Application Binary Interface for C within CLI

Rhys Weatherley, rweather@southern-storm.com.au.
Last Modified: $Date: 2002/08/23 00:06:39 $

Copyright © 2002 Southern Storm Software, Pty Ltd.
Permission to distribute copies of this work under the terms of the GNU Free Documentation License is hereby granted.

1. Introduction

This document describes an Application Binary Interface (ABI) for the C language within Common Language Infrastructure (CLI) environments that meets the following goals:

Note 1. Given the nature of C, it is always possible for a programmer to write code that depends upon platform-specific word sizes, endianness, and operating system facilities. Our goal is that C code written to commonly used C coding standards should not be aware of such platform differences.
Note 2. This doesn't preclude the application programmer from using native code facilities such as PInvoke. But the compiler itself will not use such features to implement the ABI.
Note 3. If the ABI avoids vendor-specific naming, it is more likely to be adopted by other vendors.
Some things are deliberately outside the scope of this ABI definition. We do not describe the facilities that are provided by the "libc" implementation, or the contents of standard header files, for example.

In the sections below, we suggest extended syntax for the C language to enable access to CLI-specific features. This syntax is only a suggestion. Two compilers that use different syntax for the same feature can still interoperate if they translate their syntax into the same ABI conventions.

All extension keywords begin and end in "__", following standard C practice. We recommend that compiler vendors seriously consider adopting the proposed keywords to make it easier to port source code from one compiler to another.

Note: Some of the features described in this document haven't been fully implemented by Portable.NET's C compiler yet. This document is therefore subject to change.

2. Memory models

This ABI defines two primary memory models for the CLI, which we will refer to as "Model 64" and "Model 32": They may be briefly summarised as follows: We recommend that "Model 64" be the default for all compilers that adhere to this ABI specification. Programs that use "Model 64" will work on all implementations of the CLI, be they 32-bit or 64-bit. "Model 32" programs will be more memory-efficient on 32-bit platforms, but will not run at all on 64-bit implementations of the CLI.

Note: A "Model 64" program will fail to work on a 128-bit CLI implementation, for the same reason that "Model 32" programs fail on 64-bit CLI implementations. When and if 128-bit CLI implementations become common-place, it will be easy to extend this ABI to include a "Model 128".
When the compiler builds an object file, application, or library, it MUST tag the corresponding module with the memory model. For example, the following module is tagged as "Model 64":

.module test.exe
.custom instance void [OpenSystem.C]OpenSystem.C.MemoryModelAttribute::.ctor
       (int32) = (01 00 40 00 00 00 00 00)
Linkers can use the presence of this attribute to detect that a C application is being linked, rather than a C# application, and then modify their behaviour accordingly. For example, by adding additional libraries to the link that aren't normally required by C# applications.

The two primary memory models are designed to mirror existing 64-bit and 32-bit CPU architectures. But sometimes the programmer will want to exactly match the memory model to the underlying operating system, to improve interoperability with native code. In the process, portability is sacrificed, so this must only be used when absolutely necessary.

To match an underlying operating system, the compiler chooses either "Model 64" or "Model 32", based on the size of the system's "void *" type. The compiler then applies a number of "model modifiers" to alter type alignment values to match the system. These modifiers are as follows:

0x00000001
16-bit short values are aligned on 1-byte boundaries, rather than 2-byte boundaries.
0x00000002
32-bit int values are aligned on 1-byte boundaries, rather than 4-byte boundaries.
0x00000004
32-bit int values are aligned on 2-byte boundaries, rather than 4-byte boundaries.
0x00000008
64-bit long long values are aligned on 1-byte boundaries, rather than 8-byte boundaries.
0x00000010
64-bit long long values are aligned on 2-byte boundaries, rather than 8-byte boundaries.
0x00000020
64-bit long long values are aligned on 4-byte boundaries, rather than 8-byte boundaries.
0x00000040
32-bit float values are aligned on 1-byte boundaries, rather than 4-byte boundaries.
0x00000080
32-bit float values are aligned on 2-byte boundaries, rather than 4-byte boundaries.
0x00000100
64-bit double values are aligned on 1-byte boundaries, rather than 8-byte boundaries.
0x00000200
64-bit double values are aligned on 2-byte boundaries, rather than 8-byte boundaries.
0x00000400
64-bit double values are aligned on 4-byte boundaries, rather than 8-byte boundaries.
0x00000800
long double values are aligned on 1-byte boundaries.
0x00001000
long double values are aligned on 2-byte boundaries.
0x00002000
long double values are aligned on 4-byte boundaries.
0x00004000
long double values are aligned on 8-byte boundaries.
0x00008000
long double values are aligned on 16-byte boundaries.
0x00010000
Pointer values are aligned on 1-byte boundaries.
0x00020000
Pointer values are aligned on 2-byte boundaries.
0x00040000
Pointer values are aligned on 4-byte boundaries and the primary memory model is "Model 64". This modifier should not be set if the primary memory model is "Model 32".
0x00080000
Bit fields are allocated in big-endian order rather than the default of little-endian.

These modifiers describe how the actual memory model differs from either "Model 32" or "Model 64". Programs that are compiled with non-zero modifier values are unlikely to work on runtime engines that use a different combination of flags. Programs with zero modifier values should work on all runtime engines that support the memory model.

When the object file is generated, the modifier flags are written into the "MemoryModel" attribute declaration, as an optional argument:

.module test.exe
.custom instance void [OpenSystem.C]OpenSystem.C.MemoryModelAttribute::.ctor
       (int32, int32) = (01 00 20 00 00 00 20 04 00 00 00 00)
This indicates "Model 32 with 4-byte alignment of 64-bit integers and doubles". If the second parameter is not present, the default modifier flag value is zero.

3. ABI support library

The "MemoryModelAttribute" example in the previous section demonstrated the use of the "OpenSystem.C" assembly, which provides a number of classes for tagging C applications, and for implementing ABI support facilities. The following summarises the important classes in the "OpenSystem.C" namespace:

IsConst
A modifier class for marking a type as "const".
IsFunctionPointer
A modifier class for marking a type as a function pointer, rather than as a function signature.
BitFieldAttribute
An attribute that describes the position and size of a bit field within a larger integer field.
WeakAliasForAttribute
An attribute that marks a method as defining a weak alias.
StrongAliasForAttribute
An attribute that marks a field or method as defining a strong alias.
InitializerAttribute
An attribute that marks special methods that provide static initialization logic to be executed at program startup. These methods are different from CLI static constructors (.cctor methods), in that initializers are guaranteed to be executed before main.
InitializerOrderAttribute
An attribute that defines the ordering of an initializer relative to all others. Initializers with a lower order value are called before those with higher order values. The default order value is zero.
FinalizerAttribute
An attribute that marks special methods that provide static finalization logic to be executed at program shutdown. These methods are not the same as the "Finalize" methods in garbage-collected objects. The order of garbage-collected object finalization is indeterminate with respect to C finalizers.
FinalizerOrderAttribute
An attribute that defines the ordering of a finalizer relative to all others. Finalizers with a lower order value are called after those with higher order values. The default order value is zero. Normally an initializer and its corresponding finalizer will have identical order values.
MemoryModelAttribute
An attribute that specifies the memory model for a C module.
OriginalNameAttribute
An attribute that specifies the original name of a symbol that had to be renamed to resolve link-time naming conflicts.
LongJmpException
An exception class that assists the ABI in implementing setjmp/longjmp operations.
Crt0
A class that provides utility methods to manage the startup and shutdown of C applications.
LongDouble
A value type class that wraps up the runtime engine's "native float" type. The standard C# base class library lacks a suitable class. This class has a constructor that takes a single "native float" argument, and an "Unpack" method that returns a "native float" when applied to an instance.
FloatComplex, DoubleComplex, LongDoubleComplex
Value types that correspond to the ISO C complex number types.
FloatImaginary, DoubleImaginary, LongDoubleImaginary
Value types that correspond to the ISO C imaginary number types.
CNameAttribute
An attribute that specifies the C name of a value type defined in C# code. e.g. "FloatComplex" is marked as "float _Complex".
The meaning of these classes will become clearer in later sections. Other classes may be provided to support "libc" implementations, but they are beyond the scope of this specification.

To simplify discussion, we will use an abbreviated syntax to describe attributes in CIL assembly code examples. For example, the memory model designation in the previous section can also be written as: 

.module test.exe
.custom [OpenSystem.C.MemoryModel(64)]
This abbreviation is for exposition purposes only. It isn't intended to suggest an alternative syntax for CIL assemblers.

4. Type representation

4.1. Primitive types

The size and alignment of the primitive types in "Model 64" and "Model 32" are defined as follows:

TypeModel 64
Size/Align1		 Model 32
Size/Align1		Description
void 1/1 2 1/1 Void type
_Bool 1/1 1/1 8-bit boolean value (C# "bool")
char 1/1 1/1 Signed 8-bit integer
unsigned char 1/1 1/1 Unsigned 8-bit integer
short 2/2 2/2 Signed 16-bit integer
unsigned short 2/2 2/2 Unsigned 16-bit
__wchar__ 2/2 2/2 16-bit wide character value (C# "char")
int 4/4 4/4 Signed 32-bit integer
unsigned int 4/4 4/4 Unsigned 32-bit integer
long 8/8 4/4 Signed 64-bit or 32-bit integer
unsigned long 8/8 4/4 Unsigned 64-bit or 32-bit integer
long long 8/8 8/8 Signed 64-bit integer
unsigned long long 8/8 8/8 Unsigned 64-bit integer
float 4/4 4/4 32-bit IEEE 754 floating-point
double 8/8 8/8 64-bit IEEE 754 floating-point
type * 8/8 4/4 Pointer to "type"
float _Complex 8/4 8/4 Complex number type based on float
double _Complex 16/8 16/8 Complex number type based on double
float _Imaginary 4/4 4/4 Imaginary number type based on float
double _Imaginary 8/8 8/8 Imaginary number type based on double

Note 1. These size and alignment values refer to the primary memory model. The values may be different if there are non-zero model modifier flags in effect.
Note 2. The size of "void" is 1, to be consistent with gcc.

In "Model 64", pointers are allocated 8 bytes of memory, and aligned on an 8-byte boundary, even on platforms that only support 32-bit pointers. The expression "sizeof(void *)" will always return 8. This behaviour is necessary to provide a consistent "struct" layout on all CLI implementations, as we will see in the following sections.

4.2. Type qualifiers

The "const" and "volatile" qualifiers are represented using the "OpenSystem.C.IsConst" and "System.Runtime.CompilerServices.IsVolatile" modifiers. The following table provides some examples:

DeclarationRepresentation
const int x; int32 modopt(OpenSystem.C.IsConst) x
void * volatile y; void * modreq(System.Runtime.CompilerServices.IsVolatile) y
const char *s; int8 modopt(OpenSystem.C.IsConst) * s
char * const s; int8 * modopt(OpenSystem.C.IsConst) s

The placement of the type modifier is important. A qualifier at the outer-most level of a type applies to the field or variable. A qualifier at an inner level applies to a referenced type. In the last example above, the variable "s" cannot be modified, but it points at a string that can be modified. In the second last example, the variable can be modified, but not the string.

The "IsVolatile" modifier is required, to be consistent with other CLI-compatible languages. The "IsConst" modifier is optional, because other CLI-compatible languages can safely ignore it (the programmer on the other hand probably shouldn't ignore it).

4.3. Type layout

Types in the ABI may have three kinds of layout: "fixed", "dynamic", or "unknown".

Fixed types have a constant size and alignment. The expression "sizeof(T)" can be evaluated to a constant at compile time.

Dynamic types have a constant size and alignment, but these values are not known until runtime. Native types (described in a later section) are an example, as are C# value types.

Unknown types have no known size. An example is "char[]", which cannot be used as the type of a structure field, as its storage size cannot be determined.

Traditional C compilers only have "fixed" and "unknown" types. One of the goals of this ABI is to minimize the occurence of "dynamic" types so that "struct" layout can be computed efficiently.

4.4. Struct representation

Structure types (e.g. "struct A") are converted into a value type called "struct A", with no namespace qualifier. This value type is marked as having explicit layout, with pre-computed class packing and size values. Each field within the structure has a pre-computed offset. For example, on a "Model 64" system: 
struct A
{
    int       item;
    struct A *next;
};

.class public explicit sealed ansi 'struct A' extends System.ValueType
{
    .pack 8
    .size 16
    .field [0] public int32 item
    .field [8] public 'struct A' * next
}
On a "Model 32" system, the structure would be encoded as:

.class public explicit sealed ansi 'struct A' extends System.ValueType
{
    .pack 4
    .size 8
    .field [0] public int32 item
    .field [4] public 'struct A' * next
}
Structures may only contain fields with "fixed" layout. Native structures (described later) can contain fields with both "fixed" and "dynamic" layout, but their usage is restricted.

If a structure contains sub-structures, they are converted into companion structure types:

struct A
{
    int x;
    struct
    {
        int y;
    } z;
    struct B
    {
        int w;
    } v;
};

.class public explicit sealed ansi 'struct A' extends System.ValueType
{
    .pack 4
    .size 12
    .field [0] public int32 x
    .class public explicit sealed ansi 'struct (1)'
                extends System.ValueType
    {
        .pack 4
        .size 4
        .field [0] public int32 y
    }
    .field [4] public valuetype 'struct A'/'struct (1)' z
    .field [8] public valuetype 'struct B' v
}
.class public explicit sealed ansi 'struct B' extends System.ValueType
{
    .pack 4
    .size 4
    .field [0] public int32 w
}
As can be seen, anonymous structures are assigned a unique numeric code, and are encoded as nested types. The code is unique to the surrounding structure, so that the same code will be generated each time the program is compiled.

Note: It would be desirable to allow the runtime engine to perform structure layout dynamically, rather than fix types to specific sizes and fields to specific offsets. Readers who think it may be possible to do so may like to ponder how to efficiently compile the following code so that it will work regardless of the runtime size of "void *" and the runtime alignment of "y":

struct item
{
    char x[sizeof(void *)];
    long long y;
};

long long get_y(struct item *i)
{
    return i->y;
}

4.5. Union representation

Unions (e.g. "union A") are represented as value types with the name "union A", and all fields explicitly laid out to start at offset 0. 
union A
{
    int    x;
    double y;
}

.class public explicit sealed ansi 'union A' extends System.ValueType
{
    .pack 8
    .size 8
    .field [0] public int32 x
    .field [0] public float64 y
}
Unions may only contain fields with "fixed" layout.

4.6. Representation of bit fields

Bit fields are represented as regular fields, with an attribute to indicate the field's position and size. Bits are allocated from the least significant bit, unless the big-endian modifier was specified in the memory model (see section 2 for further details).

struct A
{
    int x : 8;
    int y : 1;
    unsigned int z : 16;
    int w;
}

.class public explicit sealed ansi 'struct A' extends System.ValueType
{
    .custom [OpenSystem.C.BitField("x", ".bitfield-1", 0, 8)]
    .custom [OpenSystem.C.BitField("y", ".bitfield-1", 8, 1)]
    .custom [OpenSystem.C.BitField("z", ".bitfield-2", 0, 16)]
    .pack 4
    .size 12
    .field [0] public int32 '.bitfield-1'
    .field [4] public unsigned int32 '.bitfield-2'
    .field [8] public int32 w
}

4.7. Array types

Array types of the form "A[]" are mapped to a value type called "array A[]". For example, "int[]" is encoded as follows: 
.class public explicit sealed ansi 'array int[]'
            extends System.ValueType
{
    .pack 4
    .size 0
    .field private static specialname int32 elem__
}
The value type must have a field called "elem__", which defines the element type, and it must have the attributes "private static specialname".

Note. It will be rare to find a type of the form "A[]" in a generated object file, because such types normally decay to pointer types when used as function arguments. The encoding is specified here because the compiler does need to distinguish "A[]" from "A *" in certain circumstances.

If the array type includes a non-zero size value, then it is encoded as a value type with an explicit size defining the total size of the array. For example, "int[100] is encoded as follows: 

.class public explicit sealed ansi 'array int[100]'
            extends System.ValueType
{
    .pack 4
    .size 400
    .field [0] public specialname int32 elem__
}
The size of the array is determined by dividing ".size" by the size of the element type. The "elem__" field in this case must be "public specialname". Arrays with a zero size are encoded as follows:

.class public explicit sealed ansi 'array int[0]'
            extends System.ValueType
{
    .pack 4
    .size 0
    .field [0] public static specialname int32 elem__
}
Here, the "elem__" field is "static". This type can be distinguished from the encoding for "int[]" because the "elem__" field is "public" instead of "private".

Array element types must have "fixed" layout. The programmer can allocate arrays of "dynamic" types using "malloc" or "alloca".

The following is an example of encoding the two-dimensional array type "int [300][400]": 

.class public explicit sealed ansi 'array int[300][400]'
            extends System.ValueType
{
    .pack 4
    .size 480000  // == 300 * 400 * 4
    .field [0] public specialname valuetype 'array int[400]' elem__
}

.class public explicit sealed ansi 'array int[400]'
            extends System.ValueType
{
    .pack 4
    .size 1600  // == 400 * 4
    .field [0] public specialname int32 elem__
}

4.8. Native types

Sometimes it is necessary to access the native type representation of the underlying runtime engine, particularly when importing external library functions using PInvoke, or when accessing code written in other CLI-compliant languages.

All of the types that are described in this section have "dynamic" layout. They cannot be used as array element types, or as the members of non-native structures and unions.

The use of these native types is highly discouraged, except where it is absolutely essential to interoperate with other system components:

TypeDescription
__native__ int Signed native integer (C# "IntPtr")
unsigned __native__ int Unsigned native integer (C# "UIntPtr")
long double Native floating-point
long double _Complex Complex number type based on long double
long double _Imaginary Imaginary number type based on long double

The native integer types are "__native__ int" and "unsigned __native__ int". They may be either 4 or 8 bytes in size, depending upon the underlying platform. The expressions "sizeof(__native__ int)" and "sizeof(unsigned __native__ int)" are evaluated at run time.

The native floating point type is "long double", and is guaranteed to have precision greater than or equal to "double". The expression "sizeof(long double)" is computed at runtime.

Structures can be specified to have native layout at declaration time:

struct __native__ A
{
    int item;
    struct A *next;
};

This is represented by a sequential type definition in the program's metadata:

.class public sequential sealed ansi 'struct A' extends System.ValueType
{
    .field public int32 item
    .field public 'struct A' * next
}
The runtime engine will lay this out using platform-specific type sizes and alignment. The expression "sizeof(struct A)" will be evaluated at runtime.

Unions can also be specified to have native layout at declaration time:

union __native__ A
{
    int   x;
    void *y;
}

.class public explicit sealed ansi 'union A' extends System.ValueType
{
    .field [0] public int32 x
    .field [0] public void * y
}
The type is declared explicit, so that all fields can be defined with an offset of zero, but the type does not have an overall size.

Types with "fixed" and "dynamic" layout may be used as the members of native structures and unions.

It is recommended that the compiler issue a warning when bit fields are used in native structures and unions, and the memory model does not have an appropriate memory model modifier set. The compiler's bit order may not match the native platform's bit order, leading to problems with PInvoke'd functions.

4.9. Function pointer types

CLI metadata uses the same representation for method signatures and pointers to methods. C requires that signatures and pointers be distinct type categories. We therefore mark function pointers with the "OpenSystem.C.IsFunctionPointer" modifier: 
void (*func)(int);

.field public static method void * (int32) modopt(IsFunctionPointer) func

4.10. Argument types

When arguments are passed to a function, it is sometimes necessary to alter the type to conform with C conventions or to work around overly-strict CLI requirements.

An array argument to a function will be converted into its "decayed" pointer form. For example:

int main(int argc, char *argv[])
{
    ...
}

.method public static int32 main
        (int32 argc, int8 * * argv) cil managed
{
    ...
}

Functions that take a variable number of arguments must be declared with "vararg" calling conventions:

int printf(const char *format, ...)
{
    ...
}

.method public static vararg int32 printf
        (int8 modopt(IsConst) * format) cil managed
{
    ...
}

When arguments are passed to a variable-argument function, they must be converted into their "natural passing type" first:

TypeNatural Passing Type
_Bool _Bool
char int
unsigned char int
short int
unsigned short int
__wchar__ int
int int
unsigned int int
__native__ int long
unsigned __native__ int long
long long
unsigned long long
long long long long
unsigned long long long long
float double
double double
long double OpenSystem.C.LongDouble
type * long
struct and union Same as input type

Natural passing types help to properly implement cases where a value is passed as unsigned, but unpacked as signed, or is passed using a smaller type than the unpacking type.

The compiler must convert all variable arguments to their natural passing types at the point of the call. The "va_arg" operator is then responsible for casting the natural passing type back to the programmer's requested type.

The "va_list" type is implemented by the C# "System.ArgIterator" class, and has "dynamic" layout. The runtime engine will throw an exception if an attempt is made to unpack an argument using the wrong natural passing type.

5. Defining global fields and methods

The Common Language Infrastructure (CLI) has support for global fields and methods in the specially-defined "<Module>" type. However, there are some "undefined" issues that we now deal with.

5.1. Interoperability considerations

Microsoft's CLR does not allow references to the "<Module>" type within a foreign assembly. This appears to be a hard-wired constraint. Other CLR's (e.g. Portable.NET) make no distinction between the module type and all other types.

To achieve interoperability with Microsoft's CLR, library assemblies must use the "$Module$" type for their global field and method definitions instead of "<Module>". The "$Module$" type must have the "public" and "sealed" flags.

Executables still use the "<Module>" type, as it appears to work in all CLR's that have been tested so far. The "<Module>" type should have the "public" and "abstract" flags.

5.2. Dangling references

When a C source file is compiled to an object file, there will normally be "dangling" references to fields and methods in other object files and libraries. We need to handle this in the assembler and linker.

When the assembler sees a dangling reference to something in the "<Module>" class, it will convert it into a member reference on the "<ModuleExtern>" class. For example:

.method public static void hello() cil managed
{
    call void hello2()
}
If hello2 remains undefined at the end of the assembly process, then the resulting object file will look like this:

.method public static void hello() cil managed 
{
    call void '<ModuleExtern>'::hello2()
}
When the linker loads this object file, it will resolve references to "<ModuleExtern>" by looking for a matching definition and changing the type reference appropriately. The new reference may be to the linked executable's "<Module>" type, or to a foreign library's "$Module$" type.

The "<ModuleExtern>" type will itself be dangling. The exact means by which this is accomplished is compiler-dependent, as the ECMA specification does not define an object file format for the CLI.

Portable.NET's assembler encodes dangling types as a TypeRef, scoped to the current module, but with no corresponding TypeDef. The object file format is based on the native PE/COFF object file format, with CIL metadata stored in the ".text$il" section. Portable.NET's linker fixes up dangling TypeRef's at link time.

5.3. Access permissions

Variables or functions that are declared "static" are converted into "private" fields or methods within the "<Module>" object file's class. All other variables or functions are converted into "public" definitions.

If the "<Module>" class has any "public" members, then the class will also be declared "public". This ensures that a library will export its definitions correctly to applications that link against the library.

5.4. Renaming conflicting definitions

When two object files are linked together, it is possible that they both may have a "private" definition for the same function or variable. Alternatively, one may be "private" and the other "public".

We resolve this situation by renaming one of the "private" definitions to something else, and then redirecting all references to the original to the renamed version. From an external user's point of view, the "public" definition (if any) will become the visible definition. For example:

File 1: 
.field public static int32 x
File 2: 
.field private static float64 x
.method public static float64 getx() cil managed
{
    ldsfld float64 x
    ret
}
Result: 
.field public static int32 x
.field private static float64 'x-1'
.method public static float64 getx() cil managed
{
    ldsfld float64 'x-1'
    ret
}
If two or more object files have conflicting "public" definitions for a function or variable, then a linker error will occur.

Structure, union, and array types may also conflict when two object files are linked together. In most cases, the two definitions will be the same, because the same type is being used in both object files (e.g. "struct _IO_FILE" in glibc's stdio implementation).

When two types have identical definitions, the linker will copy one into the output file and ignore the other. When the two types have different definitions, the linker chooses one to become the primary copy, and the other is renamed.

If one of the types has the same definition as a type from a library, the linker should favour the library's definition, as it is the most likely candidate. If neither definition duplicates a library definition, the linker can choose either one, and probably should also report a warning to the programmer.

When program items are renamed, the resultant binary will not be in sync with the source code. This can make source-level debugging difficult. To alleviate this problem, the linker can add "OriginalName" attribute values to all renamed items:

.field public static int32 x
.field private static float64 'x-1'
.custom [OpenSystem.C.OriginalName("x")]
.method public static float64 getx() cil managed
{
    ldsfld float64 'x-1'
    ret
}
Normally this is only required if an object file contained debug symbol information prior to renaming.

5.5. Weak and strong aliases

C libraries such as "glibc" make heavy use of weak aliases to allow programs to replace certain functions with their own implementation. For example, the following is used in "glibc" for the definition of the "getuid" function (paraphrased a little): 
int __getuid(void)
{
    ...
}

weak_alias(__getuid, getuid)
This will be compiled as follows: 
.method public static int32 __getuid() cil managed
{
    ...
}

.field public specialname static .method int32 * () 'getuid-alias'

.method public static int32 getuid() cil managed
{
    .custom [OpenSystem.C.WeakAliasFor("__getuid")]
    .maxstack 1
    ldsfld .method int32 * () 'getuid-alias'
    tail.
    calli int32 ()
    ret
}

.method private specialname static void '.init-1'() cil managed
{
    .custom [OpenSystem.C.Initializer]
    .maxstack 1
    ldftn void __getuid()
    stsfld .method int32 * () 'getuid-alias'
    ret
}
When a program is linked against this definition, the "WeakAliasFor" attribute is used to redirect the reference to the actual definition if the system does not contain any other definitions for the function.

When a library that does not supply its own "getuid" is linked against this definition, the "getuid" method is called directly, which will then redirect control to the actual "getuid".

A program or library that defines its own "getuid" is compiled as normal: 

.method public static int32 getuid() cil managed
{
    ...
}
At link time, the linker will insert an initializer which updates the "getuid-alias" field with the new value: 
.method private specialname static void '.init-1'() cil managed
{
    .custom [OpenSystem.C.Initializer]
    .maxstack 1
    ldftn void getuid()
    stsfld .method int32 * () [library]'$Module$'::'getuid-alias'
    ret
}
where "library" is the name of the library that defines the "getuid-alias" variable.

Strong aliases for functions are defined in a similar manner:

.method public static vararg int32 _IO_printf
        (int8 modopt(OpenSystem.C.IsConst) *format) cil managed
{
    ...
}

.method public static vararg int32 printf
        (int8 modopt(OpenSystem.C.IsConst) *format) cil managed
{
    .custom [OpenSystem.C.StrongAliasFor("_IO_printf")]
}
In this case, whenever the linker sees a reference to "printf", it will redirect the caller to "_IO_printf". The body of the alias function is empty, because it will never be called at runtime.

Global variables may also have strong aliases associated with them:

char **__environ;
strong_alias(__environ, environ);

.field public static int8 * * __environ

.field public static int8 * * environ
.custom [OpenSystem.C.StrongAliasFor("__environ")]
When the linker sees a reference to "environ", it will substitute "__environ".

Weak aliases are not supported for global variables. Weak aliases exist in libc libraries primarily for legacy reasons. There are existing C programs that depend upon variables like "environ", "timezone", etc, being weak aliases, but they are rarer than programs that depend upon functions being weak aliases.

It is recommended that if the compiler sees a weak alias definition for a variable that it output a strong alias instead.

5.6. Initializers and finalizers

Initializers are compiled into static methods that have the "specialname" flag, have no parameters or return values, and are marked with the "Initializer" attribute.

Finalizers are compiled into static methods that have the "specialname" flag, have no parameters or return values, and are marked with the "Finalizer" attribute.

The linker collects up all initializers and finalizers in a program or library and does the following:

  1. It creates two "public methods in the "<Module>" class: ".init" and ".fini".
  2. The ".init" method calls the ".init" methods of all libraries that the program or library itself depends upon.
  3. The ".init" method then calls all of the locally-defined initializers.
  4. The ".fini" method calls all of the locally-defined finalizers.
  5. The ".fini" method then calls the ".fini" methods of all libraries that the program or library itself depends upon, in reverse order.
The order in which ".init" methods are called is usually indeterminable. The compiler can alter the ordering using the "InitializerOrder" attribute:

.method private specialname static void '.init-1'() cil managed
{
    .custom [OpenSystem.C.Initializer]
    .custom [OpenSystem.C.InitializerOrder(-1)]
    ...
}
This initializer will be executed before all "normal" initializers, which have a default order value of zero.

The "FinalizerOrder" attribute can used to alter the ordering of finalizers. A finalizer with an order value of -1 will be executed after the normal finalizers.

When the linker generates the ".init" and ".fini" methods, it must also insert some reference counting code. The body of the ".init" method will only be executed upon the first call, and the body of the ".fini" method will only be executed upon the last call. Appendix A contains some sample code that demonstrates this.

Usually, locally-defined initializers and finalizers are declared "private". The renaming logic described in a previous section will take care of resolving ambiguities in naming.

6. The crt0 code

When a module containing a "main" function is compiled, a small amount of CIL code is added to define the application entry point. This code calls facilities in the "OpenSystem.C.Crt0" class to initialize the application, to invoke "main", and to handle shutdown tasks when "main" exits. Using C# syntax, the startup code looks like this: 
public static void .start(String[] args)
{
    try
    {
        int argc;
        IntPtr argv;
        IntPtr envp;
        argv = Crt0.GetArgV(args, sizeof(void *), out argc);
        envp = Crt0.GetEnvironment();
        Crt0.Startup("libcNN");
        Crt0.Shutdown(main(argc, argv, envp));
    }
    catch(OutOfMemoryException)
    {
        throw;
    }
    catch(Object e)
    {
        throw Crt0.ShutdownWithException(e);
    }
}
where "libcNN" is the name of the "libc" implementation that the program was compiled against. This will normally be "libc64" for "Model 64" and "libc32" for "Model 32". The compiler will only pass those parameters to "main" that the programmer specified in their source code.

The startup code in the application is kept deliberately simple, with most of the real work being done in the "OpenSystem.C.Crt0" class. This allows the crt0 code to be modified to accomodate new "libc" requirements in the future, without needing all existing applications to be recompiled.

Appendix A. Sample initialization and finalization code

.class private sealed '.init-count' extends System.Object
{
    .field private static int32 count
}

.method public specialname static void '.init'() cil managed
{
    .maxstack 2
    .locals (class System.Type)

    // Lock down '.init-count' to synchronize access.
    ldtoken '.init-count'
    call class System.Type System.Type::GetTypeFromHandle
                 (valuetype System.RuntimeTypeHandle)
    dup
    stloc 0
    call void System.Threading.Monitor::Enter(class System.Object)
    .try
    {
        // Increase the reference count, and check for the first call.
        ldsfld '.init-count'::count
        dup
        ldc.i4.1
        add
        stsfld '.init-count'::count
        brtrue L1 
        leave runinit
    L1:
        leave exit
    }
    finally
    {
        ldloc 0
        call void System.Threading.Monitor::Exit(class System.Object)
        endfinally
    }

runinit:
    // Run the initializers for the libraries.
    call void [libc64]'<Module>'::'.init'()

    // Run the local initializers.
    call void '<Module>'::'.init-1'()
    call void '<Module>'::'.init-2'()
    ...
    call void '<Module>'::'.init-N'()

exit:
    // Initialization has finished.
    ret
}

.method public specialname static void '.fini'() cil managed
{
    .maxstack 2
    .locals (class System.Type)

    // Lock down '.init-count' to synchronize access.
    ldtoken '.init-count'
    call class System.Type System.Type::GetTypeFromHandle
                 (valuetype System.RuntimeTypeHandle)
    dup
    stloc 0
    call void System.Threading.Monitor::Enter(class System.Object)
    .try
    {
        // Decrease the reference count, and check for the last call.
        ldsfld '.init-count'::count
        ldc.i4.1
        sub
        dup
        stsfld '.init-count'::count
        brtrue L1 
        leave runfini
    L1:
        leave exit
    }
    finally
    {
        ldloc 0
        call void System.Threading.Monitor::Exit(class System.Object)
        endfinally
    }

runfini:
    // Run the local finalizers.
    call void '<Module>'::'.fini-1'()
    call void '<Module>'::'.fini-2'()
    ...
    call void '<Module>'::'.fini-N'()

    // Run the finalizers for the libraries.
    call void [libc64]'<Module>'::'.fini'()

exit:
    // Finalization has finished.
    ret
}