From 85768336b5364752a0e5d07e31814d2f95f8e2ba Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Fri, 24 Apr 2026 01:44:07 +0300
Subject: [PATCH] wasm: fix wasm backend as a shared library (fixes #17621)

---
 vlib/v/gen/wasm/mem.v                         |   2 +-
 .../gen/wasm/tests/browser_empty_main_test.v  | 120 ++++++++++++++++++
 2 files changed, 121 insertions(+), 1 deletion(-)

diff --git a/vlib/v/gen/wasm/mem.v b/vlib/v/gen/wasm/mem.v
index f9ce1f1e4..6d0830835 100644
--- a/vlib/v/gen/wasm/mem.v
+++ b/vlib/v/gen/wasm/mem.v
@@ -900,7 +900,7 @@ pub fn (mut g Gen) housekeeping() {
 		g.mod.assign_global_init(hp, wasm.constexpr_value(heap_base))
 	}
 
-	if g.pref.os == .wasi {
+	if g.pref.os == .wasi && !g.pref.is_shared {
 		mut fn_start := g.mod.new_function('_start', [], [])
 		{
 			fn_start.call('_vinit')
diff --git a/vlib/v/gen/wasm/tests/browser_empty_main_test.v b/vlib/v/gen/wasm/tests/browser_empty_main_test.v
index 02c50090d..afa0181b3 100644
--- a/vlib/v/gen/wasm/tests/browser_empty_main_test.v
+++ b/vlib/v/gen/wasm/tests/browser_empty_main_test.v
@@ -1,5 +1,16 @@
 import os
 
+struct WasmVarUint {
+	value    u32
+	next_idx int
+}
+
+struct WasmModuleSummary {
+mut:
+	exports           []string
+	has_start_section bool
+}
+
 fn test_wasm_browser_target_allows_empty_main() {
 	vexe := os.quoted_path(@VEXE)
 	wrkdir := os.join_path(os.vtmp_dir(), 'wasm_browser_tests')
@@ -24,3 +35,112 @@ fn test_wasm_browser_target_allows_empty_main() {
 		assert os.exists(output_path), 'missing output for `${flags}`'
 	}
 }
+
+fn test_wasm_shared_library_exports_custom_names_without_main() {
+	vexe := os.quoted_path(@VEXE)
+	wrkdir := os.join_path(os.vtmp_dir(), 'wasm_shared_library_tests')
+	os.mkdir_all(wrkdir)!
+	defer {
+		os.rmdir_all(wrkdir) or {}
+	}
+
+	source_path := os.join_path(wrkdir, 'my_wasm_lib.v')
+	output_path := os.join_path(wrkdir, 'my_wasm_lib.wasm')
+	source := [
+		'module my_wasm_lib',
+		'',
+		"@[export: 'myFunction']",
+		'fn my_function(a int, b int) int {',
+		'\treturn a + b',
+		'}',
+	].join_lines()
+	os.write_file(source_path, source)!
+
+	res :=
+		os.execute('${vexe} -b wasm -shared -o ${os.quoted_path(output_path)} ${os.quoted_path(source_path)}')
+	assert res.exit_code == 0, 'compilation failed: ${res.output}'
+	assert os.exists(output_path), 'missing output for shared wasm library'
+
+	wasm_bytes := os.read_bytes(output_path)!
+	summary := inspect_wasm_module(wasm_bytes) or { panic(err) }
+
+	assert 'myFunction' in summary.exports
+	assert '_start' !in summary.exports
+	assert summary.has_start_section
+}
+
+fn inspect_wasm_module(wasm_bytes []u8) !WasmModuleSummary {
+	if wasm_bytes.len < 8 {
+		return error('wasm module is too short')
+	}
+	if wasm_bytes[0] != 0x00 || wasm_bytes[1] != 0x61 || wasm_bytes[2] != 0x73
+		|| wasm_bytes[3] != 0x6d || wasm_bytes[4] != 0x01 || wasm_bytes[5] != 0x00
+		|| wasm_bytes[6] != 0x00 || wasm_bytes[7] != 0x00 {
+		return error('invalid wasm header')
+	}
+
+	mut summary := WasmModuleSummary{}
+	mut idx := 8
+	for idx < wasm_bytes.len {
+		section_id := wasm_bytes[idx]
+		idx++
+		section_len := read_wasm_u32(wasm_bytes, idx)!
+		idx = section_len.next_idx
+		section_end := idx + int(section_len.value)
+		if section_end > wasm_bytes.len {
+			return error('wasm section exceeds module bounds')
+		}
+		match section_id {
+			u8(7) {
+				mut export_idx := idx
+				exports_len := read_wasm_u32(wasm_bytes, export_idx)!
+				export_idx = exports_len.next_idx
+				for _ in 0 .. int(exports_len.value) {
+					name_len := read_wasm_u32(wasm_bytes, export_idx)!
+					export_idx = name_len.next_idx
+					name_end := export_idx + int(name_len.value)
+					if name_end > section_end {
+						return error('wasm export name exceeds section bounds')
+					}
+					summary.exports << wasm_bytes[export_idx..name_end].bytestr()
+					export_idx = name_end
+					if export_idx >= section_end {
+						return error('wasm export kind is missing')
+					}
+					export_idx++
+					export_ref := read_wasm_u32(wasm_bytes, export_idx)!
+					export_idx = export_ref.next_idx
+				}
+			}
+			u8(8) {
+				summary.has_start_section = true
+			}
+			else {}
+		}
+
+		idx = section_end
+	}
+	return summary
+}
+
+fn read_wasm_u32(wasm_bytes []u8, start int) !WasmVarUint {
+	mut value := u32(0)
+	mut shift := u32(0)
+	mut idx := start
+	for idx < wasm_bytes.len {
+		b := wasm_bytes[idx]
+		value |= u32(b & 0x7f) << shift
+		idx++
+		if b & 0x80 == 0 {
+			return WasmVarUint{
+				value:    value
+				next_idx: idx
+			}
+		}
+		shift += 7
+		if shift >= 35 {
+			break
+		}
+	}
+	return error('invalid wasm varuint32 encoding')
+}
-- 
2.39.5