accessing robot via Serial port - c

Question

I have try to access robot via serial port using gcc (i using mac).
I have made my program to send simple command:
HOME (enter) and there are some feedback from the robot: THIS IS THE
HOMING SEQUENCE COMMAND. THE ROBOT WILL SEEK ITS HOMING SWITCHES ON
EACH AXIS AND WILL EXECUTE THE HOMING FUNCTION AT THAT LOCATION. DO
YOU WANT TO PROCEED (Y/N) ?
and I send
Y(enter)
and the robot suppose move.
To access robot now is using modem/terminal Zterm.
This modem use baudrate 38400, 8N1
I use the same baudrate and 8n1
This is my code, I do not know what is wrong why my code cannot make the robot move
Thank you
Daniel
#include<stdio.h> /* Standard input/output definitions */
#include<stdlib.h>
#include<string.h> /* String function definitions */
#include<unistd.h> /* UNIX standard function definitions */
#include<fcntl.h> /* File control definitions */
#include<errno.h> /* Error number definitions */
#include<termios.h> /* POSIX terminal control definitions */
//#include<conio.h>
/*
* 'open_port()' - Open serial port 1.
*
* Returns the file descriptor on success or -1 on error.
*/
int buf_size;
char *buf;
char *buff;
int fd; /* File descriptor for the port */
int open_port(void)
{
fd = open("/dev/tty.USA28X1a2P2.2", O_RDWR | O_NOCTTY | O_NDELAY); // USA28X1a2P2.for keyspan
//fd = open("/dev/ttys000", O_RDWR | O_NOCTTY | O_NDELAY);
if (fd == -1) {
/*
* Could not open the port.
*/
perror("cannot open");
}
else
fcntl(fd, F_SETFL, 0);
struct termios options;
/*
* Get the current options for the port...
*/
tcgetattr(fd, &options);
/*
* Set the baud rates to 38400...
*/
cfsetispeed(&options, B38400);
cfsetospeed(&options, B38400);
/*
* Enable the receiver and set local mode...
*/
options.c_cflag |= (CLOCAL | CREAD);
/*
* Set the new options for the port...
*/
tcsetattr(fd, TCSANOW, &options);
options.c_cflag &= ~CSIZE; /* Mask the character size bits */
options.c_cflag &= ~PARENB;
options.c_cflag &= ~CSTOPB;
options.c_cflag &= ~CSIZE;
options.c_cflag |= CS8;
return (fd);
}
int main(int argc, char**argv) {
buf =malloc(20);
buff=malloc(20);
// strcpy(buf,"HOME");
// strcpy(buff,"Y");
open_port();
printf("type the command using Capital Letter : \n");
scanf("%s",buf);
write(fd, buf,20); //
write(fd," \r ",5);
printf("Command = %s\n", buf);
read(fd, buff,50);
printf(" %s \n",buff);
free(buf);
free(buff);
printf("type Y/N : \n");
scanf("%s",buf);
write(fd, buf,2);
write(fd,"\r",2);
// free(buf);
// free (buff);
// printf("type Y/N : \n");
// write(fd, buf,20);
// printf("You choose %s \n",buff);
// free(buf);
close(fd);
}

Answer 1

You are reading too much at a time. You allocate your buffers as 20 bytes, buff=malloc(20); But here you read 50, read(fd, buff,50); That can lead to odd behavior at best. Make sure you allocate as much space as you'll use. You don't even need to allocate it, you can use an array.
char buf[1024];
char buff[1024];
Then you free your memory before using them again.
free(buf);
free(buff);
printf("type Y/N : \n");
scanf("%s",buf);
Don't free buf or buff until after you call close(fd).
Read up on good C style, too. You're doing a few things that aren't bugs, but will make your life harder later.

Answer 2

There are so many errors in the code (like buffer overruns, etc.), but the most obvious one I could spot is here:
tcsetattr(fd, TCSANOW, &options);
options.c_cflag &= ~CSIZE; /* Mask the character size bits */
options.c_cflag &= ~PARENB;
options.c_cflag &= ~CSTOPB;
options.c_cflag &= ~CSIZE;
options.c_cflag |= CS8;
You're setting the fields of the options structure only after you already used it for setting up the file descriptor's properties. You have to put the call to tcsetattr() to the end of this whole sequence:
options.c_cflag &= ~CSIZE; /* Mask the character size bits */
options.c_cflag &= ~PARENB;
options.c_cflag &= ~CSTOPB;
options.c_cflag &= ~CSIZE;
options.c_cflag |= CS8;
tcsetattr(fd, TCSANOW, &options);
If you don't want to mess with manually setting all this yourself, try my helper function.