Question

I want to get the stack address of a thread through some function to which we can pass pthread_self(). Is it possible? The reason I am doing this is because I want to write my own assigned thread identifier for a thread somewhere in its stack. I can write near the end of the stack (end of the stack memory and not the current stack address. We can ofcourse expect the application to not get to the bottom of the stack and therefore use space from there).

In other words, I want to use the thread stack for putting a kind of thread local variable there. So, do we have some function like the following provided by pthread?

stack_address = stack_address_for_thread( pthread_self() );

I can use the syntax for thread local variables by gcc for this purpose, but I’m in a situation where I can’t use them.

Answer 1

First get the bottom of the stack and give read/write permission to it with the following code.

pthread_attr_t attr;
void * stackaddr;
int * plocal_var;
size_t stacksize;

pthread_getattr_np(pthread_self(), &attr);
pthread_attr_getstack( &attr, &stackaddr, &stacksize );

printf( "stackaddr = %p, stacksize = %d\n", stackaddr, stacksize );

plocal_var = (int*)mmap( stackaddr, 4096, PROT_READ | PROT_WRITE, 
          MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0 );
// Now try to write something 
*plocal_var = 4;

and then you can get the thread ID, with the function get_thread_id() shown below. Note that calling mmap with size 4096 has the effect of pushing the boundary of the stack by 4096, that is why we subtract 4096 when getting the local variable address.

int get_thread_id()
{
    pthread_attr_t attr;
    char * stackaddr;
    int * plocal_var;
    size_t stacksize;

    pthread_getattr_np(pthread_self(), &attr);
    pthread_attr_getstack( &attr, (void**)&stackaddr, &stacksize );

    //printf( "stackaddr = %p, stacksize = %d\n", stackaddr, stacksize );

    plocal_var = (int*)(stackaddr - 4096);

    return *plocal_var;
}