lv2forge.c

// gcc -g -o lv2forge lv2forge.c -Wall

#include <stdio.h>
#include <stdlib.h>

#include <lv2/lv2plug.in/ns/ext/atom/atom.h>
#include <lv2/lv2plug.in/ns/ext/atom/forge.h>
#include <lv2/lv2plug.in/ns/ext/uri-map/uri-map.h>
#include <lv2/lv2plug.in/ns/ext/urid/urid.h>

#define ATOM_BUFSIZE (1024)

static char**   urimap     = NULL;
static uint32_t urimap_len = 0;

static uint32_t
uri_to_id (LV2_URI_Map_Callback_Data callback_data, const char* uri)
{
	for (uint32_t i = 0; i < urimap_len; ++i) {
		if (!strcmp (urimap[i], uri)) {
			return i + 1;
		}
	}
	urimap             = (char**)realloc (urimap, (urimap_len + 1) * sizeof (char*));
	urimap[urimap_len] = strdup (uri);
	return ++urimap_len;
}

static LV2_URID_Map uri_map = { NULL, &uri_to_id };

int
main (int argc, char** argv)
{
	/* host provided port */
	LV2_Atom_Sequence* port = (LV2_Atom_Sequence*)malloc (ATOM_BUFSIZE + sizeof (uint8_t));

	/* initialize sequence (host) */
	port->atom.type = 0;
	port->atom.size = ATOM_BUFSIZE;

	/* "plugin" code starts here */
	LV2_Atom_Forge forge;
	LV2_URID_Map*  map = &uri_map;

	LV2_URID uri_1 = map->map (map->handle, "uri-1");
	LV2_URID uri_2 = map->map (map->handle, "uri-2");

	lv2_atom_forge_init (&forge, map);

	/* re-use the same Atom port two times */
	for (int i = 0; i < 2; ++i) {
		LV2_Atom_Forge_Frame seq;

		/* the 2nd time around "capacity" is the length of the data previously
		 * written to the port (!)
		 */
		const uint32_t capacity = port->atom.size;
		printf ("Port Capacity %d\n", capacity);
		lv2_atom_forge_set_buffer (&forge, (uint8_t*)port, capacity);
		lv2_atom_forge_sequence_head (&forge, &seq, 0);

		for (int j = 0; j < i; ++j) {
			LV2_Atom_Forge_Frame frame;
			lv2_atom_forge_frame_time (&forge, 0); //<< crash
			lv2_atom_forge_object (&forge, &frame, 1, uri_1);

			lv2_atom_forge_property_head (&forge, uri_2, 0);
			lv2_atom_forge_float (&forge, 1.f);

			lv2_atom_forge_pop (&forge, &frame);
		}

		/* close off sequence */
		lv2_atom_forge_pop (&forge, &seq);
	}

	return 0;
}

see also lsp-plugins/lsp-plugins#28 (comment)

I don't think that this is demonstrating a forge bug. You are giving an incorrect capacity to lv2_atom_forge_set_buffer(), the "plugin" code is broken.