rs232 read pgm is not working when i connect in loopback but write is wroking plz help... pgm #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <fcntl.h> #include <termios.h> #define _POSIX_SOURCE 1 #define BAUDRATE B9600 #define MODEMDEVICE "/dev/ttyS1" int main(void) { int ret, fd; struct termios tio; char buf[255]; /* Open device for reading and writing */ fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY | O_NDELAY); if (fd < 0) { fprintf(stderr, "Error opening device %s\n", MODEMDEVICE); goto done; } /* get current port attributes */ ret = tcgetattr(fd, &tio); if (ret < 0) { fprintf(stderr, "Error retreiving attributes, ret=%d\n", ret); goto done; } tcflush(fd, TCIFLUSH); /* set port input speed */ ret = cfsetispeed(&tio, BAUDRATE); if (ret) { fprintf(stderr, "Error setting input Baud rate, ret=%d\n", ret); goto done; } /* set port output speed */ ret = cfsetospeed(&tio, BAUDRATE); if (ret) { fprintf(stderr, "Error setting output Baud rate, ret=%d\n", ret); goto done; } tio.c_cflag &= ~CRTSCTS; /* HW flow ctl OFF */ tio.c_cflag &= ~PARENB; /* no parity */ tio.c_cflag &= ~CSTOPB; /* 1 stop bit */ tio.c_cflag &= ~CSIZE; /* 1 stop bit */ tio.c_cflag |= CS8; /* char size; 8N1 */ tio.c_cflag |= CREAD; /* enable receiver */ /* set port attributes */ ret = tcsetattr(fd, TCSANOW, &tio); if (ret < 0) { fprintf(stderr, "Error setting attributes, ret=%d\n", ret); goto done; } fcntl(fd, F_SETFL, FNDELAY); ret = write(fd, "ab\r", 3); if (ret < 0) fprintf(stderr, "write() of 3 bytes failed!, ret=%d\n", ret); printf("no of data written = %d\n",ret); usleep(5000); while (1) { ret = read(fd, buf, sizeof(char)); if (ret > 0) { printf("%c",buf[0]); } } done: close(fd); return 0; } ################################## output../a.out no of data written = 3 ######################## strace ./a.out read(3, 0xbfa8d5dd, 1) = -1 EAGAIN (Resource temporarily unavailable) read(3, 0xbfa8d5dd, 1) = -1 EAGAIN (Resource temporarily unavailable) read(3, 0xbfa8d5dd, 1) = -1 EAGAIN (Resource temporarily unavailable) read(3, 0xbfa8d5dd, 1) = -1 EAGAIN (Resource temporarily unavailable) read(3, 0xbfa8d5dd, 1) = -1 EAGAIN (Resource temporarily unavailable) read(3, 0xbfa8d5dd, 1) = -1 EAGAIN (Resource temporarily unavailable) read(3, 0xbfa8d5dd, 1) = -1 EAGAIN (Resource temporarily unavailable) read(3, 0xbfa8d5dd, 1) = -1 EAGAIN (Resource temporarily unavailable) read(3, 0xbfa8d5dd, 1) = -1 EAGAIN (Resource temporarily unavailable)