Is it allowed to name a global variable `read` or `malloc` in C++?

Question

Consider the following C++17 code:

#include <iostream>
int read;
int main(){
    std::ios_base::sync_with_stdio(false);
    std::cin >> read;
}

It compiles and runs fine on Godbolt with GCC 11.2 and Clang 12.0.1, but results in runtime error if compiled with a -static key.

As far as I understand, there is a POSIX(?) function called read (see man read(2)), so the example above actually invokes ODR violation and the program is essentially ill-formed even when compiled without -static. GCC even emits warning if I try to name a variable malloc: built-in function 'malloc' declared as non-function

Is the program above valid C++17? If no, why? If yes, is it a compiler bug which prevents it from running?

Answer 1

The code shown is valid (all C++ Standard versions, I believe). The similar restrictions are all listed in [reserved.names]. Since read is not declared in the C++ standard library, nor in the C standard library, nor in older versions of the standard libraries, and is not otherwise listed there, it's fair game as a name in the global namespace.

So is it an implementation defect that it won't link with -static? (Not a "compiler bug" - the compiler piece of the toolchain is fine, and there's nothing forbidding a warning on valid code.) It does at least work with default settings (though because of how the GNU linker doesn't mind duplicated symbols in an unused object of a dynamic library), and one could argue that's all that's needed for Standard compliance.

We also have at [intro.compliance]/8

A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any well-formed program. Implementations are required to diagnose programs that use such extensions that are ill-formed according to this International Standard. Having done so, however, they can compile and execute such programs.

We can consider POSIX functions such an extension. This is intentionally vague on when or how such extensions are enabled. The g++ driver of the GCC toolset links a number of libraries by default, and we can consider that as adding not only the availability of non-standard #include headers but also adding additional translation units to the program. In theory, different arguments to the g++ driver might make it work without the underlying link step using libc.so. But good luck - one could argue it's a problem that there's no simple way to link only names from the C++ and C standard libraries without including other unreserved names.

(Does not altering a well-formed program even mean that an implementation extension can't use non-reserved names for the additional libraries? I hope not, but I could see a strict reading implying that.)

So I haven't claimed a definitive answer to the question, but the practical situation is unlikely to change, and a Standard Defect Report would in my opinion be more nit-picking than a useful clarification.

Answer 2

Here is some explanation on why it produces a runtime error with -static only.

The https://godbolt.org/z/asKsv95G5 link in the question indicates that the runtime error with -static is Program returned: 139. The output of kill -l in Bash on Linux contains 11) SIGSEGV (and 128 + 11 = 139), so the process exits with fatal signal SIGSEGV (Segmentation fault) indicating invalid memory reference. The reason for that is that the process tries to run the contents (4 bytes) of the read variable as machine code. (Eventually std::cin >> ... calls read.) Either somethings fails in those 4 bytes accidentally interpreted as machine code, or it fails because the memory page containing those 4 bytes is not executable.

The reason why it succeeds without -static is that with dynamic linking it's possible to have multiple symbols with the same name (read): one in the program executable, and another one in the shared library (libc.so.6). std::cin >> ... (in libstdc++.so.6) links against libc.so.6, so when the dynamic linker tries to find the symbol read at program load time (to be used by libstdc++.so.6), it will look at libc.so.6 first, finding read there, and ignoring the read symbol in the program executable.