Editing Name mangling (section)

====Handling of C symbols when linking from C++====
The job of the common C++ idiom:

<syntaxhighlight lang="cpp">
#ifdef __cplusplus 
extern "C" {
#endif
    /* ... */
#ifdef __cplusplus
}
#endif
</syntaxhighlight>

is to ensure that the symbols within 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, {{code|<string.h>}}, usually contains something resembling:

<syntaxhighlight lang="cpp">
#ifdef __cplusplus
extern "C" {
#endif

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

#ifdef __cplusplus
}
#endif
</syntaxhighlight>

Thus, code such as:

<syntaxhighlight lang="cpp">
if (strcmp(argv[1], "-x") == 0) 
    strcpy(a, argv[2]);
else 
    memset (a, 0, sizeof(a));
</syntaxhighlight>

uses the correct, unmangled {{code|strcmp}} and {{code|memset}}. If the {{code|extern "C"}} had not been used, the (SunPro) C++ compiler would produce code equivalent to:

<syntaxhighlight lang="cpp">
if (__1cGstrcmp6Fpkc1_i_(argv[1], "-x") == 0) 
    __1cGstrcpy6Fpcpkc_0_(a, argv[2]);
else 
    __1cGmemset6FpviI_0_ (a, 0, sizeof(a));
</syntaxhighlight>

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

<!--
Useful links. I don't know which should end up in the finished article:
* http://www.ucmb.ulb.ac.be/documents/libg++_2.7.1/g++FAQ_35.html
* http://theoryx5.uwinnipeg.ca/gnu/gcc/gxxint_15.html    This one is ancient, should not be used.
* http://www.kegel.com/mangle.html   This one describes an old version of Visual C++.
* http://www.pgroup.com/ppro_docs/pgiws_ug/pgiug_14.htm
-->