c - Why and in what sense is pthread_t an opaque type? -

posts here on suggest pthread_t opaque type, not number, not thread index, shouldn't directly compare pthread_t's, etc. etc.

questions:

1. why? there intent support systems no numeric ids threads? when pthread_t implementation

   typedef unsigned long int pthread_t; 

   ?

2. how? there's comment before above line, it's actually

   /* thread identifiers. structure of attribute type not    exposed on purpose.  */ typedef unsigned long int pthread_t; 

   in pthreadtypes.h mean? attribute type? isn't index global table of threads?

is there intent support systems no numeric ids threads?

there different types serve numeric thread identifier. example, on systems limited resources 8-bit thread identifier used instead of unsigned long.

the structure of attribute type not exposed on purpose.

the comment not pthread_t definition, pthread_attr_t definition 1 line below:

typedef union {   char __size[__sizeof_pthread_attr_t];   long int __align; } pthread_attr_t; 

the comment states char __size[__sizeof_pthread_attr_t] used in order hide content of actual struct.

isn't [pthread_t] index global table of threads?

it not have be. fact actual type hidden allows implementer use type wishes, including pointer or struct. using struct lets implementer avoid using global table of threads in code of library (os keep such table, though).