The header file is a file that extension .h, contains a C function declaration and macro definition, and is cited by multiple source files. There are two types of header files: the header files written by the programmer and the header files comes with the compiler.
To use the header file in the program, you need to use C pre-processing instructions #include to reference it. We have seen stdio.h header files in front, it is the header file with the compiler.
The reference header file is equivalent to the contents of the copy header, but we will not copy the contents of the header file directly in the source file, because it is easy to make mistakes, especially in the program is composed of multiple source files.
A Simple Practice IN C ++ or C ++ program, it is recommended to write all constants, macros, system global variables, and function prototypes in the header file, and reference these header files at any time when needed.
Quote the grammar of the header file
Use the pre-processing instructions #include to reference the user and system header file. Its form has two types:
#include
This form is used to reference the system header file. It searches for files named File in the standard list of system catalogs. When compiling the source code, you can put the directory in front of the list through the -i option.
#include "file"
This form is used to reference the user header file. It searches for files named file in a directory containing the current file. When compiling the source code, you can put the directory in front of the list through the -i option.
Citing the operation of the header file
#include instructions indicate that the C pre-regulator browsing the specified file as an input. The output of the preprocessor contains the output that has been generated, the output generated by the reference file, and the text output after the #include instruction. For example, if you have a header file header.h, as follows:
Char * test (void);
And an main program of using the header file, as follows:
INT X; #include "Header.h" int main (void) {PUTS (TEST ());}
The compiler will see the following code information:
INT X; Char * Test (void); int main (void) {puts (test ());}
Only quote one header file
If a header file is referenced twice, the compiler handles the content of the two header files, which will generate an error. In order to prevent this, the standard practice is to put the entire content of the file in the conditional compile statement, as follows:
#ifndef header_file # define header_file the entire header file # Endif
This structure is usually the wrapper #ifndef. When the header file is retrieved again, the condition is false because header_file is defined. At this point, the preprocessor will skip the entire content of the file, and the compiler ignores it.
Conditional reference
Sometimes you need to select a reference to a program from multiple different header files. For example, you need to specify configuration parameters used on different operating systems. You can achieve this through a range of conditions, as follows:
#if system_1 # include "system_1.h" #elif system_2 # include "system_2.h" #elif system_3 ... # ENDIF
However, if the header file is more, it is very unmerder, the preprocessor uses the macro to define the name of the header file. This is the so-called conditional reference. It is not the direct parameter of the name of the header file, you only need to use the macro name instead:
#define system_h "system_1.h" ... # include system_h
System_H will extends, and the preprocessor will look for SYSTEM_1.H, just like #include initially prepared. System_h can be defined by your Makefile through the -d option. , Read full text, original title: head file in C language
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