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Name mangling

In software compiler engineering, name mangling (more properly called name decoration, although this term is less commonly used) is a technique used to solve various problems caused by the need to resolve unique names for programming entities in many modern programming languages.

It provides a way of encoding additional information about the name of a function, structure, class or another datatype in order to pass more semantic information from the compilers to linkers.

The need arises where the language allows different entities to be named with the same identifier as long as they occupy a different namespace (where a namespace is typically defined by a module, class, or explicit namespace directive).

Any object code produced by compilers is usually linked with other pieces of object code (produced by the same or another compiler) by a type of program called a linker. The linker needs a great deal of information on each program entity. For example, to correctly link a function it needs its name, the number of arguments and their types, and so on.

C name decoration in Microsoft Windows

Although in general, languages that do not support function overloading, such as C and classic Pascal, do not require or use name mangling, they use it in some cases to provide additional information about a function. For example, compilers targeted at Microsoft Windows platforms support a variety of calling conventions, which determine the manner in which parameters are sent to subroutines and results returned. Because the different calling conventions are not compatible with one another, compilers mangle symbols with codes detailing which convention should be used.

The mangling scheme was established by Microsoft, and has been informally followed by other compilers including Digital Mars, Borland, and GNU gcc. The scheme even applies to other languages, such as Pascal, D, Delphi, Fortran, and C#. This allows subroutines written in those languages to call, or be called by, existing Windows libraries using a calling convention different from their default.

When compiling the following C examples: int _cdecl f (int x) { return 0; } int _stdcall g (int y) { return 0; } int _fastcall h (int z) { return 0; } _cdecl is the default for C functions, if no calling convention is stated explicitly.

32 bit compilers emit, respectively:

_f
_g@4
@h@4

In the stdcall and fastcall mangling schemes, the function is encoded as _name@X and @name@X, for stdcall and fastcall respectively, where X is the number of bytes, in decimal, of the argument(s) in the parameter list (including those passed in registers, for fastcall).

Other common name decoration actions may involve adding prefixes, usually with an abundance of underscores (like __func__), or some standard capitalization.

Name mangling in C++

C++ compilers are the most widespread, and yet least standard, users of name mangling. The first C++ compilers were implemented as translators to C source code, which would then be compiled by a C compiler to object code; because of this, symbol names had to conform to C identifier rules. Even later, with the emergence of compilers which produced machine code or assembly directly, the system's linker generally did not support C++ symbols, and mangling was still required.

The C++ language does not define a standard decoration scheme, so each compiler uses its own. Combined with the fact that C++ decoration can become fairly complex (storing information about classes, templates, namespaces, operator overloading, etc), this means that object code produced by different compilers is not usually linkable.

Simple example

Consider the following two definitions of f() in a C++ program:

int f (void) { return 1; } int f (int) { return 0; } void g (void) { int i = f(), j = f(0); }

These are distinct functions, with no relation to each other apart from the name. If they were natively translated into C with no changes, the result would be an error — C does not permit two functions with the same name. The compiler therefore will encode the type information in the symbol name, the result being something resembling:

int __f_v (void) { return 1; } int __f_i (int) { return 0; } void __g_v (void) { int i = __f_v(), j = __f_i(0); }

Notice that g() is mangled even though there is no conflict; name mangling applies to all symbols.

Complex example

For a more complex example, we'll consider an example of a real-world name mangling implementation: that used by GNU GCC 3.x, and how it mangles the following example class. The mangled symbol is shown below the respective identifier name.

namespace wikipedia {

  class article
  {
  public:
     std::string format (void);
        /* = _ZN9wikipedia7article6formatEv */

     bool print_to (std::ostream&);
        /* = _ZN9wikipedia7article8print_toERSo */

     class wikilink
     {
     public:
        wikilink (std::string const& name);
           /* = _ZN9wikipedia7article8wikilinkC1ERKSs */
}; }; }

The name mangling scheme used here is relatively simple. All mangled symbols begin with _Z (note that an underscore followed by a capital is a reserved identifier in C and C++, so conflict with user identifiers is avoided); for nested names (including both namespaces and classes), this is followed by N, then a series of <length, id> pairs (the length being the length of the next identifier), and finally E. For example, wikipedia::article::format becomes

_ZN·9wikipedia·7article·6format·E

For functions, this is then followed by the type information; as format() is a void function, this is simply v; hence:

_ZN·9wikipedia·7article·6format·E·v

For print_to, a standard type std::ostream (or more properly std::basic_ostream<char, char_traits<char> >) is used, which has the special alias So; a reference to this type is therefore RSo, with the complete name for the function being:

_ZN·9wikipedia·7article·8print_to·E·RSo

How different compilers mangle the same functions

There isn't a standard scheme by which even trivial C++ identifiers are mangled, and consequently different compiler vendors (or even different versions of the same compiler, or the same compiler on different platforms) mangle public symbols in radically different (and thus totally incompatible) ways. Consider how different C++ compilers mangle the same functions:

!Compiler
!void h(int)
!void h (int, char)
!void h(void)
GNU GCC 3.x _Z1hi _Z1hic _Z1hv
GNU GCC 2.9x h__Fi h__Fic h__Fv
Intel C++ 8.0 for Linux _Z1hi _Z1hic _Z1hv
Microsoft VC++ v6/v7 ?h@@YAXH@Z ?h@@YAXHD@Z ?h@@YAXXZ
Borland C++ v3.1 @h$qi @h$qizc @h$qv
OpenVMS C++ V6.5 (ARM mode) H__XI H__XIC H__XV
OpenVMS C++ V6.5 (ANSI mode) CXX$__7H__FI0ARG51T CXX$__7H__FIC26CDH77 CXX$__7H__FV2CB06E8
OpenVMS C++ X7.1 IA-64 CXX$_Z1HI2DSQ26A CXX$_Z1HIC2NP3LI4 CXX$_Z1HV0BCA19V
Digital Mars C++ ?h@@YAXH@Z ?h@@YAXHD@Z ?h@@YAXXZ
SunPro CC __1cBh6Fi_v_ __1cBh6Fic_v_ __1cBh6F_v_
HP aC++ A.05.55 IA-64 _Z1hi _Z1hic _Z1hv
HP aC++ A.03.45 PA-RISC h__Fi h__Fic h__Fv
Tru64 C++ V6.5 (ARM mode) h__Xi h__Xic h__Xv
Tru64 C++ V6.5 (ANSI mode) __7h__Fi __7h__Fic __7h__Fv
Notes:

  • The Compaq C++ compiler on OpenVMS VAX and Alpha (but not IA-64) and Tru64 has two name mangling schemes. The original, pre-standard scheme is known as ARM model, and is based on the name mangling described in the C++ Annotated Reference Manual (ARM). With the advent of new features in standard C++, particularly templates, the ARM scheme became more and more unsuitable — it could not encode certain function types, or produced identical mangled names for different functions. It was therefore replaced by the newer "ANSI" model, which supported all ANSI template features, but was not backwards compatible.
  • On IA-64, a standard ABI exists (see external links), which defines (among other things) a standard name-mangling scheme, and which is used by all the IA-64 compilers. GNU GCC 3.x, in addition, has adopted the name mangling scheme defined in this standard for use on other, non-Intel platforms.

Handling of C symbols when linking from C++

The job of the common C++ idiom:

#ifdef __cplusplus
extern "C" {

  1. endif

   /* ... */

  1. ifdef __cplusplus
  2. }
  3. endif

is to ensure that the symbols following are "unmangled" – that the compiler emits a binary file with their names undecorated, as a C compiler would do. As C language definitions are unmangled, the C++ compiler needs to avoid mangling references to these identifiers.

For example, the standard strings library, <string.h> usually contains something resembling:

#ifdef __cplusplus
extern "C" {

  1. endif

void *memset (void *, int, size_t); char *strcat (char *, const char *); int strcmp (const char *, const char *); char *strcpy (char *, const char *);

  1. ifdef __cplusplus
  2. }
  3. endif

Thus, code such as:

if (strcmp(argv[1], "-x") == 0)
   strcpy(a, argv[2]);
else
   memset (a, 0, sizeof(a));

uses the correct, unmangled strcmp and memset. If the extern had not been used, the C++ compiler would produce code equivalent to:

if (__1cGstrcmp6Fpkc1_i_(argv[1], "-x") == 0)
   __1cGstrcpy6Fpcpkc_0_(a, argv[2]);
else
   __1cGmemset6FpviI_0_ (a, 0, sizeof(a));

Since those symbols do not exist in the C runtime library (e.g. libc), link errors would result.

Standardised name mangling in C++

While it is a relatively common belief that standardised name mangling in the C++ language would lead to greater interoperability between implementations, this is not really the case. Name mangling is only one of several application binary interface issues in a C++ implementation. Other ABI issues like exception handling, virtual table layout, structure padding, etc. cause differing C++ implementations to be incompatible. Further, requiring a particular form of mangling would cause issues for systems where implementation limits (e.g. length of symbols) dictate a particular mangling scheme. A standardised requirement for name mangling would also prevent an implementation where mangling was not required at all — for example, a linker which understood the C++ language.

The C++ standard therefore does not attempt to standardise name mangling. On the contrary, the Annotated C++ Reference Manual (also known as ARM, ISBN 0-201-51459-1, section 7.2.1c) actively encourages the use of different mangling schemes to prevent linking when other aspects of the ABI, such as exception handling and virtual table layout, are incompatible.

Real-world effects of C++ name mangling

Because C++ symbols are routinely exported from DLL and shared object files, the name mangling scheme is not merely a compiler-internal matter. Different compilers (or different versions of the same compiler, in many cases) produce such binaries under different name decoration schemes, meaning that symbols are frequently unresolved if the compilers used to create the library and the program using it employed different schemes. For example, if a system with multiple C++ compilers installed (e.g. GNU GCC and the OS vendor's compiler) wished to install the Boost library, it would have to be compiled twice — once for the vendor compiler and once for GCC.

For this reason name decoration is an important aspect of any C++-related ABI.

Name mangling in Java

The language, compiler, and .class file format were all designed together (and had object-orientation in mind from the start), so the primary problem solved by name mangling doesn't exist in implementations of the Java runtime. There are, however, cases where an analogous transformation and qualification of names is necessary.

Creating unique names for inner and anonymous classes

The scope of anonymous classes is confined to their parent class, so the compiler must produce a "qualified" public name for the inner class, to avoid conflict where other classes (inner or not) exist in the same namespace. Similarly, anonymous classes must have "fake" public names generated for them (as the concept of anonymous classes exists only in the compiler, not the runtime). So, compiling the following java program

public class foo {

   class bar {
       public int x;
}

   public void zark {
       Object f = new Object {
           public String toString() {
               return "hello";
} }; } }

will produce three .class files:

  • foo.class, containing the main (outer) class foo
  • foo$bar.class, containing the named inner class foo.bar
  • foo$1.class, containing the anonymous inner class (local to method foo.zark)

All of these class names are valid (as $ symbols are permitted in the JVM specification) and these names are "safe" for the compiler to generate, as the Java language definition prohibits $ symbols in normal java class definitions.

Name resolution in Java is further complicated at runtime, as fully qualified class names are unique only inside a specific classloader instance. Classloaders are ordered hierarchically and each Thread in the JVM has a so called context class loader, so in cases where two different classloader instances contain classes with the same name, the system first tries to load the class using the root (or system) classloader and then goes down the hierarchy to the context class loader.

Handling issues with the java to native interface

Java's native method support allows java language programs to call out to programs written in another language (generally either C or C++). There are two name-resolution concerns here, neither of which is implemented in a particularly standard manner.

Name mangling in Python

A Python programmer can explicitly designate that an identifier is a "private name" (its scope is confined to the class) by setting the first two characters of the identifier to be underscores. Mangling will not be performed if the identifier ends with more than one underscore, for example, __thing will be mangled, as will ___thing and __thing_, but __thing__ and __thing___ will not. On encountering these, the Python compiler or interpreter turns these private names into global symbols by prepending a string consisting of a single underscore and the name of the enclosing class.

So, for example,

class Test:

   def __private_symbol(self):
       pass
   def normal_symbol(self):
       pass

print dir(Test)

will output:

['_Test__private_symbol',
'__doc__',
'__module__',
'normal_symbol']

Name mangling in Borland's Turbo Pascal / Delphi range

To avoid name mangling in Pascal, use:

exports

 myFunc name 'myFunc', myProc name 'myProc';

Name mangling in Objective-C

Essentially two forms of method exist in Objective-C, the class ("static") method, and the instance method. A method declaration in Objective-C is of the following form
+ method name: argument name1:parameter1 ...
- method name: argument name1:parameter1 ...
Class methods are signified by +, instance methods use -. A typical class method declaration may then look like:
+ (id) initWithX: (int) number andY: (int) number;
+ (id) new;
with instance methods looking like
- (id) value;
- (id) setValue: (id) new_value;

Each of these method declarations have a specific internal representation. When compiled, each method is named according to the following scheme for class methods:

_c_Class_methodname_name1_name2_ ...
and this for instance methods:
_i_Class_methodname_name1_name2_ ...

The colons in the Objective-C syntax are translated to underscores. So, the Objective-C class method + (id) initWithX: (int) number andY: (int) number;, if belonging to the Point class would translate as _c_Point_initWithX_andY_, and the instance method (belonging to the same class) - (id) value; would translate to _i_Point_value.

Each of the methods of a class are labeled in this way. However, in order to look up a method that a class may respond to would be tedious if all methods are represented in this fashion. Each of the methods is assigned a unique symbol (such as an integer). Such a symbol is known as a selector. In Objective-C, one can manage selectors directly — they have a specific type in Objective-C — SEL.

During compilation, a table is built that maps the textual representation (such as _i_Point_value) to selectors (which are given a type SEL). Managing selectors is more efficient than manipulating the textual representation of a method. Note that a selector only matches a method's name, not the class it belongs to — different classes can have different implementations of a method with the same name. Because of this, implementations of a method are given a specific identifier too — these are known as implementation pointers, and are given a type also, IMP.

Message sends are encoded by the compiler as calls to the id objc_msgSend (id receiver, SEL selector, ...) function, or one of its cousins, where receiver is the receiver of the message, and SEL determines the method to call. Each class has its own table that maps selectors to their implementations — the implementation pointer specifies where in memory the actual implementation of the method resides. There are separate tables for class and instance methods. Apart from being stored in the SEL to IMP lookup tables, the functions are essentially anonymous.

The SEL value for a selector does not vary between classes. This enables polymorphism.

The Objective-C runtime maintains information about the argument and return types of methods. However, this information is not part of the name of the method, and can vary from class to class.

Since Objective-C does not support namespaces, there is no need for mangling of class names (that do appear as symbols in generated binaries).

Name mangling in Fortran

Name mangling is also necessary in Fortran compilers, originally because the language is case insensitive. Further mangling requirements were imposed later in the evolution of the language because of the addition of modules and other features in the Fortran 90 standard. The case mangling, especially, is a common issue that must be dealt with in order to call Fortran libraries (such as LAPACK) from other languages (such as C).

Because of the case insensitivity, the name of a subroutine or function "FOO" must be converted to a canonical case and format by the Fortran compiler so that it will be linked in the same way regardless of case. Different compilers have implemented this in various ways, and no standardization has occurred. The AIX and HP-UX Fortran compilers convert all identifiers to lower case ("foo"), while the Cray Unicos Fortran compilers converted identifiers all upper case ("FOO"). The GNU g77 compiler converts identifiers to lower case plus an underscore ("foo_"), except that identifiers already containing an underscore ("FOO_BAR") has two underscores appended ("foo_bar__"), following a convention established by f2c. Many other compilers, including SGI's IRIX compilers, gfortran, and Intel's Fortran compiler, convert all identifiers to lower case plus an underscore ("foo_" and "foo_bar_").

Identifiers in Fortran 90 modules must be further mangled, because the same subroutine name may apply to different routines in different modules.

External links

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