No, it’s a bad idea. With C++ declarations, it’s likely to introduce linker errors as identifiers get declared in the wrong namespace. With C declarations, it works, but it may hide clashes between identifiers in the global namespace (which you were trying to avoid, I guess) until link time; it doesn’t really put the identifiers in a namespace.
A better idea would be to put your own identifiers in a namespace and avoid defining anything but
main in the global one.