When you use a precompiled header, the compiler assumes the same compilation environment that was in effect when you created the .PCH file, unless you specify otherwise. You should take care to specify a consistent environment (using consistent compiler options, pragmas, and so on) for the current compilation. If the compiler detects an inconsistency, it issues a warning and identifies the inconsistency where possible. Such warnings don't necessarily indicate a problem with the .PCH file; they simply warn you of possible conflicts. The following sections explain the consistency requirements for precompiled headers.