hb.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>

int main( int argc, char * argv[] )
{
    unsigned char code[] = "happy birthday";

    void * mem = mmap(
        NULL,
        sizeof( code ),
        PROT_WRITE | PROT_EXEC,
        MAP_ANON | MAP_PRIVATE,
        -1,
        0
    );

    memcpy( mem, code, sizeof( code ) );

    void ( * hb )() = mem;

    // let's see what that's gonna do
    // (not that I expect anything good)
    hb();

    return EXIT_SUCCESS;
}