Write directly to a stream buffer

include/fmt/format-inl.h

@@ -1432,6 +1432,158 @@ FMT_FUNC auto vformat(string_view fmt, format_args args) -> std::string {
 }
 
 namespace detail {
+
+template <typename T> struct span {
+  T* data;
+  size_t size;
+};
+
+#ifdef _WIN32
+inline void flockfile(FILE* f) { _lock_file(f); }
+inline void funlockfile(FILE* f) { _unlock_file(f); }
+inline int getc_unlocked(FILE* f) { return _fgetc_nolock(f); }
+#endif
+
+// A FILE wrapper. F is FILE defined as a template parameter to make system API
+// detection work.
+template <typename F> class file_base {
+ public:
+  F* file_;
+
+ public:
+  file_base(F* file) : file_(file) {}
+  operator F*() const { return file_; }
+
+  // Reads a code unit from the stream.
+  auto get() -> int {
+    int result = getc_unlocked(file_);
+    if (result == EOF && ferror(file_) != 0)
+      FMT_THROW(system_error(errno, FMT_STRING("getc failed")));
+    return result;
+  }
+
+  // Puts the code unit back into the stream buffer.
+  void unget(char c) {
+    if (ungetc(c, file_) == EOF)
+      FMT_THROW(system_error(errno, FMT_STRING("ungetc failed")));
+  }
+};
+
+// A FILE wrapper for glibc.
+template <typename F> class glibc_file : public file_base<F> {
+ public:
+  using file_base<F>::file_base;
+
+  // Returns the file's read buffer as a string_view.
+  auto get_read_buffer() const -> span<const char> {
+    return {this->file_->_IO_read_ptr,
+            to_unsigned(this->file_->_IO_read_end - this->file_->_IO_read_ptr)};
+  }
+};
+
+// A FILE wrapper for Apple's libc.
+template <typename F> class apple_file : public file_base<F> {
+ private:
+  auto offset() const -> ptrdiff_t {
+    return this->file_->_p - this->file_->_bf._base;
+  }
+
+ public:
+  using file_base<F>::file_base;
+
+  auto is_buffered() const -> bool {
+    return (this->file_->_flags & 2) == 0;  // 2 is __SNBF.
+  }
+
+  auto get_read_buffer() const -> span<const char> {
+    return {reinterpret_cast<char*>(this->file_->_p),
+            to_unsigned(this->file_->_r)};
+  }
+
+  auto get_write_buffer() const -> span<char> {
+    if (!this->file_->_p || offset() == this->file_->_bf._size) {
+      // Force buffer initialization by placing and removing a char in a buffer.
+      fputc(0, this->file_);
+      --this->file_->_p;
+      ++this->file_->_w;
+    }
+    auto size = this->file_->_bf._size;
+    FMT_ASSERT(offset() < size, "");
+    return {reinterpret_cast<char*>(this->file_->_p),
+            static_cast<size_t>(size - offset())};
+  }
+
+  void advance_write_buffer(size_t size) {
+    this->file_->_p += size;
+    this->file_->_w -= size;
+  }
+};
+
+// A fallback FILE wrapper.
+template <typename F> class fallback_file : public file_base<F> {
+ private:
+  char next_;  // The next unconsumed character in the buffer.
+  bool has_next_ = false;
+
+ public:
+  using file_base<F>::file_base;
+
+  auto is_buffered() const -> bool { return false; }
+
+  auto get_read_buffer() const -> span<const char> {
+    return {&next_, has_next_ ? 1u : 0u};
+  }
+
+  auto get_write_buffer() const -> span<char> { return {nullptr, 0}; }
+
+  auto get() -> int {
+    has_next_ = false;
+    return file_base<F>::get();
+  }
+
+  void unget(char c) {
+    file_base<F>::unget(c);
+    next_ = c;
+    has_next_ = true;
+  }
+
+  void advance_write_buffer(size_t) {}
+};
+
+template <typename F, FMT_ENABLE_IF(sizeof(F::_p) != 0)>
+auto get_file(F* f, int) -> apple_file<F> {
+  return f;
+}
+inline auto get_file(FILE* f, ...) -> fallback_file<FILE> { return f; }
+
+using file_ref = decltype(get_file(static_cast<FILE*>(nullptr), 0));
+
+class file_print_buffer : public buffer<char> {
+ private:
+  file_ref file_;
+
+  void set_buffer() {
+    file_.advance_write_buffer(size());
+    auto buf = file_.get_write_buffer();
+    this->set(buf.data, buf.size);
+  }
+
+  static void grow(buffer<char>& buf, size_t) {
+    auto& self = static_cast<file_print_buffer&>(buf);
+    self.set_buffer();
+  }
+
+ public:
+  explicit file_print_buffer(FILE* f) : buffer(grow, size_t()), file_(f) {
+    flockfile(f);
+    set_buffer();
+  }
+  ~file_print_buffer() {
+    file_.advance_write_buffer(size());
+    funlockfile(file_);
+  }
+};
+
 #if !defined(_WIN32) || defined(FMT_WINDOWS_NO_WCHAR)
 FMT_FUNC auto write_console(int, string_view) -> bool { return false; }
 #else
@@ -1470,6 +1622,10 @@ FMT_FUNC void print(std::FILE* f, string_view text) {
 }  // namespace detail
 
 FMT_FUNC void vprint(std::FILE* f, string_view fmt, format_args args) {
+  if (detail::file_ref(f).is_buffered()) {
+    auto&& buffer = detail::file_print_buffer(f);
+    return detail::vformat_to(buffer, fmt, args);
+  }
   auto buffer = memory_buffer();
   detail::vformat_to(buffer, fmt, args);
   detail::print(f, {buffer.data(), buffer.size()});

test/CMakeLists.txt

@@ -82,7 +82,7 @@ endif()
 add_fmt_test(printf-test)
 add_fmt_test(ranges-test ranges-odr-test.cc)
 
-add_fmt_test(scan-test)
+add_fmt_test(scan-test HEADER_ONLY)
 check_symbol_exists(strptime "time.h" HAVE_STRPTIME)
 if (HAVE_STRPTIME)
   target_compile_definitions(scan-test PRIVATE FMT_HAVE_STRPTIME)

test/format-test.cc

@@ -1741,6 +1741,14 @@ TEST(format_test, print) {
                "Don't panic!\n");
 }
 
+TEST(format_test, big_print) {
+  enum {count = 5000};
+  auto big_print = []() {
+    for (int i = 0; i < count / 5; ++i) fmt::print("xxxxx");
+  };
+  EXPECT_WRITE(stdout, big_print(), std::string(count, 'x'));
+}
+
 TEST(format_test, variadic) {
   EXPECT_EQ(fmt::format("{}c{}", "ab", 1), "abc1");
 }

test/scan.h

@@ -42,9 +42,9 @@ class scan_buffer {
       : ptr_(ptr), end_(end), contiguous_(contiguous) {}
   ~scan_buffer() = default;
 
-  void set(string_view buf) {
+  void set(span<const char> buf) {
-    ptr_ = buf.begin();
+    ptr_ = buf.data;
-    end_ = buf.end();
+    end_ = buf.data + buf.size;
   }
 
   auto ptr() const -> const char* { return ptr_; }
@@ -150,83 +150,6 @@ class string_scan_buffer : public scan_buffer {
       : scan_buffer(s.begin(), s.end(), true) {}
 };
 
-#ifdef _WIN32
-void flockfile(FILE* f) { _lock_file(f); }
-void funlockfile(FILE* f) { _unlock_file(f); }
-int getc_unlocked(FILE* f) { return _fgetc_nolock(f); }
-#endif
-
-// A FILE wrapper. F is FILE defined as a template parameter to make
-// system-specific API detection work.
-template <typename F> class file_base {
- protected:
-  F* file_;
-
- public:
-  file_base(F* file) : file_(file) {}
-  operator F*() const { return file_; }
-
-  // Reads a code unit from the stream.
-  auto get() -> int {
-    int result = getc_unlocked(file_);
-    if (result == EOF && ferror(file_) != 0)
-      FMT_THROW(system_error(errno, FMT_STRING("getc failed")));
-    return result;
-  }
-
-  // Puts the code unit back into the stream buffer.
-  void unget(char c) {
-    if (ungetc(c, file_) == EOF)
-      FMT_THROW(system_error(errno, FMT_STRING("ungetc failed")));
-  }
-};
-
-// A FILE wrapper for glibc.
-template <typename F> class glibc_file : public file_base<F> {
- public:
-  using file_base<F>::file_base;
-
-  // Returns the file's read buffer as a string_view.
-  auto buffer() const -> string_view {
-    return {this->file_->_IO_read_ptr,
-            to_unsigned(this->file_->_IO_read_end - this->file_->_IO_read_ptr)};
-  }
-};
-
-// A FILE wrapper for Apple's libc.
-template <typename F> class apple_file : public file_base<F> {
- public:
-  using file_base<F>::file_base;
-
-  auto buffer() const -> string_view {
-    return {reinterpret_cast<char*>(this->file_->_p),
-            to_unsigned(this->file_->_r)};
-  }
-};
-
-// A fallback FILE wrapper.
-template <typename F> class fallback_file : public file_base<F> {
- private:
-  char next_;  // The next unconsumed character in the buffer.
-  bool has_next_ = false;
-
- public:
-  using file_base<F>::file_base;
-
-  auto buffer() const -> string_view { return {&next_, has_next_ ? 1u : 0u}; }
-
-  auto get() -> int {
-    has_next_ = false;
-    return file_base<F>::get();
-  }
-
-  void unget(char c) {
-    file_base<F>::unget(c);
-    next_ = c;
-    has_next_ = true;
-  }
-};
-
 class file_scan_buffer : public scan_buffer {
  private:
   template <typename F, FMT_ENABLE_IF(sizeof(F::_IO_read_ptr) != 0)>
@@ -243,19 +166,19 @@ class file_scan_buffer : public scan_buffer {
 
   // Fills the buffer if it is empty.
   void fill() {
-    string_view buf = file_.buffer();
+    span<const char> buf = file_.get_read_buffer();
-    if (buf.size() == 0) {
+    if (buf.size == 0) {
       int c = file_.get();
       // Put the character back since we are only filling the buffer.
       if (c != EOF) file_.unget(static_cast<char>(c));
-      buf = file_.buffer();
+      buf = file_.get_read_buffer();
     }
     set(buf);
   }
 
   void consume() override {
     // Consume the current buffer content.
-    size_t n = to_unsigned(ptr() - file_.buffer().begin());
+    size_t n = to_unsigned(ptr() - file_.get_read_buffer().data);
     for (size_t i = 0; i != n; ++i) file_.get();
     fill();
   }