Serial IO with posix and write to shared memory - c++

Question

I got a problem with a posix signal, that I use for reading serial data. First here is a small example code how I initialize my serial port, my signal and a shared memory.
#include <unistd.h>
#include <termios.h>
#include <sys/signal.h>
#include <stdint.h>
#include <cassert>
#include <cstdio>
#include <fcntl.h>
#include <iostream>
#include <QSharedMemory>
int fd;
struct termios termAttr;
struct sigaction saio;
char buff[255];
QSharedMemory QShare("IDSTRINGOFTHISMEMORY");
void signal_handler_IO (int status)
{
std::cout << "Got Serial Data!" << std::endl;
int si = read(fd, &buff[0], 255);
if(true) // Works with false here, but then the shared memory ist not used!
{
char TWrite = 't';
QShare.lock();
char *to = (char*)QShare.data();
const char *from = reinterpret_cast<char*>(&TWrite);
memcpy(to, from, 1);
QShare.unlock();
}
}
int main()
{
// Create SharedMemory:
QShare.create(1);
fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NDELAY);
if (fd == -1)
{
return -1;
}
saio.sa_handler = signal_handler_IO;
saio.sa_flags = 0;
saio.sa_restorer = NULL;
sigemptyset(&saio.sa_mask);
sigaddset(&saio.sa_mask, SIGINT);
sigaction(SIGIO,&saio,NULL);
fcntl(fd, F_SETFL, FNDELAY|FASYNC);
fcntl(fd, F_SETOWN, getpid());
tcgetattr(fd,&termAttr);
cfsetospeed(&termAttr,B57600);
cfsetispeed(&termAttr,B57600);
termAttr.c_cflag &= ~PARENB;
termAttr.c_cflag &= ~CSTOPB;
termAttr.c_cflag &= ~CSIZE;
termAttr.c_cflag |= CS8;
termAttr.c_cflag |= (CLOCAL | CREAD);
termAttr.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
termAttr.c_iflag &= ~(IXON | IXOFF | IXANY);
termAttr.c_oflag &= ~OPOST;
termAttr.c_cc[VMIN] = 5;
tcsetattr(fd,TCSANOW,&termAttr);
while(true)
{
usleep(1000000);
}
return 0;
}
I had this code in use for a while and it did work quite well. Now for a different project I want to store the serial data in shared memory block, so I can read the current datapoint from multiple processes. In the signal_handler_IO I try to write the data to a shared memory that is created by sys/shm.h. The moment I started using shared memory the posix signal stopped working. Since I was unsure about the use of the shared memory I also tried to use the QSharedMemory class from qt, which created the same result.
Can you tell me, what the shared memory does to my posix signal?