Read 0 bytes after writing to /dev/ttyS0 - c++

Question

I have been trying to perform serial communication on linux via the /dev/ttyS devices but when I try to read from them after writing I read no data.
I have the following code
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <termios.h>
#include <unistd.h>
int main() {
printf("hello world\n");
int n;
int fd;
char c;
int bytes;
char buffer[10];
char *bufptr;
int nbytes;
int tries;
int x;
struct termios options;
fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY);
if(fd == -1) {
perror("open_port: Unable to open:");
} else
tcgetattr(fd, &options);
// Set the baudrate, same speed for both I/O
cfsetispeed(&options, B150);
cfsetospeed(&options, B150);
// Enable reading
options.c_cflag |= (CLOCAL | CREAD);
// Set 'RAW' mode
cfmakeraw(&options);
// Set byte size
options.c_cflag &= ~CSIZE;
options.c_cflag |= CS8;
// Set parity
// options.c_cflag &= ~PARENB;
options.c_cflag |= PARENB;
options.c_cflag |= PARODD;
options.c_cflag &= ~CSTOPB;
// Set StopBits, #Linux no OneHalf is supproted, so OneHalf and Two are the same
options.c_cflag &= ~CSTOPB;
// Set Handshake options
// options.c_iflag |= CRTSCTS;
// options.c_iflag &= ~CRTSCTS;
// options.c_cflag &= ~( IXON | IXOFF | IXANY );
options.c_cflag |= IXON | IXOFF | IXANY;
// Set Timeouts
options.c_cc[VMIN] = 0; // read() will return after receiving character
options.c_cc[VTIME] = 10; // == 0 - infinite timeout, != 0 - sets timeout in deciseconds
tcsetattr(fd, TCSANOW, &options);
tcflush(fd, TCIOFLUSH);
bytes = write(fd, "ATZ\r",4);
printf(" wrote %d bytes\n", bytes);
bufptr = buffer;
bytes = read(fd, bufptr, sizeof(buffer));
printf("number of bytes read is %d\n", bytes);
perror ("read error:");
for (x = 0; x < 10 ; x++) {
c = buffer[x];
printf("%d ",c);
}
tcflush(fd, TCIOFLUSH);
close(fd);
printf("\n");
return (0);
}
The program output is as follows
hello world
wrote 4 bytes
number of bytes read is 0
read error:: Success
0 0 0 0 0 0 0 0 0 0
Although I expected it to read the 4 characters I just wrote it seems that read reads 0 bytes. In case i put VTIME to 0 then read blocks forever. I have tried to do echo /dev/ttyS0 but no output comes out. Any idea what might cause this and how can it be fixed?

Answer 1

Your code apparently is ok, except for the following facts:
You call perror("read error"); after calling printf(3), and not right after read(2) so the possible error (if happened) is masked to the call to read(2) by the call to printf(3). If you want to print the number of read characters, save the value of errno and the returned value from read(2) before calling printf(3), and then, if the returned error is neg, then call perror(3).
Anyway. The c_cc[VTIME] = 10 imposes a one second timeout, and this is too sort for resetting a modem. Your line settings are:
CS8, Parity ODD, one STOP bit, and 150 baudrate
Normally, modems answer to ATZ\r command after resetting, which takes some time (frequently more than a second) and in the default modem speed (because you have reset it) and not at the speed at which you have send the AT command.
Resetting a modem is, for this reason, done blindly normally, and then you send a simple AT\r command, in order to ask for an \r\nOK\r\n response. The answer to AT\r command is normally immediate, not as the response to a reset command, and lets the modem adjust its communication settings according to the received chars.
Modems always adapt their speed when the A and T sequence is received, by sampling the square pulses received at a high sampling frequency, then they switch speed to the detected and normally do speed conversion (making it possible to talk to the modem at a different speed than the negotiated remotely)