Defined in header <threads.h>
int tss_set( tss_t tss_id, void *val );
(since C11)

Sets the value of the thread-specific storage identified by tss_id for the current thread to val. Different threads may set different values to the same key.

The destructor, if available, is not invoked.


tss_id - thread-specific storage key, obtained from tss_create and not deleted by tss_delete
val - value to set thread-specific storage to

Return value

thrd_success if successful, thrd_error otherwise.


The POSIX equivalent of this function is pthread_setspecific.

Typically TSS is used to store pointers to blocks of dynamically allocated memory that have been reserved for use by the calling thread.

tss_set may be called in the TSS destructor. If the destructor exits with non-NULL value in the TSS storage, it will be retried by thrd_exit up to TSS_DTOR_ITERATIONS times, after which the storage will be lost.


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_set function (p: 282-283)
  • C11 standard (ISO/IEC 9899:2011):
    • The tss_set function (p: 387)

See also

reads from thread-specific storage

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