Introduction to C language functions

function

A piece of code with a certain function , yes C The smallest unit of management code in a language
Encapsulate the code into functions , It is convenient to manage and call code

Classification of functions

1. Standard library functions

from C The language standards committee is C Language provides some basic functions in the form of functions , Be encapsulated libc.so In the library , When using, you need to include the corresponding header file , Through the corresponding function name ( Actual parameters ) Method can call functions in the standard library
For example, the following are functions in the standard library

        libm.so
        #include 《time.h>
        time_t time(time_t *tloc);
         function ： Return from 1970-1-1 0:0:0 How many seconds have elapsed until the function is executed 
            time_t sec =time(NULL);

        #include <stdlib.h>
        int system(const char *command);
         function ： Execute system commands 
        system("clear");  

        int rand (void);
         function ： Return value a random number 
        int num=rand();

        void srand(unsigned int seed);

2. System function

It is a series of functions provided by the operating system in the form of function interface , But it's not really a function
Memory management 、 file management 、 Input and output of files 、 Process management 、 signal processing 、 Process of communication 、 Thread management 、 Thread synchronization 、 Network communication, etc

3. Third party library functions

Open source or paid code base provided by a third party
MD5 encryption algorithm
Json serialize 、 Deserialization
Xml Profile parsing

4. Custom function

In order to better manage your own code 、 Reduce code redundancy and encapsulate the code into functional form

1. Function declaration ：
   The purpose of function declaration is to tell other code the calling format of the function

   Implicitly declare ： When the function is called , There is no previous declaration or definition of this function , An implicit warning will be generated , Parameters will be guessed based on actual parameters , The return value will be guessed as int type
   Don't generate implicit declarations anyway , As long as there is a declaration or definition before the call

   return type Function name ( type 1 The name of the parameter 1, type 2 The name of the parameter 2,…);
   Be careful ：
   1、C Function names in are generally all lowercase , Underline to separate
   2、 If the return value does not need , Then write void
   3、 If you don't need formal parameters , It is suggested to also write void
   4、 Even if the formal parameter types are the same , Add the type name to each

2. Function definition ： Function implementation code
   return type Function name ( type 1 The name of the parameter 1, type 2 The name of the parameter 2,…)
   {
   // The body of the function
   return [val];
   }
   3. Function call ：
   Function name ( Actual parameters );
   Be careful ：
   1、 There is a function definition before the function call , Then the function declaration can be omitted
   2、 The return value of the function will be returned at the call location , Can be displayed immediately , You can also use variables to record
   3、 A function is recommended not to exceed 50 Line code
   4、 The content of the declaration should be completely consistent with the definition
   5、 If no return value is required 、 Parameters , Must write void
   6、 Before calling , No statement 、 Definition , Warnings that generate implicit declarations

Function arguments

1. The shape parameter 、 The variables defined in the function only belong to the function it is in , This function cannot be used anymore
2. Ordinary real participants transfer data between formal parameters by assignment ( One way value passing )
   3. return It's actually storing data in a public area ( Areas that can be accessed between functions ), If you don't write return sentence , Then the original value of this area will be read , Get a garbage data
3. When an array is used as an argument to a function , The length in brackets will be lost , You need to add an extra variable to pass the length of the array
4. When an array is passed as a function parameter , Passing is the first address of the array , be called " Address transmission ", Functions and callers of functions can share the same array

Suggestions for designing functions

1、 A function is best to solve a problem , Reduce the error rate , Improve readability
2、 Try to reduce the number of dependency layers between functions ( Reduce coupling )
3、 The data is provided by the caller , The result is returned to the caller ( Improve the generality of functions )
4、 Consider the illegal parameters of the function , You can tell the caller that your parameters are wrong by returning values , You can also write the situation in the form of notes ( Improve the robustness of functions )