A quick test function to demonstrate the Linux serial / RS485 ioctls().

rs485_ioctl_test.c

/*
 * Test program Linux RS485-mode ioctls.
 *
 * cc -o rs485_mode rs485_mode.c
 */

#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <linux/serial.h>

/* RS485 ioctls: */
#define TIOCGRS485      0x542E
#define TIOCSRS485      0x542F

int main( int argc, char **argv )
{
        unsigned int i;
        char buf[80];

        if( argc < 3 ) {
                printf("Usage:  %s [port name] [0|1]\n", argv[0]);
                exit(0);
        }

        int enable = atoi( argv[2] );
        char *port = argv[1];

        int fd = open(port, O_RDWR);
        if (fd < 0) {
          /* Error handling. See errno. */
                fprintf( stderr, "Error opening port \"%s\" (%d): %s\n", port, errno, strerror( errno ));
                exit(-1);
        }

        struct serial_rs485 rs485conf;

        if (ioctl (fd, TIOCGRS485, &rs485conf) < 0) {
                fprintf( stderr, "Error reading ioctl port (%d): %s\n",  errno, strerror( errno ));
        }

        printf("Port currently RS485 mode is %s\n", (rs485conf.flags & SER_RS485_ENABLED) ? "set" : "NOT set");

        if( enable ) {
                printf("RS485 mode will be SET\n");
                rs485conf.flags |= SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND;
        } else {
                printf("RS485 mode will be UNSET\n");
                rs485conf.flags &= ~SER_RS485_ENABLED;
        }

        if (ioctl (fd, TIOCSRS485, &rs485conf) < 0) {
                fprintf( stderr, "Error sending ioctl port (%d): %s\n",  errno, strerror( errno ));
        }

        /* Use read() and write() syscalls here... */

        if (ioctl (fd, TIOCGRS485, &rs485conf) < 0) {
                fprintf( stderr, "Error reading ioctl port (%d): %s\n",  errno, strerror( errno ));
        }

        printf("Confirm RS485 mode is %s\n", (rs485conf.flags & SER_RS485_ENABLED) ? "set" : "NOT set");

        /* Do something on the serial port... */
        for( i = 0; i < 100; ++i ) {
                snprintf( buf,79, "%d\r\n", i );
                write( fd, buf, strlen(buf));
                sleep(1);
        }

        /* Close the device when finished: */
        if (close (fd) < 0) {
                fprintf( stderr, "Error closing port (%d): %s\n", errno, strerror( errno ));
        }

        exit(0);
}

How to set the baud, parity, stop bit, etc?

How to set the baud, parity, stop bit, etc?

In program code, use tcsetattr.
At the command line, use stty.