Why using ReadOSM ?

ReadOSM implements a very simple and straightforward interface; there are only three methods:

• readosm_open(): this function is intended to establish a connection to some OSM input file.
• readosm_close(): this function is intended to terminate a previously established connection.
• readosm_parse(): a single function dispatching the whole parsing process (mainly based on callback functions).

Accordingly to the above premises, implementing a complete OSM parser is incredibly simple:

#include <readosm.h>

static int 
parse_node (const void *user_data, const readosm_node * node)
{
/* callback function consuming Node objects */
  struct some_user_defined_struct *my_struct =
    (struct some_user_defined_struct *) user_data;

  ... some smart code ...

  return READOSM_OK;
}

static int 
parse_way (const void *user_data, const readosm_way * way)
{
/* callback function consuming Way objects */
  struct some_user_defined_struct *my_struct =
    (struct some_user_defined_struct *) user_data;

  ... some smart code ...

  return READOSM_OK;
}

static int 
parse_relation (const void *user_data, const readosm_relation * relation)
{
/* callback function consuming Relation objects */
  struct some_user_defined_struct *my_struct =
    (struct some_user_defined_struct *) user_data;

  ... some smart code ...

  return READOSM_OK;
}

int main ()
{
/* the basic OSM parser implementation */
  int ret;
  const void *handle;
  struct some_user_defined_struct my_struct;

  ret = readosm_open ("path-to-some-OSM-file", &handle);

  ... error handling intentionally suppressed ...

  ret = readosm_parse (handle, &my_struct, parse_node, parse_way, parse_relation);

  ... error handling intentionally suppressed ...

  ret = readosm_close (handle);

  ... error handling intentionally suppressed ...

  return 0;
}

So the real programming work is simply the one required in order to implement the callback-functions own code.
You can usefully read and study the Examples code-samples in order to get any other relevant information about this topic.