New exception class QPDFSystemError (fixes #221)

ChangeLog

@@ -1,3 +1,9 @@
+2018-08-13  Jay Berkenbilt  <ejb@ql.org>
+
+	* Add new class QPDFSystemError, derived from std::runtime_error,
+	which is now thrown by QUtil::throw_system_error. This enables the
+	triggering errno value to be retrieved. Fixes #221.
+
 2018-08-12  Jay Berkenbilt  <ejb@ql.org>
 
 	* qpdf command line: add --no-warn option to suppress issuing

include/qpdf/QPDFSystemError.hh

@@ -0,0 +1,58 @@
+// Copyright (c) 2005-2018 Jay Berkenbilt
+//
+// This file is part of qpdf.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Versions of qpdf prior to version 7 were released under the terms
+// of version 2.0 of the Artistic License. At your option, you may
+// continue to consider qpdf to be licensed under those terms. Please
+// see the manual for additional information.
+
+#ifndef QPDFSYSTEMERROR_HH
+#define QPDFSYSTEMERROR_HH
+
+#include <qpdf/DLL.h>
+#include <qpdf/Types.h>
+
+#include <qpdf/Constants.h>
+#include <string>
+#include <stdexcept>
+
+class QPDFSystemError: public std::runtime_error
+{
+  public:
+    QPDF_DLL
+    QPDFSystemError(std::string const& description,
+                    int system_errno);
+    QPDF_DLL
+    virtual ~QPDFSystemError() throw ();
+
+    // To get a complete error string, call what(), provided by
+    // std::exception.  The accessors below return the original values
+    // used to create the exception.
+
+    QPDF_DLL
+    std::string const& getDescription() const;
+    QPDF_DLL
+    int getErrno() const;
+
+  private:
+    static std::string createWhat(std::string const& description,
+				  int system_errno);
+
+    std::string description;
+    int system_errno;
+};
+
+#endif // QPDFSYSTEMERROR_HH

include/qpdf/QUtil.hh

@@ -61,9 +61,14 @@ namespace QUtil
     QPDF_DLL
     unsigned char* unsigned_char_pointer(char const* str);
 
-    // Throw std::runtime_error with a string formed by appending to
+    // Throw QPDFSystemError, which is derived from
+    // std::runtime_error, with a string formed by appending to
     // "description: " the standard string corresponding to the
-    // current value of errno.
+    // current value of errno. You can retrieve the value of errno by
+    // calling getErrno() on the QPDFSystemError. Prior to qpdf 8.2.0,
+    // this method threw system::runtime_error directly, but since
+    // QPDFSystemError is derived from system::runtime_error, old code
+    // that specifically catches std::runtime_error will still work.
     QPDF_DLL
     void throw_system_error(std::string const& description);
 

libqpdf/QPDFSystemError.cc

@@ -0,0 +1,51 @@
+#include <qpdf/QPDFSystemError.hh>
+#include <qpdf/QUtil.hh>
+#include <string.h>
+
+QPDFSystemError::QPDFSystemError(std::string const& description,
+                                 int system_errno) :
+    std::runtime_error(createWhat(description, system_errno)),
+    description(description),
+    system_errno(system_errno)
+{
+}
+
+QPDFSystemError::~QPDFSystemError() throw ()
+{
+}
+
+std::string
+QPDFSystemError::createWhat(std::string const& description,
+                            int system_errno)
+{
+    std::string message;
+#ifdef _MSC_VER
+    // "94" is mentioned in the MSVC docs, but it's still safe if the
+    // message is longer.  strerror_s is a templated function that
+    // knows the size of buf and truncates.
+    char buf[94];
+    if (strerror_s(buf, errno) != 0)
+    {
+        message = description + ": failed with an unknown error";
+    }
+    else
+    {
+        message = description + ": " + buf;
+    }
+#else
+    message = description + ": " + strerror(errno);
+#endif
+    return message;
+}
+
+std::string const&
+QPDFSystemError::getDescription() const
+{
+    return this->description;
+}
+
+int
+QPDFSystemError::getErrno() const
+{
+    return this->system_errno;
+}

libqpdf/QUtil.cc

@@ -7,6 +7,7 @@
 # include <qpdf/InsecureRandomDataProvider.hh>
 #endif
 #include <qpdf/SecureRandomDataProvider.hh>
+#include <qpdf/QPDFSystemError.hh>
 
 #include <cmath>
 #include <iomanip>
@@ -132,22 +133,7 @@ QUtil::unsigned_char_pointer(char const* str)
 void
 QUtil::throw_system_error(std::string const& description)
 {
-#ifdef _MSC_VER
-    // "94" is mentioned in the MSVC docs, but it's still safe if the
-    // message is longer.  strerror_s is a templated function that
-    // knows the size of buf and truncates.
-    char buf[94];
-    if (strerror_s(buf, errno) != 0)
-    {
-        throw std::runtime_error(description + ": failed with an unknown error");
-    }
-    else
-    {
-        throw std::runtime_error(description + ": " + buf);
-    }
-#else
-    throw std::runtime_error(description + ": " + strerror(errno));
-#endif
+    throw QPDFSystemError(description, errno);
 }
 
 int

libqpdf/build.mk

@@ -45,6 +45,7 @@ SRCS_libqpdf = \
 	libqpdf/QPDFObjectHandle.cc \
 	libqpdf/QPDFPageDocumentHelper.cc \
 	libqpdf/QPDFPageObjectHelper.cc \
+	libqpdf/QPDFSystemError.cc \
 	libqpdf/QPDFTokenizer.cc \
 	libqpdf/QPDFWriter.cc \
 	libqpdf/QPDFXRefEntry.cc \

libtests/qutil.cc

@@ -5,8 +5,10 @@
 #include <fcntl.h>
 #include <qpdf/QUtil.hh>
 #include <qpdf/PointerHolder.hh>
+#include <qpdf/QPDFSystemError.hh>
 #include <string.h>
 #include <limits.h>
+#include <assert.h>
 
 #ifdef _WIN32
 # include <io.h>
@@ -137,9 +139,10 @@ void fopen_wrapper_test()
 	std::cout << "after fopen" << std::endl;
 	(void) fclose(f);
     }
-    catch (std::runtime_error& s)
+    catch (QPDFSystemError& s)
     {
 	std::cout << "exception: " << s.what() << std::endl;
+        assert(s.getErrno() != 0);
     }
 }