Fix fflush(NULL) to flush all output streams

libsrc/stdio/_file.h

@@ -43,6 +43,7 @@ extern int _fflush(FILE *fp);
 
 extern void _add_opened_file(FILE *fp);
 extern void _remove_opened_file(FILE *fp);
+extern void _flush_opened_files(void);
 extern int _detect_open_flag(const char *mode);
 
 static inline int _fputc(int c, FILE *fp) {

libsrc/stdio/_fileman.c

@@ -40,7 +40,7 @@ void _remove_opened_file(FILE *fp) {
 }
 
 __attribute__((destructor))
-static void flush_opened_files(void) {
+void _flush_opened_files(void) {
   struct FILE **files = __fileman.opened;
   for (int i = 0, length = __fileman.length; i < length; ++i) {
     FILE *fp = files[i];

libsrc/stdio/fflush.c

@@ -3,5 +3,20 @@
 #include "./_file.h"
 
 int fflush(FILE *fp) {
-  return (fp->flush)(fp);
+  if (fp != NULL)
+    return (fp->flush)(fp);
+
+  // C11 7.21.5.2p2: If stream is a null pointer, the fflush function
+  // performs this flushing action on all streams for which the behavior
+  // is defined above (i.e. all output streams).
+  // Note: stdin is input-only and intentionally not flushed.
+  int result = 0;
+  // Standard output streams are statically allocated and not tracked in
+  // __fileman, so flush them explicitly.
+  if ((stdout->flush)(stdout) != 0)
+    result = EOF;
+  if ((stderr->flush)(stderr) != 0)
+    result = EOF;
+  _flush_opened_files();
+  return result;
 }

libsrc/tests/file_test.c

@@ -115,4 +115,24 @@ TEST(ungetc) {
   }
 }
 
+TEST(fflush_null) {
+  // fflush(NULL) must flush all output streams (C11 7.21.5.2p2).
+  const char fn[] = "tmp_fflush_null.txt";
+  FILE *fp = fopen(fn, "w");
+  if (EXPECT_NOT_NULL(fp)) {
+    // No newline, so the data stays buffered until flushed.
+    EXPECT_EQ(5, fwrite("hello", 1, 5, fp));
+    EXPECT_EQ(0, fflush(NULL));
+    // Data must have reached the file before fclose.
+    struct stat st;
+    if (EXPECT_EQ(0, stat(fn, &st))) {
+      EXPECT_EQ(5, st.st_size);
+    }
+    EXPECT_EQ(0, fclose(fp));
+    EXPECT_EQ(0, remove(fn));
+  }
+  // No opened files: still succeeds.
+  EXPECT_EQ(0, fflush(NULL));
+}
+
 XTEST_MAIN();