Embedded-c language-7

One 、 The pointer (2)

1.0 No data type pointer ：void *

a) No data type pointer (void *) Concept ： It also Is a pointer variable , Also save an address , Again Occupy 4 byte Memory space , Just the memory area it points to The data type is unknowable , Call this pointer No data type pointer

How to write it ：void *

for example ：void *p = &a;

b) Typeless pointer characteristics ：

   1. Through this pointer variable is Cannot know the data type of the data stored in the memory area pointed to

   2. You cannot dereference a typeless pointer directly "*", Because you don't know the data type of the memory area pointed to , I don't know how many bytes of data I will get in the future , If you want to get memory data through a typeless pointer , Data type conversion is required ( For the readability of the code, it is recommended to use cast )

   3. No type pointer void * Add and subtract a few , The actual address is plus or minus

      void *p = 0;

      p++; //p=1

      p++; //p=2

   4. Case code for all cases

       for example ：

      int a = 0x12345678;
      void *p = &a; // although p Point to a, But by p Can't get a Data type of variable 
      *p = 300; //gcc Compiler error , I don't know how many of my own data , I can't help but report you an error 

       ask ： How to get through a pointer a What about the data? ？

       answer ： The way 1：

int a = 0x12345678;
int *p = &a;
printf("%#x\n", *p);

Although it can be obtained at one time 4 Bytes of data , But now I don't want to get it all at once 4 Bytes of data , Want to get one 1 Bytes of data or 2 Byte data or 4 Bytes of data , Obviously, this method cannot be done , To achieve this function , Just do the following code ：

        The way 2： Get through a typed pointer variable 1 byte ,2 byte

             obtain 1 byte ：

int a = 0x12345678;
//char *p = &a;  // Implicit conversion , Code readability is not high ,gcc And a warning 
char *p = (char *)&a; // Coercive transformation , Improve the readability of the code , take &a Of int Type conversion to char*
printf("%#x\n", *p++); //0x78
printf("%#x\n", *p++); //0x56
printf("%#x\n", *p++); //0x34
printf("%#x\n", *p++); //0x12

             obtain 2 byte ：

int a = 0x12345678;
//short *p = &a;  // Implicit conversion , Code readability is not high  gcc And a warning 
short *p = ( short *)&a; // Coercive transformation , take &a Of int Type conversion to short*
printf("%#x\n", *p++); //0x5678
printf("%#x\n", *p++); //0x1234

Conclusion ： Although this method meets the requirements , But if you do an implicit conversion ,gcc Always give a warning

      ask ： Can you remove the warning ？ There is also the ability to remove pointer variables ++ Time is related to type ？

      answer ： Through typeless pointers void * Realization

       Method 3： Through typeless pointers void * To get 1 byte ,2 byte ,4 Bytes of data

        obtain 1 byte ：   

int a = 0x12345678;
void *p = &a; // Define a typeless pointer to a, No forced rotation is required , The key is gcc No warning 
char *p1 = (char *)p; // Force the typeless pointer to char*
printf("%#x\n", *p1++); //0x78
printf("%#x\n", *p1++); //0x56
printf("%#x\n", *p1++); //0x34
printf("%#x\n", *p1++); //0x12

                    perhaps ：

printf("%#x\n", *(char *)(p+0)); //0x78, First the p Forced to char * The pointer , And then take 
printf("%#x\n", *(char *)(p+1)); //0x56
printf("%#x\n", *(char *)(p+2)); //0x34
printf("%#x\n", *(char *)(p+3)); //0x12

             obtain 1 byte ：   

int a = 0x12345678;
void *p = &a; // Define a typeless pointer to a, No forced rotation is required , The key is gcc No warning 
short *p1 = (short *)p; // Force the typeless pointer to char*
printf("%#x\n", *p1++); //0x5678
printf("%#x\n", *p1++); //0x1234

                    perhaps ：

printf("%#x\n", *(short *)(p+0)); //0x5678
printf("%#x\n", *(short *)(p+2)); //0x1234

1.1. Pointers and functions

a) The pointer is used as the formal parameter of the function

     result ： The function can access the memory pointed to by the operation through the pointer

     Reference code ： for example ：     

void A(int *px, int *py, int *pm, int *pn)
    {  // Read view 
printf("%d %d %d %d\n", *px, *py, *pm, *pn);
// Write and modify 
*px = ....
*py = ...
*pm  = ...
*pn = ...
    }

b) Pointer as the return value of the function

     result ： This kind of function is also called Pointer function , That is to say The return value of the function is a pointer , In the future, the function will return an address , It is related to the memory life cycle of the returned variable , It has nothing to do with the scope of use of variables

     Bear in mind ： Be careful not to return a wild pointer

     Grammar format ： Return value data type * Function name ( Formal parameter table ){ Function body statement }

     for example ：

    // Defining pointer functions 
    int g_a = 250; // Define global non static variables 
    static int g_b = 520; // Define global static variables 
    int *A(void)
    {
int a = 200; // Define local nonstatic variables 
static int b = 100; // Define local static variables 
return &a; // Returns a wild pointer , Because of the variable a Memory of disappears with the return of the function 
// perhaps return &b;  // Sure , Because this memory is allocated once and used for life 
// perhaps return &g_a; // Sure , Because this memory is allocated once and used for life 
// perhaps return &g_b; // Sure , Because this memory is allocated once and used for life 
  }
int main(void)
   {
int *p = A();
*p = 30000; // Change the value of the variable 
printf("%d\n", *p); // View the data of the memory pointed to 
return 0;
   }

c) function , The pointer , Array

    Function to access the formula of the array ：

   void A(int a[], int len); // among a Is the address of the array

    Equivalent to

   void A(int *p, int len);

 

Two 、C Language string related content

2.1. Review character constants ： Enclose... In single quotation marks , for example ：'A','B','1' etc.

              The actual memory stores the corresponding integer number ,ASCII code

              Place holder ：%c

2.2. String definition ： Consisting of a continuous set of characters , And use "" Include , also The last character must be '\0'

this '\0' Represents the end of a string , this '\0' Of ASCII Code is 0

'0' Of ASCII by 48

Be careful ： Studying strings ultimately studies the characters inside one by one

for example ："abcefg\0"

2.3. String characteristics

a) Placeholder for string :%s

   printf("%s\n", "abc\0"); // Directly follow the string

    perhaps

   printf("%s\n", The first address of the string );

b) The memory space occupied by the string is continuous , And each byte stores one character

    Be careful ：'\0' If the end character of the string is followed by something , Then these contents are invalid

    for example ："abc\0efg\0"

   printf("%s\n", "abc\0efg"); //abc

c) Multiple parallel strings will be represented by gcc Help you merge into a string

   "abc""efg" Merge into "abcefg"

   printf("abc""efg\n"); //abcefg

2.4. Strings and pointers

a) Define a character pointer variable and point to a string , Essentially, it points to the first address of this string , That is, it points to the... In the string 0 First address of characters ( That's the character a The first address )

    Define and initialize the string pointer variable form ：

   char *p = "abcefg";

b) If you let a character pointer variable point to a string ,  This string does not need to be followed by '\0',gcc I will help you add '\0'

d) Bear in mind ： You cannot modify the value of a string through a character pointer variable , View only

    // Read view

   printf("%s\n", p);

   *(p + 2) = 'C'; // Target will be 2 Characters c become C, Can't do , Report errors

2.5. Strings and arrays , Two ways of writing ：

a) How to write it 1：

   char a[] = {'a', 'b', 'c', '\0'};

    Be careful ： If you want to put the a As a string , You need to manually add the last '\0',( Is an array , String again )

If you do not add a It's just an array of three elements , Instead of a valid string

b) How to write it 2：

   char a[] = "abc";

    Be careful ： No need to add '\0',gcc The compiler will add one for you in the future '\0'

c) Bear in mind ： No matter which way it is written , The elements in the character array can be modified

    for example ： Will be the first 2 Elements 'c' It is amended as follows 'C':

             a[2] = 'C';

              perhaps

              *(a + 2) = 'C';

 

2.6. Write a string comparison function my_strcmp

2.7. standard C String operation functions provided by the

       If you want to use the following written string operation function , Need to add header file ：#include <string.h>

a)strlen function ： Function to obtain the effective length of the string ( barring '\0')

       for example ：printf(" length :%d\n", strlen("abc")); //3

                 perhaps

                char *p = "abc";

                 printf(" length :%d\n", strlen(p)); //3

b)strcat function ： The function is string splicing

       for example ：

      char a[10] = "abc";

      char *p = NULL;

      p = strcat(a, "xyz"); // hold xyz Spliced in abc Save the back of to the array

     printf("%s  %s\n", p, a); //abcxyz  abcxyz

c)strcmp function ： The function is a character comparison function

          for example ：

     char *p1 = "abc";

     char *p2 = "xyz";

     int ret = strcmp(p1, p2);  

     int ret = strcmp("abc", "xyz"); // In essence, what is ultimately passed is the first address of the string

d)strcpy: String copy function , Will overwrite the original string

                for example ：

               char a[10] = "abc";

               char *p = NULL;

               p = strcpy(a, "xyzmn");

               printf("%s  %s\n", p, a);

e)sprintf ： Format output function , The function turns numbers (250) Convert to string "250" Save to array

               for example ：

              char a[50] = {0};

              sprintf(a, "%d %g, %c", 250, 250.2, 'A');

              printf("%s", a);