Open, Write and Read ttyUSB0 - c++

Question

I am doing a program to open, write and read the port ttyUSB0, I have the next program and I don´t write anything. Can anyone help me please????
My question is that I have a problem with the write or read function, because I can´t read and write in the ttyUSB0 port, and I search a solution to write and read ttyUSB0 port.
#include <sys/time.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <termios.h>
#include <iostream>
#include <sys/types.h>
#include <sys/stat.h>
using namespace std;
int serial_open(char *serial_name, speed_t baud)
{
struct termios newtermios;
int fd;
fd = open(serial_name,O_RDWR | O_NOCTTY);
newtermios.c_cflag= CBAUD | CS8 | CLOCAL | CREAD;
newtermios.c_iflag=IGNPAR;
newtermios.c_oflag=0;
newtermios.c_lflag=0;
newtermios.c_cc[VMIN]=1;
newtermios.c_cc[VTIME]=0;
cfsetospeed(&newtermios,baud);
cfsetispeed(&newtermios,baud);
if (tcflush(fd,TCIFLUSH)==-1) return -1;
if (tcflush(fd,TCOFLUSH)==-1) return -1;
if (tcsetattr(fd,TCSANOW,&newtermios)==-1) return -1;
return fd;
}
void serial_send(int serial_fd, char *data, int size)
{
write(serial_fd, data, size);
}
int serial_read(int serial_fd, char *data, int size, int timeout_usec)
{
fd_set fds;
struct timeval timeout;
int count=0;
int ret;
int n;
do {
FD_ZERO(&fds);
FD_SET (serial_fd, &fds);
timeout.tv_sec = 0;
timeout.tv_usec = timeout_usec;
ret=select (FD_SETSIZE,&fds, NULL, NULL,&timeout);
if (ret==1) {
n=read (serial_fd, &data[count], size-count);
count+=n;
data[count]=0;
}
} while (count<size && ret==1);
return count;
}
void serial_close(int fd)
{
close(fd);
}
int main(int argc, char *argv[])
{
int serial_fd, n, longitud,;
char *device=”at”;
char *data;
longitud=strlen(device);
serial_fd = serial_open("/dev/ttyUSB0",B38400);
if (serial_fd == -1) {
printf ("Error opening the serial device: %s\n",argv[1]);
perror("OPEN");
exit(0);
}
printf("SERIAL OPEN:%s\n", device);
serial_send(serial_fd, device, longitud);
printf ("String sent------> %s\n",device);
n=serial_read(serial_fd,data,longitud,10000);
printf("Se ha recibido %s \n Tamaño: %d\n n:%d \n serial_fd:%d\n",data, longitud,n,serial_fd);
puts(data);
serial_close(serial_fd);
// cout << "Hello world!" << endl;
return 0;
}

Answer 1

Your program works just fine with the writing and reading, but you have another problem: Undefined behavior.
You have a pointer variable in your main function named data, and you pass this to your serial_read function. But nowhere do you make this pointer actually point anywhere, so when that pointer is dereferenced you have the undefined behavior.
Local (non-static) variables are not initialized, their value is indeterminate. You need to initialize the variable, to make it actually point somewhere. I recommend you make it into an array instead.
And then when you call serial_read you only ask to read two bytes (the length of the string "at"), instead of the actual size of the data (which currently is none), you need to pass the actual size of the buffer instead.
So to sum it up:
int main(void)
{
...
char data[256];
...
n = serial_read(serial_fd, data, sizeof(data), 10000);
...
}