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From: Benjamin Peterson <benjamin@python.org>
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Subject: value of thread-specific data immediately after initialization
Date: Thu, 13 Sep 2018 13:39:38 -0700
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POSIX says that after the creation of a thread-specific key, its associated value should be NULL in all threads. musl may not uphold this requirement if a particular key is deleted and later resued.

Here's an example of the bug:

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

int main() {
	pthread_key_t k;
	pthread_key_create(&k, NULL);
	printf("%p\n", pthread_getspecific(k));
	pthread_setspecific(k, &k);
	pthread_key_delete(k);
	pthread_key_create(&k, NULL);
	printf("%p\n", pthread_getspecific(k));
	pthread_key_delete(k);
	return 0;
}

Per POSIX, I would expect this testcase to output two NULLs. However, musl prints the address of the old data even after the second initialization:

$ musl-gcc -o test test.c
$ ./test 
0
0x7fff2ba3d004