Issues while reading Writting USB data - linux

Question

I am new to Linux operating system, i am trying to interface GSM module with USB,i interfaced USB with GSM module successfully, but i have problem while reading the data from GSM module, i can able to write data to GSM module but while reading its too difficult for me, some time i can able to read data but some time i can't able to read data, here i copy and pasted my code, please go through the code and please let me where i am failing to read data,
i need to write data normally to USB port and data should be read on interrupt based
USB configuration
int init_ttyusb0(int speed,int parity)
{
fd=open("/dev/ttyUSB1",O_RDWR|O_NDELAY|O_NOCTTY|O_APPEND,S_IRWXU|S_IRWXG|S_IRWXO);|O_NOCTTY|O_SYNC|O_NDELAY);
perror("open");
if(fd==-1)
{
perror("open");
printf("open usb driver failed\n");
//close(fd);
return 1;
}
saio.sa_handler= signal_handler_IO;
saio.sa_flags=0;
saio.sa_restorer=NULL;
sigaction(SIGIO,&saio,NULL);
fcntl(fd,F_SETFL,FNDELAY);
fcntl(fd,F_SETOWN,getpid());
tcgetattr(fd,&tty);
perror("tcgetattr");
cfsetospeed(&tty,speed);
perror("cfsetospeed");
cfsetispeed(&tty,speed);
perror("cfsetispeed");
tty.c_cflag=(tty.c_cflag&~CSIZE)|CS8; // 8bit character
tty.c_iflag&=~IGNBRK;
// tty.c_lflag=0; // No signaling break process
// tty.c_oflag=0; //No remaping no delay
tty.c_cc[VMIN]=0; // read dosen't block
tty.c_cc[VTIME]=10; //0.5 seconds
tty.c_iflag&=~(IXON|IXOFF|IXANY);
tty.c_cflag|=CLOCAL; // igniore modem controls
tty.c_cflag|=CREAD;
tty.c_cflag&=~(PARENB|PARODD);
tty.c_cflag&=~CSTOPB;
tty.c_cflag&=~CRTSCTS;
tty.c_cflag&=~ICANON; // Disabling cananicle from disbling newline
tty.c_cflag&=~ISIG; // Disabling interrupt signaling bits
// tty.c_iflag&=~(IXON|IXOFF|IXANY); // Disabling software flow control
tty.c_lflag&=~ECHO;
tty.c_lflag&=~ECHOE;
tty.c_lflag&=~ECHONL;
tty.c_lflag&=~(IXON|IXOFF|IXANY);
tty.c_oflag&=~OPOST;
tcsetattr(fd,TCSANOW,&tty);
perror("tcsetattr");
printf("UART configured.....................\n");
}
Interrupt handler
-----------------------
void signal_handler_IO(int status)
{
static int gsm_count
read(fd,&zfc_buffer[gsm_count],sizeof(zfc_buffer));
printf("%s\n",zfc_buffer);
memset(zfc_buffer,0,sizeof(zfc_buffer));
fcntl(fd,F_SETFL,!(O_ASYNC));
}
int main(void)
{
int init_ttyusb0(B9600,0)
while(1)
{
printf("\n eneter option > ");
scanf("%d",&operation);
int m=26;
switch(operation)
{
case 0:
memset(buf_rcv,0,sizeof(buf_rcv));
buf[0]='A';
buf[1]='T';
buf[2]='\r';
buf[3]='\0';
write(fd,"AT\r",3);
usleep((3+25)*100);
fcntl(fd,F_SETFL,O_ASYNC); // Enabling interrupt after write
usleep((3+25)*100);
perror("write");
break;
case 1:
write(fd,"AT+CFUN=1\r",10);
usleep(175000);
fcntl(fd,F_SETFL,O_ASYNC);// Enabling interrupt after write
break;
case 2:
memset(buf,0,sizeof(buf));
strcpy(buf,"AT+CCID\r");
write(fd,"AT+CPIN?\r",9);
usleep(150000);
fcntl(fd,F_SETFL,O_ASYNC);// Enabling interrupt after write
break;
}