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Thursday, November 18, 2004
 
C++: Name Mangling

When C++ compilers compile a C++ program, it encodes all function names and

certain other identifiers to include type and scoping information. This encoding
process is called name mangling. Linker uses these mangled names to ensure
type-safe linkage. These mangled names appear in the object files and final
executable file.

What's a symbol?
In every C++ program/library/object file, all non-static functions are represented
in the binary file as symbols. These symbols are special text strings that uniquely
identify a function in the program, library or object file

The Need for Name Mangling:
C language programs does not use name mangling, because in C no two non-static
functions can have the same name. i.e., the symbol name is the same as the
function name: the symbol of myfunc will be myfunc

Because C++ allows overloading (different functions with the same name but
different number of arguments) and has many features C does not, like classes,
member functions, exception specifications — it is not possible to simply use the
function name as the symbol name. To solve that, C++ uses name mangling, which
encodes the function name and all the necessary information (like the number
and size of the arguments) into some special string which only the compiler
knows about

eg.,
bpte4500s001:/sunbuild1/giri/testcases/% nm hide.o

hide.o:

[Index] Value Size Type Bind Other Shndx Name
[3] | 16| 56|FUNC |GLOB |3 |2 |__1cKCRectangleKset_values6Mii_v_
[4] | 0| 0|NOTY |GLOB |0 |ABS |__fsr_init_value
[1] | 0| 0|FILE |LOCL |0 |ABS |hide.cpp
[2] | 88| 32|FUNC |GLOB |2 |2 |main

"__1cKCRectangleKset_values6Mii_v_" is the mangled name

But this kind of scheme is undesirable for the developers because the names
are difficult to read & debug

Two utilities are available with Sun Studio C/C++ compiler collection to convert
the mangled names to their original source code names:
1) c++filt &
2) dem

C++filt is a filter that demangles (decodes) mangled names.
bpte4500s001% echo __1cKCRectangleKset_values6Mii_v_ | c++filt
void CRectangle::set_values(int,int)

"dem" is another utility to demangle C++ names
bpte4500s001% dem __1cKCRectangleKset_values6Mii_v_
__1cKCRectangleKset_values6Mii_v_ == void CRectangle::set_values(int,int)

Note:
C++ standard does not define how names have to be mangled; thus every compiler
mangles names in its own way. Some compilers even change their name mangling
algorithm between different versions. This could be a problem if the developers
hack & rely on how compiler mangles the C++ symbols, as the same algorithm may
not work with the next version of C++ compiler



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