[learning notes, dog learning C] string + memory function

One 、strlen Function and Simulation Implementation

1. Function USES

int main() {
    
	char arr[] = "abc";
	int len = strlen(arr);
	printf("%d\n", len);

	return 0;
}

2. Function introduction

   String to ’\0’ As an end sign ,strlen Function returns in a string ’\0’ The number of characters that appear before （ It doesn't contain ’\0’);
   The string that the argument points to must be in ‘\0’ end ;
   Note that the return value of the function is size_t, It's unsigned .

3. Function source code
   
   size_t stay <vcruntime.h> In the definition of typedef unsigned int size_t;, It's unsigned ;
   Receive one char Data of type , use const modification , Ensure that the content pointed to by the pointer cannot be changed by the pointer .
4. Simulation Implementation

// Counter mode 
int my_strlen(const char* str) {
    
	int count = 0;// Counter 
	assert(str != NULL);// Pointer assertion , Ensure the validity of the pointer 

	while (*str != '\0') {
    
		count++;
		str++;
	}
	return count;
}
// Cannot create temporary variable counter 
int my_strlen(const char * str) {
    
	if(*str == '\0')
		return 0;
	else
		return 1+my_strlen(str+1);
}
// The pointer - The way of the pointer 
int my_strlen(char *s) {
    
	char *p = s;
	while(*p != ‘\0’ )
		p++;
	return p-s; 
}
int main() {
    
	char arr[] = "abc";
	int len = my_strlen(arr);
	printf("%d\n", len);

	return 0;
}

5. And strlen Difference of function

int main() {
    
	// because strlen Returns an unsigned number , So print  >
	if (strlen("abc") - strlen("abcedf") > 0)
	{
    
		printf("strlen >\n");
	}
	else
	{
    
		printf("strlen <\n");
	}
	// Our simulated function returns a int The type is signed , So print  <
	// If we want to be with strlen As like as two peas , Then you also need to return an unsigned number 
	// hold int Change it to size_t
	if (my_strlen("abc") - my_strlen("abcedf") > 0)
	{
    
		printf("my_strlen >\n");
	}
	else
	{
    
		printf("my_strlen <\n");
	}

	return 0;
}

---

Two 、 Unlimited length string function

1、strcpy Function and Simulation Implementation

1. Function USES

int main() {
    
	char arr[20] = {
     0 };
	strcpy(arr, "hello");
	printf("%s\n", arr);

	return 0;
}

2. Function introduction

   The source string must be in ’\0’ end ;
   In the source string ‘\0’ Copy to target space ;
   The target space has to be large enough , To ensure that the source string can be stored ;
   The target space has to be variable .

3. Function source code
   
     destination It's a goal , in other words destination Put the replaced character here , That is, the target character .source It means source , Namely source Put replacement characters here , That is, the source character . therefore strcpy Receive two parameters , The first parameter is the character to be replaced , The second parameter is the replacement character ;
4. Simulation Implementation

char* my_strcpy(char* dest, char* src) {
    
	while (*dest++ = *src++) {
     ; }
}
int main() {
    
	char arr[20] = {
     0 };
	my_strcpy(arr, "hello");
	printf("%s\n", arr);

	return 0;
}

---

2、strcat Function and Simulation Implementation

1. Function USES

int main() {
    
	char arr1[20] = "hello";
	char arr2[] = "world";
	strcat(arr1, arr2);// String append （ Connect ）
	printf("%s\n", arr1);

	return 0;
}

2. Function introduction

   The source string must be in ’\0’ end ;
   The target space must be large enough , It can hold the contents of the source string ;
   The target space must be modifiable ;
  strcat Functions cannot append themselves .
   The original string will find the first of the target string \0, from \0 Start adding .
   If you add yourself , It will lead to missing \0, Make the program unable to end .

3. Function source code
   
   Receive parameters and strcpy It's the same .
4. Simulation Implementation

char* my_strcat(char* dest, const char* str) {
    
	char* ret = dest;
	assert(dest && str);// Assertion , Make sure the pointer is valid 
	// Find... In the target string \0
	while (*dest) {
    
		dest++;
	}
	// Append source string 
	while (*dest++ = *str++) {
     ; }

	return ret;//strcat Returns the starting address of the target space , We also return the starting address of the target space 
}
int main() {
    
	char arr1[20] = "hello";
	char arr2[] = "world";
	my_strcat(arr1, arr2);// String append （ Connect ）
	printf("%s\n", arr1);

	return 0;
}

---

3、strcmp Function and Simulation Implementation

1. Function USES

int main() {
    
	printf("%d\n", strcmp("abc", "abb"));
	printf("%d\n", strcmp("abc", "abc"));
	printf("%d\n", strcmp("abb", "abc"));

	return 0;
}

2. Function introduction

   The first string is larger than the second string , Return greater than 0 The number of ;
   The first string is equal to the second string , Then return to 0;
   The first string is less than the second string , Then return less than 0 The number of .

3. Function source code
   
4. Simulation Implementation

int my_strcmp(const char* s1, const char* s2) {
    
	assert(s1 && s2);
	while (*s1 == *s2) {
    
		s1++;
		s2++;
		if ('\0' == *s1 || '\0' == *s2) {
     
			break;
		}
	}
	return *s1 - *s2;
}
int main() {
    
	printf("%d\n", my_strcmp("abc", "abb"));
	printf("%d\n", my_strcmp("abc", "abc"));
	printf("%d\n", my_strcmp("abb", "abc"));

	return 0;
}

---

3、 ... and 、 Length restricted string function

1、strncpy Function and Simulation Implementation

1. Function USES

int main() {
    
	char arr1[20] = "abcdef";
	char arr2[] = "qwer";
	printf("%s\n", strncpy(arr1, arr2, 2));

	return 0;
}

2. Function introduction

   Copy n Characters from the source string to the target space ;
   If the length of the source string is less than num, After copying the source string , Add... After the target 0, until num individual .

3. Function source code
   
      It is better than strcpy One more. Count Counter , Used to determine how many characters are copied from the original string to the target space . So it is called the length limited string function , and strcpy It is called string function with unlimited length .
4. Simulation Implementation

char* my_strncpy(char* dest, char* src, int count) {
    
	while (*dest++ = *src++) {
     
		count--;
	}
	int i = 0;
	for (i = 0; i < count - 1; i++) {
    
		*dest++ = '\0';
	}
}
int main() {
    
	char arr1[20] = "abcdefdef";
	char arr2[] = "qwer";
	my_strncpy(arr1, arr2, 6);
	printf("%s\n", arr1);

	return 0;
}

---

2、strncat Function and Simulation Implementation

1. Function USES

int main() {
    
	char arr1[20] = "abcd";
	char arr2[] = "efgh";
	strncat(arr1, arr2, 2);
	printf("%s", arr1);

	return 0;
}

2. Function introduction

   Put the source string n Characters are appended to the target space , And add a terminated null character at the end ;
   If the length of the source string is less than n individual , Then only the original string including the terminating null character is appended to the target space .

3. Function source code
   
4. Simulation Implementation

char* my_strncat(char* dest, const char* str, int count) {
    
	char* ret = dest;
	assert(dest && str);// Assertion , Make sure the pointer is valid 
	// Find... In the target string \0
	while (*dest) {
    
		dest++;
	}
	int i = 0;
	for (i = 0; i < count; i++) {
    
		if ('\0' == *str) {
    
			break;
		}
		*dest++ = *str++;
	}
	*dest++ = '\0';

	return ret;
}
int main() {
    
	char arr1[20] = "abcd";
	char arr2[] = "efgh";
	my_strncat(arr1, arr2, 2);
	printf("%s", arr1);

	return 0;
}

---

3、strncmp Function and Simulation Implementation

1. Function USES

int main() {
    
	char* p = "abcde";
	char* q = "abbc";
	int ret = strncmp(p, q, 4);
	printf("%d\n", ret);

	return 0;
}

3. Function introduction

   Compare to a different character or the end of a string or n Compare all the characters .

4. Function source code
   
5. Simulation Implementation

int my_strncmp(const char* s1, const char* s2, int count) {
    
	assert(s1 && s2);
	int i = 0;
	for (i = 0; i < count; i++) {
    
		if (*s1 != *s2) {
    
			break;
		}
		s1++;
		s2++;
		if (i == count - 1 || '\0' == *s1 || '\0' == *s2) {
    
			return 0;
		}
	}
	return *s1 - *s2;
}
int main() {
    
	char* p = "abcde";
	char* q = "abc";
	int ret = my_strncmp(p, q, 4);
	printf("%d\n", ret);

	return 0;
}

---

Four 、strstr Function and Simulation Implementation

1. Function USES

int main() {
    
	char arr1[20] = "abbbdefgh";
	char arr2[] = "bbd";
	char* ret = strstr(arr1, arr2);
	if (NULL == ret) {
    
		printf(" Did not find \n");
	}
	else {
    
		printf(" eureka :%s\n", arr1);
	}

	return 0;
}

2. Function introduction
   If str2 yes str1 Subset , Then return to str1 Detected with str2 Address with the same value , If str2 No str1 Part of , Return null pointer .
   
3. Function source code
   
4. Simulation Implementation

char* my_strstr(const char* str1, const char* str2) {
    
	char* cp = (char*)str1;
	char* s1, * s2;
	if (!*str2)
		return((char*)str1);
	while (*cp)
	{
    
		s1 = cp;
		s2 = (char*)str2;
		while (*s1 && *s2 && !(*s1 - *s2))
			s1++, s2++;
		if (!*s2)
			return(cp);
		cp++;
	}
	return(NULL);
}
int main() {
    
	char arr1[20] = "abbbdefgh";
	char arr2[] = "bbd";
	char* ret = my_strstr(arr1, arr2);
	if (NULL == ret) {
    
		printf(" Did not find \n");
	}
	else {
    
		printf(" eureka :%s\n", arr1);
	}

	return 0;
}

---

5、 ... and 、strtok Function and Simulation Implementation

1. Function USES

int main() {
    
	char arr[] = "[email protected]";
	char* p = "[email protected]";
	char tmp[20] = {
     0 };
	strcpy(tmp, arr);

	char* ret = NULL;
	for (ret = strtok(tmp, p); ret != NULL; ret = strtok(NULL, p)) {
    
		printf("%s\n", ret);
	}

	return 0;
}

2. Function introduction

sep The parameter is a string , Defines the set of characters used as separators ;
The first parameter specifies a string , It contains 0 One or more by sep A mark separated by one or more separators in a string ;
strtok Function found str The next mark in , And use it \0 ending , Returns a pointer to the tag （ notes ：strtok Function changes the string being manipulated , So it's using strtok The string cut by the function is usually a temporary copy and can be modified ）;
strtok The first argument of the function is not NULL , Function will find str The first mark in ,strtok Function will hold its position in the string ;
strtok The first argument to the function is NULL , The function will start at the same position in the string that is saved , Find next tag ;
If there are no more tags in the string , Then return to NULL The pointer .

3. Function source code
   
4. Simulation Implementation
   I'm sorry , I don't deserve it. .

---

6、 ... and 、strerror Function and Simulation Implementation

1. Function USES

// When using function libraries 
// Call to library function failed , Will set the error code 
int main() {
    
	// If the file fails to open, a NULL The pointer 
	FILE* pf = fopen("test.txt", "r");
	if (NULL == pf) {
    
		printf("%s\n", strerror(errno));
		return 1;
	}
	// Close file 
	fclose(pf);
	pf = NULL;

	return 0;
}

2. Function introduction

   Return error code , The corresponding error message .

---

7、 ... and 、 Memory function

1、memcpy Function and Simulation Implementation

1. Function USES

int main() {
    
	int arr1[10] = {
     0 };
	int arr2[10] = {
     1,2,3,4,5,6,7,8,9,10 };
	memcpy(arr1, arr2, 20);

	return 0;
}

2. Function introduction

   function memcpy from source The position of begins to be copied back num Bytes of data to destination Memory location for ;
   This function is encountering ‘\0’ It doesn't stop ;
   If source and destination There is any overlap , The results of replication are undefined .

3. Function source code
   

4. Simulation Implementation

void* my_memcpy(void* dest, void* src, size_t num) {
    
	assert(dest && src);
	void* ret = dest;
	while (num--) {
    
		*(char*)dest = *(char*)src;
		dest =(char*)dest + 1;
		src = (char*)src + 1;
	}
}
int main() {
    
	int arr1[10] = {
     0 };
	int arr2[10] = {
     1,2,3,4,5,6,7,8,9,10 };
	my_memcpy(arr1, arr2, 20);

	return 0;
}

---

2、memmove Function and Simulation Implementation

1. Function USES

int main() {
    
	int arr1[10] = {
     1,2,3,4,5,6,7,8,9,10 };
	memmove(arr1 + 2, arr1, 20);

	return 0;
}

2. Function introduction

   and memcpy The difference is that memmove The source and target memory blocks processed by the function can overlap ;
   If the source space and the target space overlap , You have to use memmove Function processing .

3. Function source code

4. Simulation Implementation

void* my_memmove(void* dest, const void* src, size_t num) {
    
	assert(dest && src);
	void* ret = dest;
	if (dest < src) {
    
		while (num--) {
    
			*(char*)dest = *(char*)src;
			dest = (char*)dest + 1;
			src = (char*)src + 1;
		}
	}
	else {
    
		while (num--) {
    
			*((char*)dest + num) = *((char*)src + num);
		}
	}
	return ret;
}
int main() {
    
	int arr1[10] = {
     1,2,3,4,5,6,7,8,9,10 };
	my_memmove(arr1 + 2, arr1, 20);

	return 0;
}

---

3、memcmp function

1. Function USES

int main() {
    
	float arr1[] = {
     1.0,2.0,3.0,4.0 };
	float arr2[] = {
     1.0, 3.0 };
	int ret = memcmp(arr1, arr2, 8);
	printf("%d\n", ret);

	return 0;
}

2. Function introduction

   Compare from ptr1 and ptr2 The pointer starts with num Bytes ;
   The first memory is larger than the second memory , Return greater than 0 The number of ;
   The first memory is equal to the second memory , Then return to 0;
   The first memory is smaller than the second memory , Then return less than 0 The number of .

---

4、memset function

1. Function USES

int main() {
    
	int arr[10] = {
     0 };
	memset(arr, 1, 20);

	return 0;
}

2. Function introduction

   Set memory in bytes .

---

8、 ... and 、 Character functions

1、 Character classification function

function	If his parameters meet the following conditions, it returns true
iscntrl	Any control character
isspace	Blank character ： Space ‘ ’, Change the page ‘\f’, Line break ’\n’, enter ‘\r’, tabs ’\t’ Or vertical tabs ’\v’
isdigit	Decimal number 0~9
isxdigit	Hexadecimal number , Include all decimal digits , Lowercase letters a~f, Capital A~F
islower	Lowercase letters a~z
isupper	Capital A~Z
isalpha	Letter a~z or A~Z
isalnum	Letters or numbers ,a~z,A~Z,0~9
ispunct	Punctuation , Any graphic character that is not a number or letter （ Printable ）
isgraph	Any graphic character
isprint	Any printable character , Including graphic characters and white space characters

2、 Character conversion function

int tolower ( int c );// Convert upper case letters to lower case letters 
int toupper ( int c );// Convert lowercase letters to uppercase letters