pointer get wrong value in different thread - c++

Question

I am writing a piece of code to demonstrate the multi-threading share memory writing.
However, my code gets a strange 0xffffffff pointer I can't make out why. I haven't been writing cpp code for a while. please let me know if I get something wrong.
I compile with the command:
g++ --std=c++11 shared_mem_multi_write.cpp -lpthread -g
I get error echoes like:
function base_ptr: 0x5eebff, src_ptr: 0x7f21a9c4e010, size: 6220800
function base_ptr: 0xffffffffffffffff, src_ptr: 0x7f21a9c4e010, size: 6220800
function base_ptr: 0xbdd7ff, src_ptr: 0x7f21a9c4e010, size: 6220800
function base_ptr: 0x23987ff, src_ptr: 0x7f21a9c4e010, size: 6220800
function base_ptr: 0x11cc3ff, src_ptr: 0x7f21a9c4e010, size: 6220800
function base_ptr: 0x17bafff, src_ptr: 0x7f21a9c4e010, size: 6220800
function base_ptr: 0x1da9bff, src_ptr: 0x7f21a9c4e010, size: 6220800
Segmentation fault (core dumped)
my os is CentOS Linux release 7.6.1810 (Core) gcc version 4.8.5 and the code is posted below:
#include <chrono>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iostream>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <thread>
#include <vector>
#include <memory>
const size_t THREAD_CNT = 40;
const size_t FRAME_SIZE = 1920 * 1080 * 3;
const size_t SEG_SIZE = FRAME_SIZE * THREAD_CNT;
void func(char *base_ptr, char *src_ptr, size_t size)
{
printf("function base_ptr: %p, src_ptr: %p, size: %u\n", base_ptr, src_ptr, size);
while (1)
{
auto now = std::chrono::system_clock::now();
memcpy(base_ptr, src_ptr, size);
std::chrono::system_clock::time_point next_ts =
now + std::chrono::milliseconds(42); // 24 frame per seconds => 42 ms per frame
std::this_thread::sleep_until(next_ts);
}
}
int main(int argc, char **argv)
{
int shmkey = 666;
int shmid;
shmid = shmget(shmkey, SEG_SIZE, IPC_CREAT);
char *src_ptr = new char[FRAME_SIZE];
char *shmpointer = static_cast<char *>(shmat(shmid, nullptr, 0));
std::vector<std::shared_ptr<std::thread>> t_vec;
t_vec.reserve(THREAD_CNT);
for (int i = 0; i < THREAD_CNT; ++i)
{
//t_vec[i] = std::thread(func, i * FRAME_SIZE + shmpointer, src_ptr, FRAME_SIZE);
t_vec[i] = std::make_shared<std::thread>(func, i * FRAME_SIZE + shmpointer, src_ptr, FRAME_SIZE);
}
for (auto &&t : t_vec)
{
t->join();
}
return 0;
}

Answer 1

You forgot specify access rights for created SHM segment (http://man7.org/linux/man-pages/man2/shmget.2.html):
The value shmflg is composed of:
...
In addition to the above flags, the least significant 9 bits of shmflg specify the permissions granted to the owner, group, and others. These bits have the same format, and the same meaning, as the mode argument of open(2). Presently, execute permissions are not used by the system.
Change
shmid = shmget(shmkey, SEG_SIZE, IPC_CREAT);
into
shmid = shmget(shmkey, SEG_SIZE, IPC_CREAT | 0666);
It works for me now: https://wandbox.org/permlink/Am4r2GBvM7kSmpdO
Note that I use only a vector of threads (no shared pointers), as other suggested in comments. You can possibly reserve its space as well.

Answer 2

You forget one very important thing: Error handling!
Both the shmget and shmat functions can fail. If they fail they return the value -1.
Now if you look at the first base_ptr value, it's 0x5eebff. That just happens to be the same as FRAME_SIZE - 1 (FRAME_SIZE is 0x5eec00). That means shmat do return -1, and has failed.
Since you keep on using this erroneous value, all bets are off.
You need to check for errors, and if that happens print the value of errno to find out what have gone wrong:
void* ptr = shmat(shmid, nullptr, 0);
if (ptr == (void*) -1)
{
std::cout << "Error getting shared memory: " << std::strerror(errno) << '\n';
return EXIT_FAILURE;
}
Do something similar for shmget.
Now it's also easy to understand the 0xffffffffffffffff value. It's the two's complement hexadecimal notation for -1, and it's passed to the first thread that is created.