C++ Linkage specifications Making a C library header compatible with C++
Example
A C library header can usually be included into a C++ program, since most declarations are valid in both C and C++. For example, consider the following foo.h:
typedef struct Foo {
int bar;
} Foo;
Foo make_foo(int);
The definition of make_foo is separately compiled and distributed with the header in object form.
A C++ program can #include <foo.h>, but the compiler will not know that the make_foo function is defined as a C symbol, and will probably try to look for it with a mangled name, and fail to locate it. Even if it can find the definition of make_foo in the library, not all platforms use the same calling conventions for C and C++, and the C++ compiler will use the C++ calling convention when calling make_foo, which is likely to cause a segmentation fault if make_foo is expecting to be called with the C calling convention.
The way to remedy this problem is to wrap almost all the declarations in the header in an extern "C" block.
#ifdef __cplusplus
extern "C" {
#endif
typedef struct Foo {
int bar;
} Foo;
Foo make_foo(int);
#ifdef __cplusplus
} /* end of "extern C" block */
#endif
Now when foo.h is included from a C program, it will just appear as ordinary declarations, but when foo.h is included from a C++ program, make_foo will be inside an extern "C" block and the compiler will know to look for an unmangled name and use the C calling convention.
