strcpy、strncpy、strlen、memcpy、memset、strcat、strncat、strcmp、strncmp，strchr

1、strcpy

#include<stdio.h>
#include<assert.h>

char *mystrcpy(char *dest, const char *src)
{
    assert((dest != NULL) && (src != NULL));

    char *res = dest;
    while ((*dest++ = *src++) != '');

    return res;
}

int main()
{
    char *src = "0123456789";
    char dest[11];
    printf("%s
", src);
    char *res = mystrcpy(dest, src);
    printf("%s
", res);

    return 0;
}

2、strncpy

#include<stdio.h>
#include<assert.h>

char* mystrncpy(char *dest, const char *src, size_t size)
{
    assert((dest != NULL) && (src != NULL));

    char *res = dest;
    int i = 0;
    while (((*dest++ = *src++) != '') && (i++<size));

    *(res + size) = '';

    return res;
}

int main()
{
    char *src = "0123456789";
    printf("%s
", src);
    char dest[5];
    char *res = mystrncpy(dest, src, 4);
    printf("%s
", res);

    return 0;
}

3、strlen

（1）非递归实现

#include<stdio.h>
#include<assert.h>

int mystrlen(const char *str)
{
    assert(str != NULL);

    int len = 0;
    while ((*str++) != '')
        ++len;

    return len;
}

int main()
{
    char *str = "0123456789";
    int len = mystrlen(str);
    printf("%d
", len);

    return 0;
}

（2）递归实现

#include<stdio.h>
#include<assert.h>

int strlenRecur(const char *str)
{
    if (str == NULL || *str == '')
        return 0;
    return strlenRecur(str + 1) + 1;
}

int main()
{
    char *str = "0123456789";
    int len = strlenRecur(str);
    printf("%d
", len);

    return 0;
}

4、memcpy

 memcpy 和 strncpy 最大的区别是 memcpy不会遇到 '' 结束拷贝 。

（1）version1（不能解决数据区重叠问题）：

#include<stdio.h>
#include<assert.h>

char* mymemcpy1(char *dest, const char *src, size_t size)
{
    assert((dest != NULL) && (src != NULL));

    char *res = dest;
    while (size)
    {
        *(dest++) = *(src++);
        --size;
    }
    *(dest + size) = '';

    return res;
}

int main()
{
    char *src = "0123456789";
    char dest[5];
    char *res = mymemcpy1(dest, src,4);
    printf("%s
", res);

    return 0;
}

（2）version2（解决了数据区重叠问题）：

#include<stdio.h>
#include<assert.h>

char *mymemcpy2(char *dest, const char *src, size_t size)
{
    assert((dest != NULL) && (src != NULL));
    char *res = dest;

    if (dest > src&&dest < (src + size))
    {
        dest = dest + size - 1;
        src = src + size - 1;
        while (size--)
            *dest-- = *src--;
    }
    else
        while (size--)
            *dest++ = *src++;
    *(dest + size) = '';

    return res;
}

int main()
{
    char *src = "0123456789";
    char dest[5];
    char *res = mymemcpy2(dest, src,5);
    printf("%s
", res);

    return 0;
}

 5、memset

#include<stdio.h>
#include<assert.h>

void *mymemset(void *buf, const char c, size_t size)
{
    assert(buf != NULL);
    char *res = (char*)buf;
    char *str = res;
    while (size--)
        *str++ = c;

    return res;
}

int main()
{
    char *src = "0123456789";
    printf("%s
", src);
    char dest[5];
    void *buf = (char*)dest;
    char *res = (char *)(mymemset(buf, 'a', 5));
    printf("%s
", res);

    return 0;
}

6、strcat

#include<stdio.h>
#include<assert.h>

char *mystrcat(char *dest, const char *src)
{
    assert((dest != NULL) && (src != NULL));
    char *res=dest;
    while (*dest != '')
        ++dest;
    while (*src != '')
        *dest++ = *src++;

    return res;
}

int main()
{
    char dest[15] = "123";
    char *src = "abc";
    char *res = mystrcat(dest, src);
    printf("%s
", res);

    return 0;
}

7、strncat

#include<stdio.h>
#include<assert.h>

char *mystrncat(char *dest, const char *src,size_t size)
{
    assert((dest != NULL) && (src != NULL));
    char *res=dest;
    while (*dest != '')
        ++dest;
    while ((*src != '')&&(size--))
        *dest++ = *src++;
    *dest = '';

    return res;
}

int main()
{
    char dest[15] = "123";
    char *src = "abcdef";
    char *res = mystrncat(dest, src,3);
    printf("%s
", res);

    return 0;
}

8、strcmp

#include<stdio.h>
#include<assert.h>

int mystrcmp(const char *str1, const char *str2)
{
    assert((str1 != NULL) && (str2 != NULL));
    while (*str1&&*str2 && (*str1++ == *str2++));

    return (*str1 - *str2);
}

int main()
{
    char *str1 = "abcde";
    char *str2 = "abdf";
    int res = mystrcmp(str1, str2);
    printf("%d
", res);

    return 0;
}

9、strncmp

#include<stdio.h>
#include<assert.h>

int mystrncmp(const char *str1, const char *str2,size_t size)
{
    assert((str1 != NULL) && (str2 != NULL));
    while (*str1&&*str2 && (*str1++ == *str2++)&&size--);

    return (*str1 - *str2);
}

int main()
{
    char *str1 = "abcde";
    char *str2 = "abdfsd";
    int res = mystrncmp(str1, str2,4);
    printf("%d
", res);

    return 0;
}

10、strchr

#include<stdio.h>
#include<assert.h>

char *mystrchr(const char *str, char c)
{
    assert(str != NULL);
    while (*str != '' && (*str != c))
        ++str;

    if (*str == c)
        return (char*)str;
    else
        return NULL;
}

int main()
{
    char *str = "abcde";
    char *res = mystrchr(str,'c');
    printf("%s
", res);

    return 0;
}