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12.1. Header FilesThere are two types of header files in Mac OS X.
Header files serve four functions:
Unix developers will find the ordinary header files familiar, since they follow the BSD convention. The C preprocessor directive #include includes a header file in a C source file. There are essentially three forms of this syntax:
You can use #include, followed by a macro, which, when expanded, must be in one of the aforementioned forms. As noted in the previous chapter, frameworks in Mac OS X are common when you step outside of the BSD portions of the operating system. To include a framework header file in Objective-C code, use the following format: #import <frameworkname/headerfilename.h> where frameworkname is the name of the framework without the extension and headerfilename is the name of the header file. For example, the included declaration for a Cocoa application would look like: #import <Cocoa/Cocoa.h> Note that you must use #include rather than #import when including a framework in Carbon code. When preprocessing header files or any preprocessor directives, the following three actions are always taken:
The following rules must be kept in mind:
12.1.1. Precompiled Header FilesMac OS X's Xcode Tools support and provide extensive documentation on building and using precompiled header files. This section highlights a few of the issues that may be of interest to Unix developers new to Mac OS X when it comes to working with precompiled headers. Precompiled header files are binary files that have been generated from ordinary C header files and preprocessed and parsed using cc. When such a precompiled header is created, both macros and declarations present in the corresponding ordinary header file are sorted, resulting in a faster compile time, a reduced symbol table size, and consequently, faster lookup. Precompiled header files are given a .h.gch extension and are produced from ordinary header files that end with a .h extension. There is no risk that a precompiled header file will get out of sync with the .h file, because the compiler checks the timestamp of the actual header file. When using precompiled header files , you should not refer to the .h.gch version of the name, but rather to the .h version in the #include directive. If a precompiled version of the header file is available, it is used automatically; otherwise, the real header file (.h) is used. So, to include foo.h.gch, specify foo.h. The fact that cc is using a precompiled header is totally hidden from you. You can create precompiled header files either using the cc -precomp or cc -x c-header -c compile driver flags. For example, the following command illustrates this process in its simplest, context-independent form: cc -precomp header.h The following command has the same effect: cc -x c-header -c header.h In either case, the resulting precompiled header is named header.h.gch. If there is context dependence (for example, some conditional compilation), the -Dsymbol flag is used. In this case, the command to build a precompiled header file (with the FOO symbol defined) is: cc -precomp -DFOO header.h -o header.h.gch The -x switch supplies the language (see "Supported Languages" in Chapter 11): gcc -x c c-header header.h Then, you can compile main.c as usual: gcc -o main main.c Example 12-1 shows header.h, and Example 12-2 shows main.c. Example 12-1. The header .h file/* header.h: a trivial header file. */ #define x 100 Example 12-2. The main .c application/* main.c: a simple program that includes header.h. */ #include "header.h" #include <stdio.h> int main( ) { printf("%d\n", x); return 0; } There are a few issues to keep in mind when you use a precompiled header file.
For more details on building and using precompiled header files, read the documentation stored in /Developer/ADC Reference Library/documentation/DeveloperTools/gcc-4.0.0/ gcc/Preccompiled-Headers.html.
12.1.2. malloc.hmake may fail to compile some types of Unix software if it cannot find malloc.h. Software designed for older Unix systems may expect to find this header file in /usr/include; however, malloc.h is not present in this directory. The set of malloc( ) function prototypes is actually found in stdlib.h. For portability, your programs should include stdlib.h instead of malloc.h. (This is the norm; systems that require malloc.h are the rare exception these days.) GNU autoconf will detect systems that require malloc.h and define the HAVE_MALLOC_H macro. If you do not use GNU autoconf, you will need to detect this case on your own and set the macro accordingly. You can handle such cases with this code: #include <stdlib.h> #ifdef HAVE_MALLOC_H #include <malloc.h> #endif For a list of libraries that come with Mac OS X, see the "Interesting and Important Libraries" section, later in this chapter. 12.1.3. poll.hIn pre-Tiger versions of Mac OS X, one issue in porting software from a System V platform to a BSD platform such as Mac OS X was the lack of the poll( ) system call function, which provides a mechanism for I/O multiplexing. Panther provided this function through emulation, which made use of its BSD analog select( ). In Tiger, poll( ) is provided as a native function. The associated header file, /usr/include/poll.h, is included with Panther as well as Tiger. 12.1.4. wchar.h and iconv.hAnother issue in porting Unix software to pre-Panther versions of Mac OS X was the relatively weak support for wide (i.e., more than 8-bits) character datatypes (e.g., Unicode). Panther and Tiger provide better support for wide character data types by including the GNU libiconv, which provides the iconv( ) function to convert between various text encodings. Additionally, the wchar_t type is supported in both Panther and Tiger. The header files iconv.h and wchar.h are also included. Alternatively, you can use the APIs available in the CoreFoundation's String services , which are described in CFString.h. 12.1.5. dlfcn.hThis header file, associated with dl-functions like dlopen( ), is included in Tiger. The functions such as dlopen( ) are actually included in libSystem. 12.1.6. alloc.hAlthough this header file is not included with Mac OS X, its functionality is provided by stdlib.h. If your code makes a specific request to include alloc.h, you have several choices. One option is to remove the #include <alloc.h> statement in your source code. This may be cumbersome, however, if your include statement appears in many files. Another alternative is to create your own version of alloc.h. A sample alloc.h is suggested in The Apple Developer Connection's Technical Note TN2071 (http://developer.apple.com/technotes/tn2002/tn2071.html). 12.1.7. lcyrpt.hAlthough lcrypt.h is not included in Mac OS X, its functionality is provided in unistd.h . 12.1.8. values.hThe values.h file, another header file found on many Unix systems, is not included in Mac OS X. Its functionality, however, is provided by limits.h . |
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