code_seg (__declspec)


The latest version of this topic can be found at code_seg (__declspec).

Microsoft Specific**

The code_seg declaration attribute names an executable text segment in the .obj file in which the object code for the function or class member functions will be stored.

__declspec(code_seg("segname")) declarator  

The __declspec(code_seg(...)) attribute enables the placement of code into separate named segments that can be paged or locked in memory individually. You can use this attribute to control the placement of instantiated templates and compiler-generated code.

A segment is a named block of data in an .obj file that is loaded into memory as a unit. A text segment is a segment that contains executable code. The term section is often used interchangeably with segment.

Object code that's generated when declarator is defined is put in the text segment specified by segname, which is a narrow-string literal. The name segname does not have to be specified in a section pragma before it can be used in a declaration. By default, when no code_seg is specified, object code is put in a segment named .text. A code_seg attribute overrides any existing #pragma code_seg directive. A code_seg attribute applied to a member function overrides any code_seg attribute applied to the enclosing class.

If an entity has a code_seg attribute, all declarations and definitions of the same entity must have identical code_seg attributes. If a base-class has a code_seg attribute, derived classes must have the same attribute.

When a code_seg attribute is applied to a namespace-scope function or a member function, the object code for that function is put in the specified text segment. When this attribute is applied to a class, all member functions of the class and nested classes—this includes compiler-generated special member functions—are put in the specified segment. Locally defined classes—for example, classes defined in a member function body—do not inherit the code_seg attribute of the enclosing scope.

When a code_seg attribute is applied to a template class or template function, all implicit specializations of the template are put in the specified segment. Explicit or partial specializations do not inherit the code_seg attribute from the primary template. You may specify the same or a different code_seg attribute on the specialization. A code_seg attribute can’t be applied to an explicit template instantiation.

By default, compiler-generated code such as a special member function is put in the .text segment. The #pragma code_seg directive does not override this default. Use the code_seg attribute on the class, class template, or function template to control where compiler-generated code is put.

Lambdas inherit code_seg attributes from their enclosing scope. To specify a segment for a lambda, apply a code_seg attribute after the parameter-declaration clause and before any mutable or exception specification, any trailing return-type specification, and the lambda body. For more information, see Lambda Expression Syntax. This example defines a lambda in a segment named PagedMem:

auto Sqr = [](int t) __declspec(code_seg("PagedMem")) -> int { return t*t; };  

Be careful when you put specific member functions—especially virtual member functions—in different segments. If you define a virtual function in a derived class that resides in a paged segment when the base class method resides in a non-paged segment, other base class methods or user code may assume that invoking the virtual method will not trigger a page fault.

This example shows how a code_seg attribute controls segment placement when implicit and explicit template specialization is used:

// code_seg.cpp  
// Compile: cl /EHsc /W4 code_seg.cpp  
// Base template places object code in Segment_1 segment  
template<class T>  
class __declspec(code_seg("Segment_1")) Example  
   virtual void VirtualMemberFunction(T /*arg*/) {}  
// bool specialization places code in default .text segment  
class Example<bool>   
   virtual void VirtualMemberFunction(bool /*arg*/) {}  
// int specialization places code in Segment_2 segment  
class __declspec(code_seg("Segment_2")) Example<int>   
   virtual void VirtualMemberFunction(int /*arg*/) {}  
// Compiler warns and ignores __declspec(code_seg("Segment_3"))  
// in this explicit specialization  
__declspec(code_seg("Segment_3")) Example<short>; // C4071  
int main()  
   // implicit double specialization uses base template's  
   // __declspec(code_seg("Segment_1")) to place object code  
   Example<double> doubleExample{};  
   // bool specialization places object code in default .text segment  
   Example<bool> boolExample{};  
   // int specialization uses __declspec(code_seg("Segment_2"))  
   // to place object code  
   Example<int> intExample{};  

END Microsoft Specific