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Using #pragma once allows the C preprocessor to include a header file when it is needed and to ignore an #include directive otherwise. This has the effect of altering the behavior of the C preprocessor itself, and allows programmers to express file dependencies in a simple fashion, obviating the need for manual management.
It is the responsibility of the programmer to write code that never invokes undefined behavior, although compiler implementations are allowed to issue diagnostics when this happens. Compilers nowadays have flags that enable such diagnostics, for example, -fsanitize=undefined enables the "undefined behavior sanitizer" in gcc 4.9 [2] and in clang ...
The discussion of pragma once in gcc in this article is not up-to date and should be rectified. I am using gcc version 4.3.3 and it warns me about pragma once being deprecated. The GNU C++ Manual also warns: #pragma once is now obsolete and should not be used at all. Ben T / C 12:08, 26 November 2009 (UTC) Update.
In computer programming, a directive or pragma (from "pragmatic") is a language construct that specifies how a compiler (or other translator) should process its input. Depending on the programming language , directives may or may not be part of the grammar of the language and may vary from compiler to compiler.
GCC provides #include_next for chaining headers of the same name. [13] Import. Unlike C and C++, Objective-C includes an #import directive that is like #include but results in a file being included only once – eliminating the need for include guards and #pragma once.
In computer programming, a precompiled header (PCH) is a (C or C++) header file that is compiled into an intermediate form that is faster to process for the compiler.Usage of precompiled headers may significantly reduce compilation time, especially when applied to large header files, header files that include many other header files, or header files that are included in many translation units.
For #include guards to work properly, each guard must test and conditionally set a different preprocessor macro. Therefore, a project using #include guards must work out a coherent naming scheme for its include guards, and make sure its scheme doesn't conflict with that of any third-party headers it uses, or with the names of any globally visible macros.
gcc and clang offer the -mno-red-zone flag to disable red-zone optimizations. If the callee is a variadic function , then the number of floating point arguments passed to the function in vector registers must be provided by the caller in the AL register.