char *p = ....; /* shared variable */
pthread_mutex_t lp ; /* protects ‘p’  but not len!!!*/

int getplen() {
    pthread_mutex_lock (&lp);
    int len = strlen(p);
    pthread_mutex_unlock (&lp);
    return len;
}

int some_function() {
    int nchars = getplen(); // obtain length 
    char *copy = malloc(nchars + 1); // commit to allocate memory

    ------
    
    pthread_mutex_lock (&lp);
    strcpy(copy, p); 
    pthread_mutex_unlock (&lp);
}