If you do this on win32 I don't get this "double free" error 
#include <pthread.h>

void *thread(void *arg) {
    char *msg = (char*)arg;
    printf("thread: %s\n", msg);
    free(msg);
    return NULL;
}

#include "EXTERN.h"
...

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In reply to Re^5: XS: free() outside of the main thread causes crash on Windows by Anonymous Monk
in thread XS: free() outside of the main thread causes crash on Windows by OlegG

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