12-27-2012
What they're talking about with the pointer is, whenever you create a thread, you are giving it a pointer to something. The function "called" by the new thread has that value passed into it -- that's why it takes any value at all, instead of being void.
It can be whatever you want, and is always there. If you're not passing data into it, you're probably just giving it NULL. If you wanted to pass each thread something different, be sure to not give them the same variable -- they'll all end up pointing to the same thing, they won't get unique values. To be different it needs to literally be different memory. You could give them different elements of an array, for example. Memory fresh from malloc() also works.
Of course, threads have their own unique ID's in the first place, so it's a bit redundant to give it your own unique number, but that's why this is an exercise.
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LEARN ABOUT SUNOS
pthread_key_create
pthread_key_create(3C) pthread_key_create(3C)
NAME
pthread_key_create - create thread-specific data key
SYNOPSIS
cc -mt [ flag... ] file... -lpthread [ library... ]
#include <pthread.h>
int pthread_key_create(pthread_key_t *key, void (*destructor, void*));
This function creates a thread-specific data key visible to all threads in the process. Key values provided by pthread_key_create() are
opaque objects used to locate thread-specific data. Although the same key value may be used by different threads, the values bound to the
key by pthread_setspecific() are maintained on a per-thread basis and persist for the life of the calling thread.
Upon key creation, the value NULL is associated with the new key in all active threads. Upon thread creation, the value NULL is associ-
ated with all defined keys in the new thread.
An optional destructor function may be associated with each key value. At thread exit, if a key value has a non-NULL destructor pointer,
and the thread has a non-NULL value associated with that key, the function pointed to is called with the current associated value as its
sole argument. Destructors can be called in any order.
If, after all the destructors have been called for all keys with non-NULL values, there are still some keys with non-NULL values, the
process will be repeated. If, after at least PTHREAD_DESTRUCTOR_ITERATIONS iterations of destructor calls for outstanding non-NULL values,
there are still some keys with non-NULL values, the process is continued, even though this might result in an infinite loop.
If successful, the pthread_key_create() function stores the newly created key value at *key and returns 0. Otherwise, an error number is
returned to indicate the error.
The pthread_key_create() function will fail if:
EAGAIN The system lacked the necessary resources to create another thread-specific data key, or the system-imposed limit on the
total number of keys per process PTHREAD_KEYS_MAX has been exceeded.
ENOMEM Insufficient memory exists to create the key.
The pthread_key_create() function will not return an error code of EINTR.
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Interface Stability |Standard |
+-----------------------------+-----------------------------+
|MT-Level |MT-Safe |
+-----------------------------+-----------------------------+
pthread_getspecific(3C), pthread_setspecific(3C), pthread_key_delete(3C), attributes(5), standards(5)
23 Mar 2005 pthread_key_create(3C)