SLAU132 User guide

SLAU132V October 2004 – February 2020

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5.15 Object File Symbol Naming Conventions (Linknames)

Each externally visible identifier is assigned a unique symbol name to be used in the object file, a so-called linkname. This name is assigned by the compiler according to an algorithm which depends on the name, type, and source language of the symbol. This algorithm may add a prefix to the identifier (typically an underscore), and it may mangle the name.

User-defined symbols in C code and in assembly code are stored in the same namespace, which means you are responsible for making sure that your C identifiers do not collide with your assembly code identifiers. You may have identifiers that collide with assembly keywords (for instance, register names); in this case, the compiler automatically uses an escape sequence to prevent the collision. The compiler escapes the identifier with double parallel bars, which instructs the assembler not to treat the identifier as a keyword. You are responsible for making sure that C identifiers do not collide with user-defined assembly code identifiers.

Name mangling encodes the types of the parameters of a function in the linkname for a function. Name mangling only occurs for C++ functions which are not declared 'extern "C"'. Mangling allows function overloading, operator overloading, and type-safe linking. Be aware that the return value of the function is not encoded in the mangled name, as C++ functions cannot be overloaded based on the return value.

For example, the general form of a C++ linkname for a function named func is:

func__Fparmcodes

Where parmcodes is a sequence of letters that encodes the parameter types of func.

For this simple C++ source file:

int foo(int i){ } //global C++ function

This is the resulting assembly code:

foo__Fi

The linkname of foo is foo__Fi, indicating that foo is a function that takes a single argument of type int. To aid inspection and debugging, a name demangling utility is provided that demangles names into those found in the original C++ source. See Section 8 for more information.

The mangling algorithm follows that described in the Itanium C++ ABI (http://www.codesourcery.com/cxx-abi/abi.html).

int foo(int i) { } would be mangled "_Z3fooi"