Read from serial port in C LINUX - c

Question

I know There are lot of questions about this in Internet. But none of them works for me. I'm trying to read from serial port, which is connected to MODEM.To be more specific. I send AT command and save response(OK or ERROR) to variable. But when i try to read from serial port,response always changes. like AT, AT=, AT= O etc.
Here's code that send command:
#include <stdio.h> /* Standard input/output definitions */
#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 */
int main(int argc, char const *argv[])
{
int fd;
struct termios options;
/* open the port */
fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY | O_NDELAY);
fcntl(fd, F_SETFL, FNDELAY);
/* get the current options */
tcgetattr(fd, &options);
/* set raw input, 1 second timeout */
options.c_cflag |= (CLOCAL | CREAD);
options.c_lflag &= (ICANON | ECHO | ECHOE | ISIG);
options.c_oflag &= ~OPOST;
options.c_cc[VMIN] = 0;
options.c_cc[VTIME] = 10;
init_modem();
/* set the options */
tcsetattr(fd, TCSANOW, &options);
return 0;
}
int /* O - 0 = MODEM ok, -1 = MODEM bad */
init_modem(int fd) /* I - Serial port file */
{
char buffer[255]; /* Input buffer */
char *bufptr; /* Current char in buffer */
int nbytes; /* Number of bytes read */
int tries; /* Number of tries so far */
write(fd, "AT\r", 3);
return (-1);
}
UPDATE:
Thanks for the replies.So i set canonical input and raw output. That helped me a lot and i found this code: And this outputs:AT OK But i want only OK. What should i change ?
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <stdio.h>
/* baudrate settings are defined in <asm/termbits.h>, which is
included by <termios.h> */
#define BAUDRATE B38400
/* change this definition for the correct port */
#define MODEMDEVICE "/dev/ttyUSB0"
#define _POSIX_SOURCE 1 /* POSIX compliant source */
#define FALSE 0
#define TRUE 1
volatile int STOP=FALSE;
main()
{
int fd,c, res;
struct termios oldtio,newtio;
char buf[255];
/*
Open modem device for reading and writing and not as controlling tty
because we don't want to get killed if linenoise sends CTRL-C.
*/
fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY );
if (fd <0) {perror(MODEMDEVICE); exit(-1); }
tcgetattr(fd,&oldtio); /* save current serial port settings */
bzero(&newtio, sizeof(newtio)); /* clear struct for new port settings */
/*
BAUDRATE: Set bps rate. You could also use cfsetispeed and cfsetospeed.
CRTSCTS : output hardware flow control (only used if the cable has
all necessary lines. See sect. 7 of Serial-HOWTO)
CS8 : 8n1 (8bit,no parity,1 stopbit)
CLOCAL : local connection, no modem contol
CREAD : enable receiving characters
*/
newtio.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD;
/*
IGNPAR : ignore bytes with parity errors
ICRNL : map CR to NL (otherwise a CR input on the other computer
will not terminate input)
otherwise make device raw (no other input processing)
*/
newtio.c_iflag = IGNPAR | ICRNL;
/*
Raw output.
*/
newtio.c_oflag = 0;
/*
ICANON : enable canonical input
disable all echo functionality, and don't send signals to calling program
*/
newtio.c_lflag = ICANON;
/*
initialize all control characters
default values can be found in /usr/include/termios.h, and are given
in the comments, but we don't need them here
*/
newtio.c_cc[VINTR] = 0; /* Ctrl-c */
newtio.c_cc[VQUIT] = 0; /* Ctrl-\ */
newtio.c_cc[VERASE] = 0; /* del */
newtio.c_cc[VKILL] = 0; /* # */
newtio.c_cc[VEOF] = 4; /* Ctrl-d */
newtio.c_cc[VTIME] = 0; /* inter-character timer unused */
newtio.c_cc[VMIN] = 1; /* blocking read until 1 character arrives */
newtio.c_cc[VSWTC] = 0; /* '\0' */
newtio.c_cc[VSTART] = 0; /* Ctrl-q */
newtio.c_cc[VSTOP] = 0; /* Ctrl-s */
newtio.c_cc[VSUSP] = 0; /* Ctrl-z */
newtio.c_cc[VEOL] = 0; /* '\0' */
newtio.c_cc[VREPRINT] = 0; /* Ctrl-r */
newtio.c_cc[VDISCARD] = 0; /* Ctrl-u */
newtio.c_cc[VWERASE] = 0; /* Ctrl-w */
newtio.c_cc[VLNEXT] = 0; /* Ctrl-v */
newtio.c_cc[VEOL2] = 0; /* '\0' */
/*
now clean the modem line and activate the settings for the port
*/
tcflush(fd, TCIFLUSH);
tcsetattr(fd,TCSANOW,&newtio);
/*
terminal settings done, now handle input
In this example, inputting a 'z' at the beginning of a line will
exit the program.
*/
write(fd,"AT\r",3);
while (STOP==FALSE) { /* loop until we have a terminating condition */
/* read blocks program execution until a line terminating character is
input, even if more than 255 chars are input. If the number
of characters read is smaller than the number of chars available,
subsequent reads will return the remaining chars. res will be set
to the actual number of characters actually read */
res = read(fd,buf,255);
buf[res]=0; /* set end of string, so we can printf */
buf[res-1]=0;
printf("%s\n", buf);
if (buf[0]=='\r') STOP=TRUE;
}
/* restore the old port settings */
tcsetattr(fd,TCSANOW,&oldtio);
}