The error says:
error: expected ',' or '...' before '(' token
int exec_wait(const std::string& path, std::string& stdout, ...)
^
Anyone has any clues why it's complaining about a ( token when pointing at the s? I have no idea even where to start...
The error occurs practically with no other code. The minimal example would be:
#include <string>
int exec_wait(const std::string& path, std::string& stdout, ...)
{
}
stdout is a macro defined in standard library: https://en.cppreference.com/w/cpp/io/c/std_streams. It's defined in cstdio, but any standard library header is allowed to #include any other standard library header, and in this case it seems string includes cstdio.
To answer title question: compiler sees code after it is preprocessed (so all macros are expanded). Definition of stdout depends on compiler, but it's likely something like (stuff_here), so compiler sees int exec_wait(const std::string& path, std::string& (stuff_here), ...) and errors on that.
The solution is to choose a different name for your argument.
In the future, avoid any names from this list: https://en.cppreference.com/w/cpp/symbol_index/macro (note that they are mostly UPPERCASE or start with underscore, so they aren't too hard to avoid).