string函數分析

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string函數分析
string函數包含在string.c檔案中，經常被C檔案使用。
1.strcpy
函數原型: char* strcpy(char* str1,char* str2);
函數功能: 把str2指向的字串拷貝到str1中去
函數返回: 返回str1,即指向str1的指標
/**
* strcpy - Copy a %NUL terminated string
* @dest: Where to copy the string to
* @src: Where to copy the string from
*/
char * strcpy(char *dest, const char *src)
{
char *tmp = dest;

while((*dest++ = *src++) != ‘\0’);

return tmp;
}
2.strncpy
/**
* strncpy - Copy a length-limited, %NUL-terminated string
* @dest: Where to copy the string to
* @src: Where to copy the string from
* @count: The maximum number of bytes to copy
*
* Note that unlike userspace strncpy, this does not %NUL-pad the buffer.
* However, the result is not %NUL-terminated if the source exceeds
* @count bytes.
*/
char * strncpy(char * dest,const char *src,size_t count)
{
char *tmp = dest;

while (count-- && (*dest++ = *src++) != ‘\0‘)
/* nothing */;

return tmp;
}
3.strcat
函數原型: char* strcat(char * str1,char * str2);
函數功能: 把字串str2接到str1後面,str1最後的‘\0‘被取消
函數返回: str1
/**
* strcat - Append one %NUL-terminated string to another
* @dest: The string to be appended to
* @src: The string to append to it
*/
char * strcat(char * dest, const char * src)
{
char *tmp = dest;

while (*dest)
dest++;
while ((*dest++ = *src++) != ‘\0‘)
;

return tmp;
}
4.strncat
/**
* strncat - Append a length-limited, %NUL-terminated string to another
* @dest: The string to be appended to
* @src: The string to append to it
* @count: The maximum numbers of bytes to copy
*
* Note that in contrast to strncpy, strncat ensures the result is
* terminated.
*/
char * strncat(char *dest, const char *src, size_t count)
{
char *tmp = dest;

if (count) {
while (*dest)
dest++;
while ((*dest++ = *src++)) {
if (--count == 0) {
*dest = ‘\0‘;
break;
}
}
}

return tmp;
}
5.strcmp
匹配返回0，不匹配返回非0。
函數原型: int strcmp(char * str1,char * str2);
函數功能: 比較兩個字串str1,str2.
函數返回: str1<str2,返回負數; str1=str2,返回 0; str1>str2,返回正數.
/**
* strcmp - Compare two strings
* @cs: One string
* @ct: Another string
*/
int strcmp(const char * cs,const char * ct)
{
register signed char __res;

while (1) {
if ((__res = *cs - *ct++) != 0 || !*cs++)
break;
}

return __res;
}
6.strncmp
匹配返回0，不匹配返回非0。
/**
* strncmp - Compare two length-limited strings
* @cs: One string
* @ct: Another string
* @count: The maximum number of bytes to compare
*/
int strncmp(const char * cs,const char * ct,size_t count)
{
register signed char __res = 0;

while (count) {
if ((__res = *cs - *ct++) != 0 || !*cs++)
break;
count--;
}

return __res;
}
7.strchr
函數原型: char* strchr(char* str,char ch);
函數功能: 找出str指向的字串中第一次出現字元ch的位置
函數返回: 返回指向該位置的指標,如找不到,則返回null 指標
參數說明: str-待搜尋的字串，ch-尋找的字元
/**
* strchr - Find the first occurrence of a character in a string
* @s: The string to be searched
* @c: The character to search for
*/
char * strchr(const char * s, int c)
{
for(; *s != (char) c; ++s)
if (*s == ‘\0‘)
return NULL;
return (char *) s;
}
8.strrchr
/**
* strrchr - Find the last occurrence of a character in a string
* @s: The string to be searched
* @c: The character to search for
*/
char * strrchr(const char * s, int c)
{
const char *p = s + strlen(s);
do {
if (*p == (char)c)
return (char *)p;
} while (--p >= s);
return NULL;
}
9.strlen
函數原型: unsigned int strlen(char * str);
函數功能: 統計字串str中字元的個數(不包括終止符‘\0‘)
函數返回: 返回字串的長度.
/**
* strlen - Find the length of a string
* @s: The string to be sized
*/
size_t strlen(const char * s)
{
const char *sc;

for (sc = s; *sc != ‘\0‘; ++sc)
/* nothing */;
return sc - s;
}
10.strnlen
/**
* strnlen - Find the length of a length-limited string
* @s: The string to be sized
* @count: The maximum number of bytes to search
*/
size_t strnlen(const char * s, size_t count)
{
const char *sc;

for (sc = s; count-- && *sc != ‘\0‘; ++sc)
/* nothing */;
return sc - s;
}

11.memset
函數原型: void *memset(void *s, int c, size_t n)
函數功能: 字串中的n個位元組內容設定為c
函數返回:
參數說明: s-要設定的字串，c-設定的內容，n-長度
所屬檔案: <string.h>,<mem.h>
/**
* memset - Fill a region of memory with the given value
* @s: Pointer to the start of the area.
* @c: The byte to fill the area with
* @count: The size of the area.
*
* Do not use memset() to access IO space, use memset_io() instead.
*/
void * memset(void * s,int c,size_t count)
{
char *xs = (char *) s;

while (count--)
*xs++ = c;

return s;
}
12.bcopy
/**
* bcopy - Copy one area of memory to another
* @src: Where to copy from
* @dest: Where to copy to
* @count: The size of the area.
*
* Note that this is the same as memcpy(), with the arguments reversed.
* memcpy() is the standard, bcopy() is a legacy BSD function.
*
* You should not use this function to access IO space, use memcpy_toio()
* or memcpy_fromio() instead.
*/
char * bcopy(const char * src, char * dest, int count)
{
char *tmp = dest;

while (count--)
*tmp++ = *src++;

return dest;
}

13.memcpy
函數原型: void *memcpy(void *dest, const void *src, size_t n)
函數功能: 字串拷貝
函數返回: 指向dest的指標
參數說明: src-源字串，n-拷貝的最大長度
所屬檔案: <string.h>,<mem.h>
/**
* memcpy - Copy one area of memory to another
* @dest: Where to copy to
* @src: Where to copy from
* @count: The size of the area.
*
* You should not use this function to access IO space, use memcpy_toio()
* or memcpy_fromio() instead.
*/
void * memcpy(void * dest,const void *src,size_t count)
{
char *tmp = (char *) dest, *s = (char *) src;

while (count--)
*tmp++ = *s++;

return dest;
}
14.memcmp
/**
* memcmp - Compare two areas of memory
* @cs: One area of memory
* @ct: Another area of memory
* @count: The size of the area.
*/
int memcmp(const void * cs,const void * ct,size_t count)
{
const unsigned char *su1, *su2;
int res = 0;

for( su1 = cs, su2 = ct; 0 < count; ++su1, ++su2, count--)
if ((res = *su1 - *su2) != 0)
break;
return res;
}
15.memscan
/**
* memscan - Find a character in an area of memory.
* @addr: The memory area
* @c: The byte to search for
* @size: The size of the area.
*
* returns the address of the first occurrence of @c, or 1 byte past
* the area if @c is not found
*/
void * memscan(void * addr, int c, size_t size)
{
unsigned char * p = (unsigned char *) addr;

while (size) {
if (*p == c)
return (void *) p;
p++;
size--;
}
return (void *) p;
}
16.strstr
/**
* strstr - Find the first substring in a %NUL terminated string
* @s1: The string to be searched
* @s2: The string to search for
*/
char * strstr(const char * s1,const char * s2)
{
int l1, l2;

l2 = strlen(s2);
if (!l2)
return (char *) s1;
l1 = strlen(s1);
while (l1 >= l2) {
l1--;
if (!memcmp(s1,s2,l2))
return (char *) s1;
s1++;
}
return NULL;
}

17.memchr
/**
* memchr - Find a character in an area of memory.
* @s: The memory area
* @c: The byte to search for
* @n: The size of the area.
*
* returns the address of the first occurrence of @c, or %NULL
* if @c is not found
*/
void *memchr(const void *s, int c, size_t n)
{
const unsigned char *p = s;
while (n-- != 0) {
if ((unsigned char)c == *p++) {
return (void *)(p-1);
}
}
return NULL;
}

string函數分析