Fix scoping. Use vectors for buffers.

Writing data to a buffer obtained by data() is undefined behavior. Fix that.

Author: Sterling-Augustine

libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/xsgetn.pass.cpp

@@ -15,57 +15,58 @@
 
 // streamsize xsgetn(char_type*, streamsize) override;
 
-// This isn't a required override by the standard, but most implementations override it, since it allows for
-// significantly improved performance in some cases. All of this code is required to work, so this isn't a libc++
-// extension
+// This isn't a required override by the standard, but most implementations
+// override it, since it allows for significantly improved performance in some
+// cases. All of this code is required to work, so this isn't a libc++ extension
 
 #include <cassert>
 #include <fstream>
+#include <vector>
 
 #include "test_macros.h"
 
 int main(int, char**) {
-  {
-    char buffer[10];
-    std::ifstream fs("xsgetn.test.dat");
-    std::filebuf* fb = fs.rdbuf();
-    fb->pubsetbuf(buffer, 10);
+  std::vector<char> stream_buffer(10);
+  std::ifstream fs("xsgetn.test.dat");
+  std::filebuf* fb = fs.rdbuf();
+  fb->pubsetbuf(stream_buffer.data(), 10);
 
-    // Ensure that the buffer is set up
-    assert(fb->sgetc() == 't');
+  // Ensure that the buffer is set up
+  assert(fb->sgetc() == 't');
 
-    std::string str(5, '\0');
+  std::vector<char> test_buffer(5);
+  test_buffer[0] = '\0';
 
-    { // Check that a read smaller than the buffer works fine
-      assert(fb->sgetn(str.data(), 5) == 5);
-      assert(str == "this ");
-    }
-    { // Check that reading up to the buffer end works fine
-      assert(fb->sgetn(str.data(), 5) == 5);
-      assert(str == "is so");
-    }
-    { // Check that reading from an empty buffer, but more than the buffer can hold works fine
-      str.resize(12);
-      assert(fb->sgetn(str.data(), 12) == 12);
-      assert(str == "me random da");
-    }
-    { // Check that reading from a non-empty buffer, and more than the buffer can hold works fine
-      // Fill the buffer up
-      str.resize(2);
-      assert(fb->sgetn(str.data(), 2) == 2);
-      assert(str == "ta");
+  { // Check that a read smaller than the buffer works fine
+    assert(fb->sgetn(test_buffer.data(), 5) == 5);
+    assert(std::string(test_buffer.data()) == "this ");
+  }
+  { // Check that reading up to the buffer end works fine
+    assert(fb->sgetn(test_buffer.data(), 5) == 5);
+    assert(std::string(test_buffer.data(), 5) == "is so");
+  }
+  { // Check that reading from an empty buffer, but more than the buffer can
+    // hold works fine
+    test_buffer.resize(12);
+    assert(fb->sgetn(test_buffer.data(), 12) == 12);
+    assert(std::string(test_buffer.data(), 12) == "me random da");
+  }
+  { // Check that reading from a non-empty buffer, and more than the buffer can
+    // hold works fine Fill the buffer up
+    test_buffer.resize(2);
+    assert(fb->sgetn(test_buffer.data(), 2) == 2);
+    assert(std::string(test_buffer.data(), 2) == "ta");
 
-      // Do the actual check
-      str.resize(12);
-      assert(fb->sgetn(str.data(), 12) == 12);
-      assert(str == " to be able ");
-    }
-    { // Check that trying to read more than the file size works fine
-      str.resize(30);
-      assert(fb->sgetn(str.data(), 30) == 24);
-      str.resize(24);
-      assert(str == "to test buffer behaviour");
-    }
+    // Do the actual check
+    test_buffer.resize(12);
+    assert(fb->sgetn(test_buffer.data(), 12) == 12);
+    assert(std::string(test_buffer.data(), 12) == " to be able ");
+  }
+  { // Check that trying to read more than the file size works fine
+    test_buffer.resize(30);
+    assert(fb->sgetn(test_buffer.data(), 30) == 24);
+    test_buffer.resize(24);
+    assert(std::string(test_buffer.data(), 24) == "to test buffer behaviour");
   }
 
   return 0;