XS is Perl-aware glue, and as such, Perl redefines certain functions to Perl-aware functions.

I'd start by separating your Perl-incompatible stuff (non-Perl threads, and the non-Perl memory allocation) from XS. A separate file would be good, but the following might suffice. 

#include <pthread.h>

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

void test_thread(const char *msg) {
    char *thread_arg = malloc((strlen(msg)+1)*sizeof(char));
    strcpy(thread_arg, msg);
    pthread_t tid;
    pthread_create(&tid, NULL, thread, (void*)thread_arg);
        
    void *rv;
    pthread_join(tid, &rv);
}

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include "ppport.h"

MODULE = My    PACKAGE = My

void
test_thread(const char *msg)

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Note that mid-block variable declarations (like thread_arg, tid and rv in your code, and the latter two in mine) aren't legal C89 which Perl supports. Of course, it doesn't mean you have to support it.

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

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