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matters needing attention ：
1. String must be in ‘\0’ end , because strlen The integer returned is '\0' The number of characters that appear before
2. Notice that the function returns size_t Type of , It's an unsigned integer
3. Learn to strlen Simulation Implementation of

Simulation Implementation ：

// Pointer subtraction 
int strlen(const char* s)
{
	assert(s);
	char* p = s;
	while (*p++);
	return p - s-1;
}
// Recursive method 
int my_strlen2(const char* s)
{
    assert(s);
	if (*s == '\0')
		return 0;
	else
		return my_strlen2(s+1)+1;
}
// Counting method 
int my_strlen3(const char* s)
{
    assert(s);
	int count = 0;
	while (*s)
	{
		count++;
		s++;
	}
	return count;
}

3、 ... and 、strcpy

char* strcpy(char * destination, const char * source );

1. This function will souce The string of is overwritten in destination On
2. Will souce Medium \0 Copy in
3. The target space has to be large enough , Used to put down the source string
4. The target space must be visible, not const type
5. Learn to simulate

Simulation Implementation ：

char* my_strcpy(char* destination, const char* source)
{
	assert(destination&&source);
	char* s = destination;
	while (*source)
	{
		*destination = *source;
		source++;
		destination++;
	}
	return s;
}

Four 、strcat

char * strcat ( char * destination, const char * source );

1. Append a copy of the source string to the destination string . Terminate the null character in destination Is overwritten by the first character of the source , And contain a null character at the end of the new string , Connect the two to destination To form , The return value is after the connection destination The first address .
2. The source string must be in '\0' end
3. The target space has to be large enough
4. The target space can be modified
5. What happens if the string appends itself ？ Will fall into a dead cycle . The following will explain ：

Simulation Implementation

char* my_strcat(char* destination, const char* source)
{
	assert(destination&&source);
	char* p = destination;
	while (*destination)
		destination++;
	while (*source)
	{
		*destination = *source;
		destination++;
		source++;
	}
	return p;
}

If I do something ：

5、 ... and 、strcmp

int strcmp ( const char * str1, const char * str2 );

1. This function starts comparing the first character of each string . If they are equal , Just continue to compare the next character , Until the characters are different or until the termination reaches ‘\0’
2. The standard stipulates ：
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. Learn to simulate

Simulation Implementation ：

int my_strcmp(const char* str1, const char* str2)
{
	assert(str1 && str2);
	while (*str1 == *str2)
	{
		if (*str1 == '\0')
			return 0;// equal 
		str1++;
		str2++;
	}
	if (*str1 > *str2)
		return 1;
	else
		return -1;
}

6、 ... and 、strncpy

char * strncpy ( char * destination, const char * source, size_t num );

1. Than strcpy There is an additional length limit , the source Before num Copy characters to destination in
2. If source The length of is less than num, Then it will be copied again source Continue copying after '\0' Know to achieve num individual

7、 ... and 、strncat

char * strncat ( char * destination, const char * source, size_t num );

Than strcat There is an additional length limit , the source Before num Characters to destination On .

Simulation Implementation ：

char* my_strncpy(char* destination, const char* source, size_t num)
{
	assert(destination&&source);
	char* p=destination;
	while (num)
	{
		*destination = *source;
		destination++;
		source++;
	}
	return p;
}

8、 ... and 、strncmp

int strncmp ( const char * str1, const char * str2, size_t num );

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

Nine 、 String search

strstr

char * strstr ( const char *str1, const char * str2);

strstr The function looks up a string , If in str1 Appeared completely in str2 This string , Then return to str2 stay str1 The address of the first letter in . If it is not found, it returns a null pointer .

Simulation Implementation ：

char* my_strstr(const char* str1, const char* str2)
{
	assert(str1&&str2);
	char* p = str1;
	char* s = str2;
	while (*p)
	{
		str1 = p;
		str2 = s;
		while (*str1 && *str2 && *str1 == str2)
		{
			str1++;
			str2++;
		}
		p++;
		if (*str2 == '\0')
			return p;
	}
	return NULL;
}

strtok

char * strtok ( char * str, const char * sep );

matters needing attention ：
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 token separated by one or more separators in a string .
strtok Function found str Middle the next delimiter and use \0 replace . And will the \0 Save the address of so that the next time you call this function, you can directly start from the last bit here to find the separator （ notes ：strtok Will change the string being manipulated , So use syrtok Function segmented strings are generally temporary copies and can be modified ）.
strtok The first argument of the function is not NULL, The function will find the first tag ,strtok Function will hold its position in the string . And return the address of this section of 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 , And return the address of this section of string .
When the separators of a string are found, it returns NULL.

Examples of use ：

Explanation of principle ：

The second call, and so on .

Ten 、strerror Error message report

char * strerror ( int errnum );

Return the error message corresponding to the error code （ That is to return a string ）

Example ：

#include <stdio.h>
#include <string.h>
#include <errno.h>// The header file that must be included 
int main ()
{
  FILE * pFile;
  pFile = fopen ("unexist.ent","r");
  if (pFile == NULL)
    printf ("Error opening file unexist.ent: %s\n",strerror(errno));
    //errno: Last error number
  return 0;
}
Edit & Run

The principle is that once the program runs, it will produce one errno value , Use the following collocation strerror（errno） Will return a string address , This string is the error message of the program .

errno The specific value of is as follows （ Part of the ）