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An external variable can be accessed by all the functions in all the modules of a program. It is a global variable.For a function to be able to use the variable, a declaration or the definition of the external variable must lie before the function definition in the source code.
Differences between C and C++ linkage and calling conventions can also have subtle implications for code that uses function pointers. Some compilers will produce non-working code if a function pointer declared extern "C" points to a C++ function that is not declared extern "C". [22] For example, the following code:
This is also valid in C++. (C++ 98/03 deprecated this usage in favor of anonymous namespaces, but is no longer deprecated in C++ 11.) Also, C++ implicitly treats any const namespace-scope variable as having internal linkage unless it is explicitly declared extern, unlike C. A name's linkage is related to, but distinct from, its scope. The scope ...
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 standardized requirement for name mangling would also prevent an implementation where mangling was not required at all – for example, a linker that understood the C++ ...
The primary effect of the extern "C" declaration in C++ is to disable C++ name mangling. With other languages, separate utils or middleware are used, examples include: GNOME project: GObject Introspection; SWIG; Chromium project: Blink and V8 engine use an interface description language (IDL) compiler for standard JavaScript interfaces; Other ...
For example, a binary is dynamically linked against libraries libfoo.so and libbar.so. libfoo defines symbol f and declares it as weak. libbar also defines f and declares it as strong. Depending on the library ordering on the link command line (i.e. -lfoo -lbar ) the dynamic linker uses the weak f from libfoo.so although a strong version is ...
Such extern declarations are often placed in a shared header file, since it is common practice for all .c files in a project to include at least one .h file: the standard header file errno.h is an example, making the errno variable accessible to all modules in a project.
In C++, a function defined inline will, if required, emit a function shared among translation units, typically by putting it into the common section of the object file for which it is needed. The function must have the same definition everywhere, always with the inline qualifier. In C++, extern inline is the same as inline.