VMS Help POSIX Threads, PTHREAD routines, pthread_cancel, Return Values  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    If an error condition occurs, this routine returns an integer
    indicating the type of error. Possible return values are as
    follows:

    Return      Description

    0           Successful completion.
    [EINVAL]    The specified thread is invalid.
    [ESRCH]     The thread argument does not specify an existing
                thread.