[ECOS] Re: ecos hal symbol table creation mechanism

Sergei Gavrikov sergei.gavrikov@gmail.com
Wed Nov 18 15:59:00 GMT 2015


On Wed, 18 Nov 2015, Robert H wrote:

> 2015-11-17 Sergei Gavrikov wrote:
> > On Mon, 16 Nov 2015, Robert H wrote:

[snip]

> >> I would be very pleased to know more about how the mechanism to get
> >> ecos symbols works in hal_tables.h
> >
> > Generally and particularly (objloader case) the table boundaries are
> > known
> >
> > https://sourceware.org/viewvc/ecos/packages/services/objloader/current/src/objelf.c?view=markup#l62
> >
> > Look up example, objelf.c:cyg_ldr_external_address():
> >
> > https://sourceware.org/viewvc/ecos/packages/services/objloader/current/src/objelf.c?view=markup#l219

> I probably still don't get it by looking at the source code how
> entry->handler gets filled with the address of the symbol_name in the
> ecos image.
>
> cyg_ldr_external_address is for accessing the symbol table in that
> section until the entry-pointer reached the cyg_ldr_table_end-pointer,
> but to me that still doesn't explain how entry->handler gets filled by
> the right address.

Notice, that eCos table macros is a quite smart way to declare the
arrays of some C structure and keep them in a special memory region.

For your case the array's item (or "row" of the table) is C struct
cyg_ldr_table_entry:

  https://sourceware.org/viewvc/ecos/packages/services/objloader/current/include/objelf.h?revision=1.7&view=markup#l133

Similar in plain C

  extern int bar();
  extern int foo();

  struct s {
    char *name;
    void *call;
  };

  struct s table[] = { { "bar", bar }, {"foo", foo}, };
  struct s *table_end = &table[sizeof table / sizeof table[0]];

  struct s *entry = table;

  while(entry != table_end) {
    if(strcmp(entry->name, "foo") == 0) {
      return entry->call;
    }
    entry++;
  }

Hope, that no magic anymore.

Sergei

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