Warning: Could not open bridge dump file `%s` for writing: %

Error message

Warning: Could not open bridge dump file `%s` for writing: %s\n

What it means

In dump_graph (sgen-new-bridge.c), when a bridge dump prefix is set, SGen writes the object graph of each bridge cycle to <prefix>.<counter>.gexf. If fopen(filename, "w") fails it logs this warning with the filename and strerror(errno) then returns, skipping that dump; the collection and bridge processing continue unaffected. The prefix is configured via set_dump_prefix, exposed through MONO_GC_DEBUG=bridge-dump=<prefix> (parsed in sgen-bridge.c) or the sgen_bridge_set_dump_prefix API.

Source

Thrown at src/mono/mono/metadata/sgen-new-bridge.c:598

#endif

static void
dump_graph (void)
{
	static int counter = 0;

	MonoObject *obj;
	HashEntry *entry;
	size_t prefix_len = strlen (dump_prefix);
	char *filename = g_newa (char, prefix_len + 64);
	FILE *file;
	int edge_id = 0;

	sprintf (filename, "%s.%d.gexf", dump_prefix, counter++);
	file = fopen (filename, "w");

	if (file == NULL) {
		fprintf (stderr, "Warning: Could not open bridge dump file `%s` for writing: %s\n", filename, strerror (errno));
		return;
	}

	fprintf (file, "<gexf xmlns=\"http://www.gexf.net/1.2draft\" xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xsi:schemaLocation=\"http://www.gexf.net/1.2draft http://www.gexf.net/1.2draft/gexf.xsd\" version=\"1.2\">\n");

	fprintf (file, "<graph defaultedgetype=\"directed\">\n"
			"<attributes class=\"node\">\n"
			"<attribute id=\"0\" title=\"class\" type=\"string\"/>\n"
			"<attribute id=\"1\" title=\"bridge\" type=\"boolean\"/>\n"
			"</attributes>\n");

	fprintf (file, "<nodes>\n");
	SGEN_HASH_TABLE_FOREACH (&hash_table, MonoObject *, obj, HashEntry *, entry) {
		MonoVTable *vt = SGEN_LOAD_VTABLE (obj);
		fprintf (file, "<node id=\"%p\"><attvalues><attvalue for=\"0\" value=\"%s.%s\"/><attvalue for=\"1\" value=\"%s\"/></attvalues></node>\n",
			 obj, m_class_get_name_space (vt->klass), m_class_get_name (vt->klass), entry->is_bridge ? "true" : "false");
	} SGEN_HASH_TABLE_FOREACH_END;
	fprintf (file, "</nodes>\n");

View on GitHub (pinned to 60108ba66e)

Solutions

Create and make writable the parent directory of the prefix: mkdir -p <dir> && chmod u+w <dir>
Use a prefix in a known-writable location such as /tmp/bridge or another temp dir owned by the runtime user
Remove 'bridge-dump=' from MONO_GC_DEBUG if the dumps are not actually needed

Example fix

# before
export MONO_GC_DEBUG=bridge-dump=/var/log/bridge/dump   # warning: dir missing

# after
mkdir -p /var/log/bridge
export MONO_GC_DEBUG=bridge-dump=/var/log/bridge/dump

Defensive patterns

Strategy: validation

Validate before calling

// Validate the bridge-dump prefix is writable BEFORE enabling bridge dumps.
// Run this before the runtime writes its first bridge cycle.
#include <glib.h>
#include <sys/stat.h>
#include <errno.h>
#include <string.h>
gboolean bridge_dump_prefix_ok (const char *prefix) {
    gchar *dir = g_path_get_dirname (prefix);
    gboolean ok = g_file_test (dir, G_FILE_TEST_IS_DIR) && access (dir, W_OK) == 0;
    if (!ok)
        g_printerr ("bridge-dump dir '%s' not writable: %s\n", dir, strerror (errno));
    g_free (dir);
    return ok;
}

Prevention

Prefer an absolute prefix in a temp dir owned by the runtime user (e.g. /tmp/bridge) to avoid CWD/permission surprises
mkdir -p the prefix's parent directory in your launch script before setting MONO_GC_DEBUG
Remember the warning is non-fatal and repeats each bridge cycle, so an unfixable path can flood stderr - disable bridge-dump if you don't need it
On mobile/sandboxed runtimes, confirm the sandbox grants write access to the chosen path before enabling dumps

When it happens

Trigger: Enabling 'bridge-dump=<prefix>' in MONO_GC_DEBUG where the prefix's parent directory does not exist, is not writable by the runtime user, is on a read-only filesystem, or the path is otherwise invalid. The warning recurs on every bridge cycle while the prefix remains set.

Common situations: iOS/Android-style bridge debugging where the prefix points to a sandbox/container directory not yet created; running the process as a user without write permission to the prefix path; a typo'd or relative prefix resolved against an unexpected working directory.

Related errors

AI-assisted analysis of dotnet/runtime@60108ba66e (2026-08-10). Data as JSON: /api/errors/4e99e995e42ca3dc. Report an issue: GitHub.