fcntl(fd,S_SETFL,0);

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <stdio.h>

int  open_port(void);

void main()
{
   int fd = open_port();
   if (fd < 0)
      exit(-1)
   int n ;
   while(n != -1)
   {
        if (n == -1)
           break;
       // Получается считывать только по-байтно
       n = read(fd,&ch,1);
       ....
    }         
}

int  open_port(void)
{
    int fd;
    fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY |O_NONBLOCK);
    if (fd == -1)
    perror("open_port: Unable to open /dev/ttyS0 - "); 
    else
    {
    struct termios options; 
    tcgetattr(fd, &options);
    cfsetispeed(&options, B115200);
    cfsetospeed(&options, B115200);
    options.c_cflag &= ~PARENB; 
    options.c_cflag &= ~CSTOPB;
    options.c_cflag &= ~CSIZE;
    options.c_cflag |= CS8;
    tcsetattr(fd, TCSANOW, &options);
    }
    /*
    Если не вставить эту стороку в ридере - вылет из цикла
    while по break - С чем это связано???????
    */                
    fcntl(fd, F_SETFL, 0);  
     return (fd);
}

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <stdbool.h>
#include <fcntl.h>
#include <termios.h>
#include <unistd.h>
#include <errno.h>
#include <time.h>
#ifndef _STDIO_H
#include <stdio.h>
#endif
#ifndef _STDLIB_H
#include <stdlib.h>
#endif
#ifndef _STRING_H
#include <string.h>
#endif
#ifndef __USE_MISC
typedef unsigned int uint;
#endif
#define MSGSIZE 72
#define CAS_SIZE 10
#define CAS_NUM 10
#define MINI(n1,n2) ((n1<=n2)?n1:n2)
typedef unsigned char uchar;
int readto(int,char*);
extern int errno;
int main(int argc,char **argv){
struct timespec tds;    //,tdw;
struct timeval tv1,tv2;
struct timezone tz1,tz2;
struct termios oldcomio,newcomio;
fd_set rdsetcur,rdsetold;
int ibdrt[20]={B0,B50,B75,B110,B134,B150,B200,B300,B600,B1200,B1800,B2400,
    B4800,B9600,B19200,B38400,B57600,B115200,B230400,0},i=argc,ibr=14,
    ibrrl[10]={300,600,1200,2400,4800,9600,19200,38400,57600,115200},
    icom=1,fd,fdcom,n1,n2,n3,n4,ifd,istr,ivt=60,numcm,numex,nd=2,
    nw=4,ncyc=0,nt1,idv=1,idsr,j;
clock_t time;
uint iw=1,nz;
bool bds;
char nmcom[16],buf[80],bufrm[80];
strcpy(buf,"/usr/local/etc");if(i>1){strcpy(buf,argv[1]);fprintf(stdout,"%d %s\n",strlen(buf),buf);}
strcpy(buf+strlen(buf),"/chpa2_d.in");
FILE *fin=fopen(buf,"r");
if(fin!=NULL){
    fscanf(fin,"%d%[^\n]",&ibr,buf+32);
    fscanf(fin,"%d%[^\n]",&icom,buf+32);
    fscanf(fin,"%d%[^\n]",&nd,buf+32);
    fscanf(fin,"%d%[^\n]",&nw,buf+32);
    fscanf(fin,"%d%[^\n]",&ivt,buf+32);
    fscanf(fin,"%d%[^\n]",&idv,buf+32);
    fclose(fin);
}
strcpy(buf+30,"origcom ");
if(idv==1){strcpy(nmcom,"/dev/ttySx");sprintf(nmcom+9,"%1d",icom-1);//to COM
sprintf(bufrm," - COM-port, %1d",icom);buf[37]=nmcom[9];}
else{strcpy(nmcom,"/dev/USB0");strcpy(bufrm, " USB<->COM,   ");buf[37]='u';}
nz=(uint)((11.*1.e+9)/(double)ibrrl[ibr-8]);
fprintf(stdout,"%s%s %d%8d%8d%8u %d %s\n",
nmcom,bufrm,ibr,ibdrt[ibr],ibrrl[ibr-8],nz,ivt,buf);
sprintf(buf,"U1I%02d %02d\r",nw,nd);nd*=10;nw*=10;strcpy(buf+10,"U1R\r");
strcpy(buf+15,"U1N\r");strcpy(buf+20,"U1T\r");strcpy(buf+25,"U1P\r");
tds.tv_sec=0;tds.tv_nsec=100000000L;fprintf(stdout,"%d %d\n",nd,nw);
FD_ZERO(&rdsetold);FD_SET(0,&rdsetold);
fd=open(nmcom,O_RDWR|O_NOCTTY|O_ASYNC );
if(fd<0){perror(nmcom);exit(-1); }
if((fdcom=open(buf+30,O_WRONLY|O_CREAT|O_EXCL,0644))!=-1){
    tcgetattr(fd,&oldcomio); /* save current port settings */
    write(fdcom,(void*)&oldcomio,sizeof(oldcomio));
    close(fdcom);}
else if(errno==EEXIST){
    fdcom=open(buf+30,O_RDONLY);
    read(fdcom,(void*)&oldcomio,sizeof(oldcomio));
    tcsetattr(fd,TCSANOW,&oldcomio);
    close(fdcom);}
bzero(&newcomio, sizeof(newcomio));
newcomio.c_cflag = ibdrt[ibr] | CS8 | CLOCAL | CREAD;
newcomio.c_iflag = IGNPAR | IGNBRK;
/* set input mode (non-canonical, no echo,...) */
newcomio.c_cc[VEOF]=0;newcomio.c_cc[VEOL]=0;newcomio.c_cc[VEOL2]=0;
newcomio.c_cc[VTIME]=ivt;   /* inter-character timer unused */
newcomio.c_cc[VMIN]=0;   /* blocking read until 5 chars received */
cfsetispeed(&newcomio,ibdrt[ibr]);cfsetospeed(&newcomio,ibdrt[ibr]);
tcflush(fd,TCIOFLUSH);
tcsetattr(fd,TCSANOW,&newcomio);//fcntl(fd,F_SETFL,O_ASYNC);
FD_ZERO(&rdsetold);FD_SET(0,&rdsetold);
while(iw==1){
  rdsetcur=rdsetold;
  fprintf(stdout,"%s","enter istr, numcom and numex ");fflush(stdout);
  ifd=select(1,&rdsetcur,NULL,NULL,NULL);
  if(ifd>0){
    if(FD_ISSET(0,&rdsetcur)){
        read(0,bufrm,MSGSIZE);sscanf(bufrm,"%d%d%d",&istr,&numcm,&numex);}
    else{iw=0;istr=0;}}
  else{iw=0;istr=0;}
  fprintf(stdout,"%3d entered:%2d %d %d\n",ifd,istr,numcm,numex);
  if(istr==1){n1=0;n3=4;
    switch(numcm){
    case 1:case 2:case 4:case 5:if(numcm==1)n3=9;else n1=10+(numcm-2)*5;
      n2=write(fd,(void*)(buf+n1),n3);if(n2==-1){
      fprintf(stdout,"w %d %s\n",errno,strerror(errno));exit(-1);}
      sprintf(buf+30," !!!");n2=readto(fd,buf+30);if(n2==-1){
      fprintf(stdout,"r %d %d %s\n",numcm,errno,strerror(errno));exit(-1);}
      fprintf(stdout,"numcm=%d %d %s\n",numcm,n2,buf+30);break;
    case 3:
        ncyc=numex;gettimeofday(&tv1,&tz1);bds=0;
      for(i=0;i<ncyc;i++){gettimeofday(&tv2,&tz2);sprintf(buf+30," !!!");
        nt1=(tv2.tv_sec-tv1.tv_sec)*1000000+tv2.tv_usec-tv1.tv_usec;tv1=tv2;
        tcflush(fd,TCIOFLUSH);n2=write(fd,(void*)(buf+15),4);if(n2==-1){fprintf(
        stdout,"w %d %s\n",errno,strerror(errno));exit(-1);}n4=readto(fd,buf+30);
        if(n4==-1){fprintf(stdout,"r %d %d %s\n",numcm,errno, strerror(errno));
        exit(-1);}fprintf(stdout,"numcm=%d %d %d %d%s %d\n",numcm,i,n2,n4,buf+30
        ,nt1/1000);time=clock();j=0;if(i==0)goto lbds;while(bds&&clock()-time<60
        *CLOCKS_PER_SEC){ioctl(fd,TIOCMGET,&idsr);bds=idsr&TIOCM_DSR;if(bds){
        sprintf(buf+30,"**");n4=readto(fd,buf+30);if(n4>0){fprintf(stdout,
        "1 inf %d %d %s\r",n4,j,buf+31);j++;nanosleep(&tds,NULL);}}}if(bds){fputs
        ("\nDSR not reset!\n",stdout);break;}lbds:while(!bds&&clock()
        -time<60*CLOCKS_PER_SEC){ioctl(fd,TIOCMGET,&idsr);bds=TIOCM_DSR&idsr;if(
        !bds){sprintf(buf+30,"**");n4=readto(fd,buf+30);if(n4>0){fprintf(stdout,
        "0 inf %d %d %s\r",n4,j,buf+31);j++;nanosleep(&tds,NULL);}}}if(!bds
        ){fputs("\nNo DSR!\n",stdout);break;}
      }break;
    default:;}
      }
    else iw=0;
}
    tcsetattr(fd,TCSANOW,&oldcomio);
    close(fd);
return 0;}

int readto(int fd,char* buf){
//reads from fd to buf+1 bytes until '\r' with timeout VTIME/10.
int nb=1;
ssize_t nr;
  while(*(buf+nb-1)!='\r'){nr=read(fd,(void*)(buf+nb),1);
    if(nr<=0){if(nb==1){if(buf[0]==' ')fprintf(stdout,"timeout at 1st\n");
    nb=0;}break;}
    else{if(buf[0]!=' ')fprintf(stdout,"readto: %d %d\n",nb,buf[nb]);nb++;}}
  if(nb>1){if(buf[nb-1]=='\r')buf[nb-1]=0;else buf[nb]=0;}
return nb-1;}


// chpa2_d.in

14  ibr, 15 - 38400
1   icom, 1 - ttyS0
2   nd in sec
4   nw in sec
60  ivt in 0.1s
1   idv, 1/2 - COM/USB

int  open_port(void)
{
    int fd;
    fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY |O_NONBLOCK);
    if (fd == -1)
    perror("open_port: Unable to open /dev/ttyS0 - "); 
    else
    {
    struct termios options; 
    tcgetattr(fd, &options);
    cfsetispeed(&options, B115200);
    cfsetospeed(&options, B115200);
    options.c_cflag &= ~PARENB; 
    options.c_cflag &= ~CSTOPB;
    options.c_cflag &= ~CSIZE;
    options.c_cflag |= CS8;
    tcsetattr(fd, TCSANOW, &options);
    }                
    fcntl(fd, F_SETFL, 0);
     return (fd);
}
...
int n = write(fd,"STROKA\n",6);

...
int n ;
while(n != -1)
{
       n = read(fd,&ch,1);
       ....
}