共享内存，如何从另一个cpp文件访问

Question

What I have done so far:

main.cpp

using namespace std;

bool *failOver = static_cast<bool*>(mmap(0,sizeof(failOver),
                                    PROT_READ|PROT_WRITE,
                                    MAP_SHARED|MAP_ANONYMOUS,-1,0));

int main()
{
   call of function from function.cpp.
}

function.cpp

extern bool* failOver;

function test()
{
    //sem_wait(shared sem)
    modify failOver;

    //sem_post(shared sem)
}

If I try to compile it, it returns an error: "cannot convert bool to bool* in assignment". Also accessing with std::failOver does not work.

How do I access the shared memory variable from other files?

Answer 1

You are mixing together a few different problems.

1- bool* failOver is not a bool , it is a pointer to bool ; in other words, failOver represents the memory address where the bool you are interested in is stored. If you wanna read the bool value pointed by failOver , you have to *dereference` the pointer, ie

bool my_value = *failOver;

So, I don't know how mmap works, but if it returns a void* , you may want to cast that to bool* and then to dereference it. Furthermore, I think you don't want to read sizeof(failOver) bytes (ie sizeof(bool*) bytes), but sizeof(bool) bytes.

2- You have to define your variable (ie making room for it in memory) if you want it to be found by another translation unit. To define it, you have to initialize it in the global scope :

bool* failOver = 0;

Then, in the other file you can declare it extern and use it.

Answer 2

In main.cpp it should be

bool *failOver = static_cast<bool*>(mmap(0,sizeof(*failOver),PROT_READ|PROT_WRITE,MAP_SHARED|MAP_ANONYMOUS,-1,0));

the size was wrong. In function.cpp

extern bool* failOver;

function test()
{
    //sem_wait(shared sem)
    *failOver = false;
    //sem_post(shared sem)
}

Is it as simple as you missing the * in *failOver ? Reading Paolo latest answer he's come to the same conclusions.