Throw SystemError if fwrite fails.

format-test.cc

@@ -100,7 +100,7 @@ using fmt::pad;
     } \
     catch (expected_exception const& e) { \
       gtest_caught_expected = true; \
-      if (std::strcmp(message, e.what()) != 0) \
+      if (std::string(message) != e.what()) \
         throw; \
     } \
     catch (...) { \
@@ -1561,6 +1561,18 @@ TEST(FormatterTest, FileSink) {
   EXPECT_EQ("test", ReadFile("out"));
 }
 
+TEST(FormatterTest, FileSinkWriteError) {
+  FILE *f = std::fopen("out", "r");
+  fmt::FileSink fs(f);
+  int result = std::fwrite(" ", 1, 1, f);
+  int error_code = errno;
+  EXPECT_EQ(0, result);
+  std::string error_message =
+      str(Format("{}: {}") << "cannot write to file" << strerror(error_code));
+  EXPECT_THROW_MSG(fs(Writer() << "test"), fmt::SystemError, error_message);
+  std::fclose(f);
+}
+
 // The test doesn't compile on older compilers which follow C++03 and
 // require an accessible copy constructor when binding a temporary to
 // a const reference.

format.cc

@@ -33,11 +33,17 @@
 
 #include "format.h"
 
+#include <string.h>
+
 #include <cctype>
 #include <climits>
 #include <cmath>
 #include <cstdarg>
 
+#ifdef _WIN32
+# include <windows.h>
+#endif
+
 using fmt::ULongLong;
 
 #if _MSC_VER
@@ -161,6 +167,55 @@ void fmt::internal::ReportUnknownType(char code, const char *type) {
         << static_cast<unsigned>(code) << type));
 }
 
+void fmt::internal::FormatSystemErrorMessage(
+    fmt::Writer &out, int error_code, fmt::StringRef message) {
+#ifndef _WIN32
+  Array<char, INLINE_BUFFER_SIZE> buffer;
+  buffer.resize(INLINE_BUFFER_SIZE);
+  char *system_message = 0;
+  for (;;) {
+    errno = 0;
+# ifdef _GNU_SOURCE
+    system_message = strerror_r(error_code, &buffer[0], buffer.size());
+# else
+    strerror_r(error_code, system_message = &buffer[0], buffer.size());
+# endif
+    if (errno == 0)
+      break;
+    if (errno != ERANGE) {
+      // Can't get error message, report error code instead.
+      out << message << ": error code = " << error_code;
+      return;
+    }
+    buffer.resize(buffer.size() * 2);
+  }
+  out << message << ": " << system_message;
+#else
+  class String {
+   private:
+    LPWSTR str_;
+
+   public:
+    String() : str_() {}
+    ~String() { LocalFree(str_); }
+    LPWSTR *ptr() { return &str_; }
+    LPCWSTR c_str() const { return str_; }
+  };
+  String system_message;
+  if (FormatMessageW(FORMAT_MESSAGE_ALLOCATE_BUFFER |
+      FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, 0,
+      error_code, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
+      reinterpret_cast<LPWSTR>(system_message.ptr()), 0, 0)) {
+    UTF16ToUTF8 utf8_message;
+    if (!utf8_message.Convert(system_message.c_str())) {
+      out << message << ": " << utf8_message;
+      return;
+    }
+  }
+  // Can't get error message, report error code instead.
+  out << message << ": error code = " << error_code;
+#endif
+}
 
 // Fills the padding around the content and returns the pointer to the
 // content area.
@@ -688,6 +743,12 @@ void fmt::BasicWriter<Char>::FormatParser::Format(
   writer.buffer_.append(start, s);
 }
 
+void fmt::SystemErrorSink::operator()(const fmt::Writer &w) const {
+  Writer message;
+  internal::FormatSystemErrorMessage(message, error_code_, w.c_str());
+  throw SystemError(message.c_str(), error_code_);
+}
+
 void fmt::ANSITerminalSink::operator()(
     const fmt::BasicWriter<char> &w) const {
   char escape[] = "\x1b[30m";

format.h

@@ -31,6 +31,7 @@
 #include <stdint.h>
 
 #include <cassert>
+#include <cerrno>
 #include <cstddef>  // for std::ptrdiff_t
 #include <cstdio>
 #include <algorithm>
@@ -120,11 +121,71 @@ FMT_GCC_EXTENSION typedef unsigned long long ULongLong;
 template <typename Char>
 class BasicWriter;
 
+typedef BasicWriter<char> Writer;
+typedef BasicWriter<wchar_t> WWriter;
+
 template <typename Char>
 class BasicFormatter;
 
 struct FormatSpec;
 
+/**
+  \rst
+  A string reference. It can be constructed from a C string, ``std::string``
+  or as a result of a formatting operation. It is most useful as a parameter
+  type to allow passing different types of strings in a function, for example::
+
+    Formatter<> Format(StringRef format);
+
+    Format("{}") << 42;
+    Format(std::string("{}")) << 42;
+    Format(Format("{{}}")) << 42;
+  \endrst
+ */
+template <typename Char>
+class BasicStringRef {
+ private:
+  const Char *data_;
+  mutable std::size_t size_;
+
+ public:
+  /**
+    Constructs a string reference object from a C string and a size.
+    If *size* is zero, which is the default, the size is computed with
+    `strlen`.
+   */
+  BasicStringRef(const Char *s, std::size_t size = 0) : data_(s), size_(size) {}
+
+  /**
+    Constructs a string reference from an `std::string` object.
+   */
+  BasicStringRef(const std::basic_string<Char> &s)
+  : data_(s.c_str()), size_(s.size()) {}
+
+  /**
+    Converts a string reference to an `std::string` object.
+   */
+  operator std::basic_string<Char>() const {
+    return std::basic_string<Char>(data_, size());
+  }
+
+  /**
+    Returns the pointer to a C string.
+   */
+  const Char *c_str() const { return data_; }
+
+  /**
+    Returns the string size.
+   */
+  std::size_t size() const {
+    if (size_ == 0) size_ = std::char_traits<Char>::length(data_);
+    return size_;
+  }
+};
+
+typedef BasicStringRef<char> StringRef;
+typedef BasicStringRef<wchar_t> WStringRef;
+
 namespace internal {
 
 // The number of characters to store in the Array object, representing the
@@ -396,71 +457,36 @@ void FormatDecimal(Char *buffer, UInt value, unsigned num_digits) {
 template <typename Char, typename T>
 void FormatCustomArg(
   BasicWriter<Char> &w, const void *arg, const FormatSpec &spec);
-}
+
+// Formats a system error message writing the output to out.
+void FormatSystemErrorMessage(Writer &out, int error_code, StringRef message);
+
+}  // namespace internal
 
 /**
-  \rst
-  A string reference. It can be constructed from a C string, ``std::string``
-  or as a result of a formatting operation. It is most useful as a parameter
-  type to allow passing different types of strings in a function, for example::
-
-    Formatter<> Format(StringRef format);
-
-    Format("{}") << 42;
-    Format(std::string("{}")) << 42;
-    Format(Format("{{}}")) << 42;
-  \endrst
+  A formatting error such as invalid format string.
  */
-template <typename Char>
-class BasicStringRef {
- private:
-  const Char *data_;
-  mutable std::size_t size_;
-
- public:
-  /**
-    Constructs a string reference object from a C string and a size.
-    If *size* is zero, which is the default, the size is computed with
-    `strlen`.
-   */
-  BasicStringRef(const Char *s, std::size_t size = 0) : data_(s), size_(size) {}
-
-  /**
-    Constructs a string reference from an `std::string` object.
-   */
-  BasicStringRef(const std::basic_string<Char> &s)
-  : data_(s.c_str()), size_(s.size()) {}
-
-  /**
-    Converts a string reference to an `std::string` object.
-   */
-  operator std::basic_string<Char>() const {
-    return std::basic_string<Char>(data_, size());
-  }
-
-  /**
-    Returns the pointer to a C string.
-   */
-  const Char *c_str() const { return data_; }
-
-  /**
-    Returns the string size.
-   */
-  std::size_t size() const {
-    if (size_ == 0) size_ = std::char_traits<Char>::length(data_);
-    return size_;
-  }
-};
-
-typedef BasicStringRef<char> StringRef;
-typedef BasicStringRef<wchar_t> WStringRef;
-
 class FormatError : public std::runtime_error {
  public:
   explicit FormatError(const std::string &message)
   : std::runtime_error(message) {}
 };
 
+/**
+  An error returned by the operating system or the language runtime,
+  for example a file opening error.
+ */
+class SystemError : public std::runtime_error {
+ private:
+  int error_code_;
+
+ public:
+  SystemError(StringRef message, int error_code)
+  : std::runtime_error(message), error_code_(error_code) {}
+
+  int error_code() const { return error_code_; }
+};
+
 enum Alignment {
   ALIGN_DEFAULT, ALIGN_LEFT, ALIGN_RIGHT, ALIGN_CENTER, ALIGN_NUMERIC
 };
@@ -1197,9 +1223,6 @@ BasicFormatter<Char> BasicWriter<Char>::Format(StringRef format) {
   return f;
 }
 
-typedef BasicWriter<char> Writer;
-typedef BasicWriter<wchar_t> WWriter;
-
 // The default formatting function.
 template <typename Char, typename T>
 void Format(BasicWriter<Char> &w, const FormatSpec &spec, const T &value) {
@@ -1323,15 +1346,14 @@ template <typename Char>
 inline const Char *c_str(const BasicWriter<Char> &f) { return f.c_str(); }
 
 /**
-  Returns the content of the output buffer as an `std::string`.
+  Converts a string reference an `std::string`.
  */
 inline std::string str(StringRef s) {
   return std::string(s.c_str(), s.size());
 }
 
 /**
-  Returns a pointer to the output buffer content with terminating null
-  character appended.
+  Returns the pointer to a C string.
  */
 inline const char *c_str(StringRef s) {
   return s.c_str();
@@ -1468,6 +1490,28 @@ inline Formatter<NullSink, wchar_t> Format(WStringRef format) {
   return f;
 }
 
+
+/**
+  A sink that gets the system error message corresponding to the error code
+  and throws SystemError.
+ */
+class SystemErrorSink {
+ private:
+  int error_code_;
+
+ public:
+  explicit SystemErrorSink(int error_code) : error_code_(error_code) {}
+
+  void operator()(const Writer &w) const;
+};
+
+/** Throws SystemError with a code and a formatted message. */
+inline Formatter<SystemErrorSink> ThrowSystemError(
+    int error_code, StringRef format) {
+  Formatter<SystemErrorSink> f(format, SystemErrorSink(error_code));
+  return f;
+}
+
 /** A sink that writes output to a file. */
 class FileSink {
  private:
@@ -1478,8 +1522,8 @@ class FileSink {
 
   /** Writes the output to a file. */
   void operator()(const BasicWriter<char> &w) const {
-    // TODO: check error
-    std::fwrite(w.data(), w.size(), 1, file_);
+    if (std::fwrite(w.data(), w.size(), 1, file_) == 0)
+      ThrowSystemError(errno, "cannot write to file");
   }
 };