C语言基础字符串函数源代码

#include

#include

/*

* 作用：获取串str的长度

*/

int strlen(const char *pcStr)

{

assert(pcStr != NULL);

int iLen = 0;

while ('\0' != *pcStr)

{

pcStr++;

iLen++;

}

return iLen;

}

/*

* 前提：strDes指向的内存至少能够容纳(strlen(strDes) + strlen(strSrc) + 1) bytes

* 作用：将串strSrc连接到串strDes的末尾，函数本身不保证安全(使用不当会内存溢出)

*/

char *strcat(char *pcStrDest, const char *pcStrSrc)

{

assert((pcStrDest != NULL) && (pcStrSrc != NULL));

char *pcTemp = pcStrDest;

/* pcStrDes指针指向串pcStrDes的末尾 */

while ('\0' != *pcStrDest)

{

pcStrDest++;

}

/* 依次从源串pcStrSrc不断拷贝字符到目的串pcStrDes中 */

while ('\0' != *pcStrSrc)

{

*pcStrDest = *pcStrSrc;

pcStrSrc++;

pcStrDest++;

}

*pcStrDest = *pcStrSrc;

return pcTemp;

}

/*

* 前提：strDes指向的内存至少能够容纳(strlen(strSrc) + 1) bytes

* 作用：将串strSrc拷贝到strDes指向的内存空间，函数本身不保证安全(使用不当会内存溢出)

*/

char *strcpy(char *pcStrDest, const char *pcStrSrc)

{

assert((pcStrDest != NULL) && (pcStrSrc != NULL));

char *pcTemp = pcStrDest;

while ('\0' != *pcStrSrc)

{

*pcStrDest = *pcStrSrc;

pcStrSrc++;

pcStrDest++;

}

*pcStrDest = *pcStrSrc;

return pcTemp;

}

/*

* 作用：按字典序比较串s和串t的大小

*/

int strcmp(const char *pcStrA, const char *pcStrB)

{

assert((pcStrA != NULL) && (pcStrB != NULL));

while (('\0' != *pcStrA) && ('\0' != *pcStrB) && (*pcStrA == *pcStrB))

{

pcStrA++;

pcStrB++;

}

return (int)(*pcStrA - *pcStrB);

}

/*

* 前提：strDes指向的内存至少能够容纳(strlen(strDes) + count + 1) bytes

* 作用：将串strSrc的前count个字符连接到串strDes的末尾，函数本身不保证安全(使用不当会内存溢出)

*/

char *strncat(char *pcStrDest, const char *pcStrSrc, int iCount)

{

assert((pcStrDest != NULL) && (pcStrSrc != NULL));

char *pcTemp = pcStrDest;

while ('\0' != *pcStrDest)

{

pcStrDest++;

}

while ((iCount > 0) && ('\0' != *pcStrSrc))

{

*pcStrDest = *pcStrSrc;

pcStrSrc++;

pcStrDest++;

iCount--;

}

*pcStrDest = '\0';

return pcTemp;

}

/*

* 前提：strDes指向的内存至少能够容纳(count + 1) bytes

* 作用：将串strSrc的前count个字符拷贝到strDes指向的内存空间，函数本身不保证安全(使用不当会内存溢出)

*/

char *strncpy(char *pcStrDest, const char *pcStrSrc, int iCount)

{

assert((pcStrDest != NULL) && (pcStrSrc != NULL));

char *pcTemp = pcStrDest;

while ((iCount > 0) && ('\0' != pcStrSrc))

{

*pcStrDest = *pcStrSrc;

pcStrSrc++;

pcStrDest++;

iCount--;

}

return pcTemp;

}

/*

* 作用：按字典序比较串s和串t的前count个字符的大小

*/

int strncmp(const char *pcStrA, const char *pcStrB, int iCount)

{

assert((pcStrA != NULL) && (pcStrB != NULL));

while (('\0' != *pcStrA) && ('\0' != *pcStrB) && (*pcStrA == *pcStrB) &&

(iCount > 0))

{

iCount--;

if (0 == iCount)

{

break;

}

pcStrA++;

pcStrB++;

}

return (int)(*pcStrA - *pcStrB);

}

/*

* 前提：dest指向的内存能够容纳count bytes

* 作用：将src所指向内存的count bytes拷贝到的dest指向的内存

*/

void *memcpy(void *pvDest, const void *pvSrc, int iCount)

{

assert((pvDest != NULL) && (pvSrc != NULL));

void *pvTemp = pvDest;

char *pcDest = (char *)pvDest;

const char *pcSrc = (const char *)pvSrc;

while (iCount > 0)

{

*pcDest = *pcSrc;

pcDest++;

pcSrc++;

iCount--;

}

return pvTemp;

}

/*

* 作用：将str的前count个字节置为字符c

*/

void *memset(void *pvSrc, int iCh, int iCount)

{

assert(pvSrc != NULL);

char *pcSrc = (char *)pvSrc;

while (iCount > 0)

{

*pcSrc = (char)iCh;

pcSrc++;

iCount--;

}

return pvSrc;

}

/*

* 作用：将src所指向内存的count bytes拷贝到的dest指向的内存(解决了memcpy未解决的内存重叠问题)

*/

void *memmove(void *pvDest, const void *pvSrc, int iCount)

{

assert((pvDest != NULL) && (pvSrc != NULL));

void *pvTemp = pvDest;

char *pcDest = (char *)pvDest;

const char *pcSrc = (const char *)pvSrc;

// 逻辑：

// 1)在src小于dest前提下，然后判断src+count指向的内存是否与dest指向的内存重叠

// 2) 如果重叠则从src+count开始复制，复制到dest+count，从后往前拷贝count字节

if ((pvSrc < pvDest) && (pvSrc + iCount > pvDest))

{

pcDest = pcDest + iCount - 1;

pcSrc = pcSrc + iCount - 1;

while (iCount > 0)

{

*pcDest = *pcSrc;

pcDest--;

pcSrc--;

iCount--;

}

}

else

{

while (iCount > 0)

{

*pcDest = *pcSrc;

*pcDest++;

*pcSrc++;

iCount--;

}

}

return pvTemp;

}

int main()

{

int iLen = 0;

int iRet = 0;

char acBuffer[4] = {0};

char *pcTestStr1 = "Today is sunday.";

char acTestStr2[] = "Today";

char acTestStr3[] = "Today is sunday.";

char acTestStr4[sizeof("Today is ") + sizeof("sunday.") - 1] = "Today is ";

iLen = strlen(pcTestStr1);

printf("iLen = %d\n", iLen);

(void)strncpy(acBuffer, pcTestStr1, sizeof(acBuffer) - 1);

printf("acBuffer:%s\n", acBuffer);

iRet = strncmp(pcTestStr1, acTestStr2, strlen(acTestStr2));

printf("strncmp iRet = %d\n", iRet);

iRet = strcmp(pcTestStr1, acTestStr2);

printf("strcmp iRet = %d\n", iRet);

(void)memmove(&acTestStr3[1], &acTestStr3[0], strlen("Today"));

printf("memmove acTestStr3:%s\n", acTestStr3);

printf("before strncat acTestStr4:%s\n", acTestStr4);

(void)strncat(acTestStr4, "sunday.111",

sizeof(acTestStr4) - strlen(acTestStr4) - 1);

printf("after strncat acTestStr4:%s\n", acTestStr4);

return 0;

}

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