simple character device in rtems

I am new to RTEMS and I want to write a very simple character device for it but when I search in internet I can't find anything.

I can write character device for linux but doesn't have any experience in RTEMS.

I want to write a character device having 3 function: init(), exit() , read().

and when i call read just return a string.

Is there any reference to give me a hint to write this simple device driver?

This maybe depends a bit what you mean by "simple character device". I assume that you want something that is accessible via a device interface (similar to the Linux character devices).

In RTEMS there are some frameworks for I2C, SPI, serial ports, and some more simple interfaces. Quite a bit of drivers is pulled in from FreeBSD via libbsd and therefore uses the FreeBSD interface. Hardware that doesn't fit one of the frameworks uses a self defined interface most of the time. That can also be a file like interface.

If you want to use a file like interface you want to take a look at IMFS_make_generic_node. You can take a look at the I2C driver for a fully working example: https://git.rtems.org/rtems/tree/cpukit/dev/i2c/i2c-dev.c?id=94481cedc4165f6a49ef5287098251740922fee1#n147

A bit of a more minimal example could look like follows: Note that I write the following more or less from my head so there can be quite some typos and there are quite sure missing headers:

#include <rtems/imfs.h>

static ssize_t my_read(
  rtems_libio_t *iop,
  void *buffer,
  size_t count
)
{
  memset(buffer, 42, count);
  return count;
}

static const rtems_filesystem_file_handlers_r my_handler = {
    .open_h = rtems_filesystem_default_open,
    .close_h = rtems_filesystem_default_close,
    .read_h = my_read,
    .write_h = rtems_filesystem_default_write,
    .ioctl_h = rtems_filesystem_default_ioctl,
    .lseek_h = rtems_filesystem_default_lseek_file,
    .fstat_h = rtems_filesystem_default_fstat,
    .ftruncate_h = rtems_filesystem_default_ftruncate,
    .fsync_h = rtems_filesystem_default_fsync_or_fdatasync,
    .fdatasync_h = rtems_filesystem_default_fsync_or_fdatasync,
    .fcntl_h = rtems_filesystem_default_fcntl,
    .kqfilter_h = rtems_filesystem_default_kqfilter,
    .mmap_h = rtems_filesystem_default_mmap,
    .poll_h = rtems_filesystem_default_poll,
    .readv_h = rtems_filesystem_default_readv,
    .writev_h = rtems_filesystem_default_writev
};

static const IMFS_node_control my_node_control = IMFS_GENERIC_INITIALIZER(
  &my_handler,
  IMFS_node_initialize_generic,
  IMFS_node_destroy_default,
);

void my_Initialize( void )
{
  IMFS_make_generic_node(
    "/dev/my_dev",
    S_IFCHR | S_IRWXU | S_IRWXG | S_IRWXO,
    &my_node_control,
    NULL
  );
}

Calling my_Initialize should give you a "/dev/my_dev" where you can read any amount of 42 that you want.