return address of dlsym and Address of Function Pointer assigned

Question

void* l = dlsym(lib,"_ZN11Environment9LibLogger14log_processingEiNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEjS6_z");
*(void **)&log_fcn   = l;
std::cout<<"Address"<<<<l<<"LOG_FCN "<<log_fcn<<std::endl;

I am trying to print address of l and log_fcn but both are not same. Why is that, and how can I get address of dlsym assign to function pointer?

Output example:

l= 0x7efe10eabaa0 LOG_FCN 1

void (*log_fcn)(int level, std::string frmt, unsigned int line_no, std::string file_name, ...); function decleration

@KonradRudolph void (*log_fcn)(int level, std::string frmt, unsigned int line_no, std::string file_name, ...); — M N Adarsh Kumar
– M N Adarsh Kumar, Commented May 10, 2022 at 10:06
(void*)log_fcn? the stream seems to convert it to bool instead of pointer otherwise — numzero
– numzero, Commented May 10, 2022 at 10:10
The compiler tries to convert log_fcn to something that can be printed, and because function pointers are not convertible to void*, the only valid match is bool — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented May 10, 2022 at 10:11
You should convert l to decltype(log_fcn), not assign to log_fcn through a void* lvalue. The latter is an aliasing violation. — user17732522
– user17732522, Commented May 10, 2022 at 10:13

molbdnilo · Accepted Answer · 2022-05-10 10:32:58Z

3

There is an operator << for void*, but not for function pointers.
Function pointers are implicitly converted to bool, not void*, and bool prints as 0 or 1 by default.

Your 1 output indicates that log_fcn is not the null pointer.

Convert it when printing:

std::cout << "Address" << l << "LOG_FCN "<< reinterpret_cast<void*>(log_fcn) << std::endl;

I would recommend that you also do the assignment in the conventional form, by casting the right-hand side:

log_fcn = reinterpret_cast<decltype(log_fcn)>(l);

edited May 10, 2022 at 10:32

answered May 10, 2022 at 10:13

molbdnilo

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4 Comments

numzero Over a year ago

The unnatural casting on the left side comes from the dlsym manual page. It states it’s the workaround for that in C99, casting void* to a function pointer is undefined.

molbdnilo Over a year ago

This is C++, not C99.

numzero Over a year ago

Well, yes. In C++ rules are stricter. Don’t remember if there is any standard-conforming way in C++.

user17732522 Over a year ago

@numzero reinterpret_cast<decltype(log_fcn)>(l) and decltype(log_fcn)(l) are standard-conforming in C++, although conditionally-supported.

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