusplus" is defined whenever it's a C++ compiler,
// not a C compiler, that is doing the compiling.
extern "C" {
#endif
// Exchange "void" with the real return type for these functions
void yajl_tree_parse(char const*, char*, unsigned long);
void yajl_tree_get(yajl_val_s*, char const**, yajl_type);
void yajl_tree_free(yajl_val_s*);
#ifdef __cplusplus
}
#endif

Answer 2

If extern "C" does not fix it, try this.
I had the same problem trying to link against the static library- libyajl_s.a.
It is possible to work around the problem by compiling the yajl source into the executable,
but I really needed the static library.
On closer inspection of the yajl source you will see that the source is looking for headers in two separate folders, e.g.
#include <api/yajl_tree.h>
#include <api/yajl_parse.h>
#include "yajl_parser.h"
The linker seems to resolve yajl_tree_parse, yajl_tree_get and yajl_tree_free to their prototypes in yajl_tree.h and not to their definitions in yajl_tree.c. In other words- they remain undefined in the resulting library.
The solution was to move all headers from the api folder into the src folder and remove all references to <api/...>. The linker then picked up the yajl_tree_xxx function definitions correctly.