Add a portable thread-safe version of strerror.

format-test.cc

@@ -248,6 +248,22 @@ TEST(UtilTest, UTF8ToUTF16) {
 // TODO: test UTF16ToUTF8::Convert
 #endif  // _WIN32
 
+TEST(UtilTest, StrError) {
+  using fmt::internal::StrError;
+  EXPECT_DEBUG_DEATH(StrError(EDOM, 0, 0), "Assertion");
+  char buffer[BUFFER_SIZE];
+  EXPECT_DEBUG_DEATH(StrError(EDOM, buffer, 0), "Assertion");
+  buffer[0] = 'x';
+  const char *message = StrError(-1, buffer, 1);
+  EXPECT_EQ(ERANGE, errno);
+  EXPECT_STREQ("", message);
+  message = StrError(-1, buffer, BUFFER_SIZE);
+  EXPECT_EQ(0, errno);
+  EXPECT_GE(BUFFER_SIZE - 1, std::strlen(message));
+  message = StrError(-1, buffer, std::strlen(message));
+  EXPECT_EQ(ERANGE, errno);
+}
+
 class TestString {
  private:
   std::string value_;

format.cc

@@ -204,6 +204,26 @@ int fmt::internal::UTF16ToUTF8::Convert(fmt::WStringRef s) {
 
 #endif
 
+char *fmt::internal::StrError(
+    int error_code, char *buffer, std::size_t buffer_size) {
+  assert(buffer != 0 && buffer_size != 0);
+  errno = 0;
+#ifdef _GNU_SOURCE
+  char *message = strerror_r(error_code, buffer, buffer_size);
+  if (message == buffer && strlen(buffer) == buffer_size - 1)
+    errno = ERANGE;  // The buffer is full so the message is probably truncated.
+  return message;
+#elif _WIN32
+  errno = strerror_s(buffer, buflen, error_code);
+  if (errno == 0 && std::strlen(buffer) == buffer_size - 1)
+    errno = ERANGE;  // The buffer is full so the message is probably truncated.
+  return buffer;
+#else
+  strerror_r(error_code, buffer, buffer_size);
+  return buffer;
+#endif
+}
+
 void fmt::internal::FormatSystemErrorMessage(
     fmt::Writer &out, int error_code, fmt::StringRef message) {
 #ifndef _WIN32

format.h

@@ -489,6 +489,16 @@ class UTF16ToUTF8 {
 };
 #endif
 
+// Portable thread-safe version of strerror.
+// Returns a pointer to a string describing the error code. This can be
+// either a pointer to a string stored in buffer, or a pointer to some
+// static immutable string. Sets errno to one of the following values:
+//   0      - success
+//   ERANGE - buffer is not large enough to store the error message
+//   other  - failure
+// Buffer should be at least of size 1.
+char *StrError(int error_code, char *buffer, std::size_t buffer_size);
+
 // Formats a system error message writing the output to out.
 void FormatSystemErrorMessage(Writer &out, int error_code, StringRef message);
 
@@ -1521,7 +1531,6 @@ 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.