Why PTHREAD_COND_INITIALIZER may be used to initialize a condition variable only when it is declared?

Question

Since the PTHREAD_COND_INITIALIZER is actually a structure initializer, it may be used to initialize a condition variable only when it is declared.

From: Multi-Threaded Programming With POSIX Threads

Question: Couldn't understand the above quote.
It is just a macro, why can't I use it to initialize the condition variable on run time?
What has its being a structure initializer to do with anything?

Solution

Because it is a structure initializer, you cannot use it to init the structure in a statement apart from its declaration.

It is defined on my system like so:

#define PTHREAD_COND_INITIALIZER {_PTHREAD_COND_SIG_init, {0}}

Expanded and used, we see:

pthread_cond_t p = PTHREAD_COND_INITIALIZER; // << ok!
p = PTHREAD_COND_INITIALIZER; // << compiler error =\

That is,

p = PTHREAD_COND_INITIALIZER;

expands to:

p = {_PTHREAD_COND_SIG_init, {0}};

Comments

[Aquarius_Girl]

Since you've shown the code, I understand the meaning. Thanks, didn't know the "term" structure initializer. Very helpful.

[justin]

@AnishaKaul you're welcome. note also that you could reinitialize it using C99 compound literals, provided of course it either failed initialization or has been destroyed, and your compiler supports C99 compound literals (not always C++ friendly). Here's the form: p = (pthread_cond_t)PTHREAD_COND_INITIALIZER; -- which expands to p = (pthread_cond_t){_PTHREAD_COND_SIG_init, {0}};.