clang: init functions

Author: nalgeon

doc/spec.md

@@ -1078,3 +1078,19 @@ There's no automatic order for declarations within a package. You need to declar
 
 - If a function F uses a constant C or a variable V, you must declare V and C before F.
 - If type B refers to type A, you must declare A before B.
+
+### Init functions
+
+Each package can have an `init()` function (with no arguments or return values) that runs automatically before `main()`. Unlike Go, only one `init` function is allowed per package.
+
+Init functions can be used to initialize package-level variables with non-static values.
+
+```go
+var state int
+
+func init() {
+    state = 42
+}
+```
+
+If the program has multiple packages, each with its own `init` function, the order in which the `init` functions are called is not guaranteed.

internal/clang/emit.go

@@ -82,6 +82,7 @@ type Generator struct {
 	symbols  []symbol              // pre-collected top-level declarations
 	embeds   Embeds                // embedded C files from //so:embed
 	comments ast.CommentMap        // all comments across all files
+	initFunc *ast.FuncDecl         // package init() function, if any
 	panicked bool                  // true after first panic caught in Visit
 }
 
@@ -129,6 +130,7 @@ func (g *Generator) emitImpl(w io.Writer) {
 		}
 		ast.Walk(g, file)
 	}
+	g.emitInitFunc(w)
 }
 
 // emitEmbeds writes the content of embedded files, separated by blank lines.
@@ -143,6 +145,27 @@ func (g *Generator) emitEmbeds(w io.Writer, files []embedFile) {
 	}
 }
 
+// emitInitFunc emits the package init() function as a GCC constructor
+// that runs automatically before main().
+func (g *Generator) emitInitFunc(w io.Writer) {
+	if g.initFunc == nil {
+		return
+	}
+	decl := g.initFunc
+	g.state.funcSig = g.funcSig(decl)
+	g.state.tempCount = 0
+
+	fmt.Fprintf(w, "\nstatic void __attribute__((constructor)) %s_init() {\n", g.pkg.Name)
+	g.state.indent++
+	g.walkStmts(decl.Body.List)
+	g.emitDeferredCalls()
+	g.state.indent--
+	fmt.Fprintf(w, "}\n")
+
+	g.state.defers = nil
+	g.state.funcSig = nil
+}
+
 // collectComments builds a merged CommentMap from all source files.
 func (g *Generator) collectComments() {
 	g.comments = ast.CommentMap{}

internal/clang/function.go

@@ -96,6 +96,9 @@ func (g *Generator) emitFuncDecl(decl *ast.FuncDecl) {
 	if decl.Body == nil || g.hasExtern("", externFuncKey(decl)) {
 		return
 	}
+	if decl.Name.Name == "init" {
+		return
+	}
 	if decl.Recv != nil {
 		g.emitMethodDecl(decl)
 		return

internal/clang/symbols.go

@@ -69,6 +69,13 @@ func (g *Generator) collectSymbols() {
 				if d.Body == nil || d.Name.Name == "main" {
 					continue
 				}
+				if d.Name.Name == "init" {
+					if g.initFunc != nil {
+						g.fail(d.Name, "multiple init functions in package %s", g.pkg.Name)
+					}
+					g.initFunc = d
+					continue
+				}
 				if g.hasExtern("", externFuncKey(d)) {
 					continue
 				}

so/os/extern.go

@@ -6,9 +6,6 @@ package os
 //so:embed os.h
 var os_h string
 
-//so:embed os.c
-var os_c string
-
 //so:extern
 var errno int
 
@@ -27,6 +24,13 @@ const (
 //so:extern
 type os_file struct{}
 
+//so:extern
+var (
+	stdin  *os_file
+	stdout *os_file
+	stderr *os_file
+)
+
 // FILE *fopen(const char *restrict filename, const char *restrict mode);
 //
 //so:extern

so/os/file.go

@@ -10,20 +10,10 @@ type File struct {
 	closed bool
 }
 
-// Stdin is the standard input file descriptor.
-//
-//so:extern
-var Stdin = File{}
-
-// Stdout is the standard output file descriptor.
-//
-//so:extern
-var Stdout = File{}
-
-// Stderr is the standard error file descriptor.
-//
-//so:extern
-var Stderr = File{}
+// Standard input, output, and error streams.
+var Stdin File
+var Stdout File
+var Stderr File
 
 // Read reads up to len(b) bytes from the file and stores them in b.
 // It returns the number of bytes read and any error encountered.
@@ -135,3 +125,9 @@ func (f *File) Close() error {
 	f.closed = true
 	return nil
 }
+
+func init() {
+	Stdin = File{fd: stdin}
+	Stdout = File{fd: stdout}
+	Stderr = File{fd: stderr}
+}

so/os/os.c

@@ -10,12 +10,6 @@ typedef struct {
     bool closed;
 } os_File;
 
-// Stdin, Stdout and Stderr are the standard
-// input, output and error file descriptors.
-extern os_File os_Stdin;
-extern os_File os_Stdout;
-extern os_File os_Stderr;
-
 // Error codes.
 #define os_EACCES EACCES
 #define os_EEXIST EEXIST

so/os/os.h

@@ -358,7 +358,7 @@ func subMono(t, u int64) Duration {
 func Since(t Time) Duration {
 	if (t.wall & hasMonotonic) != 0 {
 		// Common case optimization: if t has monotonic time, then Sub will use only it.
-		return subMono(time_mono()-time_monoStart, t.ext)
+		return subMono(time_mono()-monoStart, t.ext)
 	}
 	return Now().Sub(t)
 }
@@ -368,7 +368,7 @@ func Since(t Time) Duration {
 func Until(t Time) Duration {
 	if (t.wall & hasMonotonic) != 0 {
 		// Common case optimization: if t has monotonic time, then Sub will use only it.
-		return subMono(t.ext, time_mono()-time_monoStart)
+		return subMono(t.ext, time_mono()-monoStart)
 	}
 	return t.Sub(Now())
 }

so/time/arithm.go

@@ -5,9 +5,6 @@ import _ "embed"
 //so:embed time.h
 var time_h string
 
-//so:embed time.c
-var time_c string
-
 //so:extern
 type time_tm struct {
 	tm_sec   int
@@ -53,6 +50,8 @@ func time_mono() int64 { return 0 }
 // which appears to have a default resolution of 15ms),
 // we avoid ever reporting a monotonic time of 0.
 // (Callers may want to use 0 as "time not set".)
-//
-//so:extern
-var time_monoStart int64 = time_mono() - 1
+var monoStart int64
+
+func init() {
+	monoStart = time_mono() - 1
+}