Defined in header <threads.h>
int tss_create( tss_t* tss_key, tss_dtor_t destructor );
(since C11)

Creates new thread-specific storage key and stores it in the object pointed to by tss_key. Although the same key value may be used by different threads, the values bound to the key by tss_set are maintained on a per-thread basis and persist for the life of the calling thread.

The value NULL is associated with the newly created key in all existing threads, and upon thread creation, the values associated with all TSS keys is initialized to NULL.

If destructor is not a null pointer, then also associates the destructor which is called when the storage is released by thrd_exit (but not by tss_delete and not at program termination by exit).

A call to tss_create from within a thread-specific storage destructor results in undefined behavior.


tss_key - pointer to memory location to store the new thread-specific storage key
destructor - pointer to a function to call at thread exit


The POSIX equivalent of this function is pthread_key_create.

Return value

thrd_success if successful, thrd_error otherwise.


int thread_func(void *arg) {
    tss_t key;
    if (thrd_success == tss_create(&key, free)) {
        tss_set(key, malloc(4)); // stores a pointer on TSS
        // ...
} // calls free() for the pointer stored on TSS


  • C17 standard (ISO/IEC 9899:2018):
    • The tss_create function (p: 281-282)
  • C11 standard (ISO/IEC 9899:2011):
    • The tss_create function (p: 386)

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