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Asked: 2026-06-17T18:41:43+00:00

I have difficulties setting the serial port parameters. Program that works fine on same

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I have difficulties setting the serial port parameters.
Program that works fine on same device, leaves followings stty output:

$ stty -a -F /dev/ttyUSB0  
speed 1200 baud; rows 0; columns 0; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>;     swtch = <undef>; start = ^Q;
stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; min = 0; time =     0;
-parenb -parodd cs8 -hupcl -cstopb cread clocal -crtscts
ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany -imaxbel -iutf8
-opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig -icanon -iexten -echo -echoe -echok -echonl noflsh -xcase -tostop -echoprt -echoctl -echoke

my attempt goes like this:

tcgetattr(fd, &options);

cfsetispeed(&options, B9600);
cfsetospeed(&options, B9600);
options.c_cflag |= (CLOCAL | CREAD);
options.c_cflag &= ~PARENB;
options.c_cflag &= ~CSTOPB;
options.c_cflag &= ~CSIZE;
options.c_cflag |= CS8;
options.c_cflag &= ~( ICANON | ECHO | ECHOE |ISIG );
options.c_iflag &= ~(IXON | IXOFF | IXANY );
options.c_iflag |= IGNBRK;
options.c_oflag |= ONLCR;

options.c_oflag &= ~(OCRNL|ONLRET|NLDLY|CRDLY|TABDLY|BSDLY|VTDLY|FFDLY);

tcsetattr(fd, TCSANOW, &options);

I’ve tried setting speed both B1200 and B9600 but that didn’t work (in manual it says that should be B9600)

What is wrong in this options?

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1 Answer

  1. Editorial Team

    The stty settings appear to be for raw mode, whereas your tcgetattr()/tcsetattr() code tries to use non-canonical mode but is incomplete for output processing (OPOST is not cleared).
    The result is that the serial port is in a half-raw mode for output.

    Try using cfmakeraw() to setup non-canonical mode.

    cfmakeraw() sets the terminal to something like the “raw” mode of the old Version 7 terminal driver: input is available character by character, echoing is disabled, and all special processing of terminal input and output characters is disabled. The terminal attributes are set as follows:

    termios_p->c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP
            | INLCR | IGNCR | ICRNL | IXON);
termios_p->c_oflag &= ~OPOST;
termios_p->c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
termios_p->c_cflag &= ~(CSIZE | PARENB);
termios_p->c_cflag |= CS8;

    For working sample code see this.
    Note that the return codes from system calls during initialization are checked.

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