pthread_t pointer as argument of pthread_create

Question

The first argument of pthread_create is a pthread_t pointer. In the hello program below, if the first argument is a pointer to pthread_t (pthread_t*) instead of a pthread_t (pthread_t) the program ends with Segmentation fault...why?

I don't remember seeing pthread_t* as the declared type of the first argument of pthread_create.
And chapter 2 of Butenhof's book Programming with POSIX Threads says:

To create a thread, you must declare a variable of type pthread_t [not pthread_t*].

But according to the specification the first argument of pthread_create is a pointer to pthread_t, so why the segmentation fault?

---

Segmentation fault

pthread_t* thr;
pthread_create(thr, NULL, &hello, NULL);

---

Runs OK

pthread_t thr;
pthread_t* pntr = &thr;
pthread_create(pntr, NULL, &hello, NULL);

---

hello program:

#include <pthread.h>
#include <stdio.h>

void * 
hello(void *arg){
  printf("Hello\n");
  pthread_exit(NULL);
}

int 
main(int argc, char **argv){
  pthread_t thr = 1;
  pthread_create(&thr, NULL, &hello, NULL);



  pthread_join(thr, NULL);

  return 0;
}

---

pthread_create prototype:

int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
void *(*start_routine)(void*), void *arg);

Solution 1

pthread_t* thr;
pthread_create(thr, NULL, &hello, NULL);

declares a pointer to a pthread_t without allocating storage for it. When you call pthread_create, it'll try writing to *thr. This is at an undefined location and will almost certainly fail.

pthread_t thr;
pthread_t* pntr = &thr;
pthread_create(pntr, NULL, &hello, NULL);

works because you've declare storage (thr on the stack) for a pthread_t.

Note that the second, working, version can be simplified to what is used in your hello program

pthread_t thr;
pthread_create(&thr, NULL, &hello, NULL);

...which declares a pthread_t on the stack then passes a pointer to it into pthread_create.

Solution 2

It is because if you simply declare a pointer, you can't expect it to point to allocated, initialised memory.

When you instead declare a pthread_t it gets allocated its own little block of memory that you can then get the address to with the & operator and pass it to pthread_create.

Comments

[user454322]

See the difference between defined and declared in stackoverflow.com/questions/1433204/…

[user454322]

Ohhh... It is because & allocates space! I couldn't spot such a silly thing. Thanks

[user454322]

Note that the second, working, version can be simplified to as is in the hello program.

[simonc]

Oh, yes, I'm afraid I hadn't read that far down the question. I'll update my answer

[user454322]

Right. Without & the pointer is not initialized. Thank you.

[Admin]

The & operator takes the address of the pthread_t as a pointer of type pthread_t * and this can then be used to call pthread_create. It doesn't initialise the pointer so much as simply create a valid pointer to an allocated area of memory.

[user454322]

Perhaps I wasn't clear. With initializing the pointer, I meant taking the address of (point to, i.e., assigning the address of thr to pntr) the memory where the pthread_t refers. Note that: just between pthread_t* pntr = &thr; and the call to pthread_create pntr is pointing to UNallocated memory (because thr has not been initialized).

[Admin]

pntr is pointing to allocated memory because it points to &thr; thr could very well be uninitialised, though.

[user454322]

I see your point. Then pntr points to &thr which in turn points to unallocated memory... rigth?

[Admin]

&thr is the address of (pointer to the data of) thr, which is allocated memory on the stack (automatically allocated for you) created by the variable declaration pthread_t thr;. However, because you are merely declaring thr, it is not initialised which means that both pntr and thr are allocated but only pntr is initialised: It contains the value &thr. In the example that works, nothing is pointing to unallocated memory, which is exactly why it works.

[user454322]

Thanks for the followup. With thr, which is allocated memory on the stack (automatically allocated for you) created by the variable declaration pthread_t thr do you mean that the solely pthread_t thr allocates memory? Before pthread_create what thr is pointing to?