C++ 11 thread and pointer parameter

I want to print strings with contents 1,2,3,4,5 using 5 threads, My working environment:

OS version : CentOS Linux release 7.0.1406 (Core)

g++ version: g++ (GCC) 4.8.2 20140120 (Red Hat 4.8.2-16)

compile cmd: g++ -o thread_sz -std=c++11 -pthread thread_sz.cpp

// file name: thread_sz.cpp
#include <iostream>
#include <string>
#include <thread>

#include <unistd.h>

using namespace std;

void myFunc1(char* sz)
{
    printf("sz = %s\n", sz);
}

void myFunc2(const char* const sz)
{
    printf("const char* const sz = %s\n", sz);
}

void myFunc3(string str)
{
    printf("string str = %s\n", str.c_str());
}

int main(int argc, char* argv[])
{
    for(int i = 1; i <= 5; i++) {
        char sz[16];
        sprintf(sz, "%2d", i);
        const string str(sz);

        std::thread t1(myFunc1, sz);
        std::thread t2(myFunc2, (const char* const)str.c_str());
        std::thread t3(myFunc3, str);

        t1.detach();
        t2.detach();
        t3.detach();
    }
    usleep(1000000); // sleep 1 second
    return 0;
}

Output something like:

const char* const sz =  1
sz =  3
const char* const sz =  2
string str =  3
sz =  4
sz =  4
string str =  2
sz =  5
const char* const sz =  4
sz =  5
string str =  1
const char* const sz =  5
string str =  4
string str =  5
const char* const sz =  5

I run the program for lots of times, and the result seems:

string str output: 100% correct
sz output: ~10% correct
const char* const sz output: ~50% correct

My questions are:

1, Why MyFunc1 and MyFunc2 are not 100% correct, what's the differences btw them. How to fix it.

2, Can MyFunc3 work 100% correctly.

3, If I comment out this line usleep(1000000); // sleep 1 second MyFunc1 may output empty value like sz =, Why?

Thank you very much.