C语言str部分函数实现

int m_strstr(const char*dst,const char*sub)

{

if (dst == NULL || sub == NULL)

return NULL;

int d_length = strlen(dst) + 1;

int s_length = strlen(sub) + 1;

int dst_index;

int sub_index;

int j;

for (dst_index = 0; dst_index < d_length; ++dst_index)

{

sub_index = 0;

if (dst[dst_index] == sub[sub_index])

{

j = dst_index;

while (dst[j] == sub[sub_index])

{

++sub_index;

++j;

if (sub_index == (s_length - 1))

return dst_index;

}

}

}

return -1;

}

char* m_strcpy(char* dst, const char* src)

{

if (dst == NULL || src == NULL)

return NULL;

char* dst_t = dst;

while (*dst_t++ = *src++);

return dst;

}

void* m_memmove(void* dst,const void* src,int count)

{

if (dst == NULL || src == NULL)

return NULL;

char* dst_t = (char*)dst;

char* src_t = (char*)src;

int s_length = strlen(src_t) + 1;

if ((int)dst < (int)src || (int)dst >(int)src + s_length)

{

while (count--)

{

*dst_t++ = *src_t++;

}

}

else

{

while (count)

{

*(dst_t + count) = *(src_t + count);

count--;

}

}

return dst;

}

int m_strcmp(const char* dst, const char* src)

{

assert(dst == NULL);

assert(src == NULL);

while ((*dst++ == *src++)&&(*dst != '\0')&&(*src != '\0'));

return *dst - *src;

}