c - UART blocks on read -

i have problem uart read (on raspberry pi). works ok stops in loop , waits on data... made option o_ndelay stops so.

i use 2 terminal windows:

• one used uart program

• on second write:

  echo '123445' > /dev/ttyama0 (raspberry pi uart port)

complete code below.

#include <stdio.h> #include <unistd.h>             //used uart #include <fcntl.h>              //used uart #include <termios.h>        //used uart   int main(int argv, char * argc[]) { // setting uart      //-------------------------     //----- setup usart 0 -----     //-------------------------     //at bootup, pins 8 , 10 set uart0_txd, uart0_rxd (ie         alt0 function) respectively     int uart0_filestream = -1;      //open uart     //the flags (defined in fcntl.h):     //  access modes (use 1 of these):     //      o_rdonly - open reading only.     //      o_rdwr - open reading , writing.     //      o_wronly - open writing only.     //     //  o_ndelay / o_nonblock (same function) - enables nonblocking mode. when set read requests on file can return failure status     //                                          if there no input available (instead of blocking). likewise, write requests can return     //                                          failure status if output can't written immediately.     //     //  o_noctty - when set , path identifies terminal device, open() shall not cause terminal device become controlling terminal process.       uart0_filestream = open("/dev/ttyama0", o_rdwr | o_noctty | o_ndelay);      //open in non blocking read/write mode     if (uart0_filestream == -1)     {         //error - can't open serial port         printf("error - unable open uart.  ensure not in use application\n");     }      //configure uart     //the flags (defined in termios.h - see http://pubs.opengroup.org/onlinepubs/007908799/xsh/termios.h.html):     //  baud rate:- b1200, b2400, b4800, b9600, b19200, b38400, b57600, b115200, b230400, b460800, b500000, b576000, b921600, b1000000, b1152000, b1500000, b2000000, b2500000, b3000000, b3500000, b4000000     //  csize:- cs5, cs6, cs7, cs8     //  clocal - ignore modem status lines     //  cread - enable receiver     //  ignpar = ignore characters parity errors     //  icrnl - map cr nl on input     //  parenb - parity enable     //  parodd - odd parity (else even)     struct termios cfg;    //get existing configuration setup   tcgetattr(uart0_filestream, &cfg);    //fcntl(devicefd, f_setfl, fndelay);   fcntl(uart0_filestream, f_setfl, 0);    ////set both incoming , outgoing baud rates...   cfsetispeed(&cfg, b115200);   cfsetospeed(&cfg, b115200);    cfg.c_cflag |= (clocal | cread);    ////8n1 (8 data bits, no parity, 1 stop bit)   cfg.c_cflag &= ~parenb;   cfg.c_cflag &= ~cstopb;   cfg.c_cflag &= ~csize;   cfg.c_cflag |= cs8;    cfg.c_cflag &= ~crtscts;  //~cnew_rtscts; //disable hardware flow control    //use raw unbuffered data mode (eg, not canonical mode)   cfg.c_lflag &= ~(icanon | echo | echoe | isig | ignbrk);    cfg.c_iflag &= ~(ignpar | ixon | ixoff | ixany);    //raw (unprocessed) output mode   cfg.c_oflag &= ~opost;    tcsetattr(uart0_filestream, tcsanow, &cfg);      //transmitting bytes      //----- tx bytes -----     unsigned char tx_buffer[20];     unsigned char *p_tx_buffer;      p_tx_buffer = &tx_buffer[0];     *p_tx_buffer++ = 'h';     *p_tx_buffer++ = 'e';     *p_tx_buffer++ = 'l';     *p_tx_buffer++ = 'l';     *p_tx_buffer++ = 'o';      if (uart0_filestream != -1)     {         int count = write(uart0_filestream, &tx_buffer[0], (p_tx_buffer - &tx_buffer[0]));      //filestream, bytes write, number of bytes write         if (count < 0)         {             printf("uart tx error\n");         }     }     //receiving bytes      //----- check rx bytes -----     while(1) {      printf("loop\n");      if (uart0_filestream != -1)     {         // read 255 characters port if there         unsigned char rx_buffer[256];         int rx_length = read(uart0_filestream, (void*)rx_buffer, 255);      //filestream, buffer store in, number of bytes read (max)           if (rx_length < 0)         {             //an error occured             printf("uart rx error\n");         }         else if (rx_length == 0)         {             //no data waiting           printf("no data uart rx   test commit\n");         }         else         {             //bytes received             rx_buffer[rx_length] = '\0';             printf("%i bytes read : %s\n", rx_length, rx_buffer);             //break;         }     }     sleep(1); } //closing uart if no longer needed      //----- close uart -----     close(uart0_filestream);    return 0; } 

i used have same problem, , problem not related code , solution disable login on serial port, there file @ /etc/inittab open file nano , find line:

 t0:23:respawn:/sbin/getty -l ttyama0 115200 vt100 

and put # char @ begining line save , reboot , working weirdly.