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

rs485_ioctl_test.c

/*
 * Test program Linux RS485-mode ioctls.
 * Build:
 * 	- source /opt/fsl-imx-xwayland/5.15-kirkstone/environment-setup-armv8a-poky-linux
 * 	- $CC -o test-rs485 rs485_ioctl_test.c
 * Usage:
 * 	- ./test-rs485 /dev/ttymxc2
 */

#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];

        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");

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

        exit(0);
}