c语言字符串操作实现

复试前，用字符串相关的操作来练练手。敲一遍总该用些用处。

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

int strlen(const char *str)
{
    assert(str!=NULL);
    int len = 0;
    while(*str++)
        ++len;
    return len;
}

char *strcpy(char *to, const char *from)
{
    assert((to!=NULL)&&(from!=NULL));
    char *result = to;
    while(*to++ = *from++);
    return result;
}

char *strncpy(char *to, const char *from, size_t count)
{
    assert((to!=NULL)&&(from!=NULL));
    char *result = to;
    while (count--)
    {
        if (*from)
            *to++=*from++;
        else
        {
            *to++='\0';
            break;
        }
    }
    return result;
}

void *memset(void *buffer, int c, size_t count)
{
    assert(buffer!=NULL);
    char *p = (char *)buffer;
    while(count--)
        *p++ = (char)c;
    return buffer;
}

char *strchr(char *str, int c)
{
    assert(str!=NULL);
    for (;*str!=(char)c;++str)
    {
        if(!(*str))
            return NULL;
    }
    return str;
}

//need to release the block by hand in your outside code
char *strcat(char *strDes, const char *strSrc)
{
    assert((strDes!=NULL)&&(strSrc!=NULL));
    char *address = strDes;
    int len = 0;
    while(*strDes)
    {
        len++;
        strDes++;
    }
    char *strResult = (char *)malloc(len+1+strlen(strSrc));
    char *strRtmp = strResult; 
    while(*strResult++=*address++);
    strResult--;
    while(*strResult++=*strSrc++);
    return strRtmp;
}


char * strncat(char *strDes, const char *strSrc, unsigned int count)
{
    assert((strDes!=NULL)&&(strSrc!=NULL));
    char *address = strDes;
    while(*strDes)
        ++strDes;
    while(count--&&*strSrc)
        *strDes++=*strSrc++;
    *strDes = '\0';
    return address;
}

char *strstr(const char *strSrc, const char *str)
{
    assert(strSrc!=NULL&&str!=NULL);
    const char *s = strSrc;
    const char *t = str;
    for (;*strSrc;++strSrc)
    {
        for(s=strSrc,t=str;*t&&*s==*t;++s,++t);
        if(!(*t))
            return (char *)strSrc;
    }
    return NULL;
}

//need to release the block by hand in your outside code
char *strdup(char *strSrc)
{
    if (strSrc!=NULL)
    {
        char *start = strSrc;
        int len = 0;
        while(*strSrc++)
            len++;
        char *address =(char *)malloc(len+1);
        assert(address!=NULL);
        while((*address++=*start++));
        return address-(len+1);
    }
    return NULL;
}

void *memcpy(void *to, const void *from, size_t count)
{
    assert((to!=NULL)&&(from!=NULL));
    void *result = to;
    char *pto = (char *)to;
    char *pfrom = (char *)from;
    while(count--)
        *pto++=*pfrom++;
    return result;
}

void *memmove(void *to, const void *from, size_t count)
{
    assert((to!=NULL)&&(from!=NULL));
    void *result = to;
    char *pto = (char *)to;
    char *pfrom = (char *)from;
    assert(pto<pfrom||pto>pfrom+count-1);
    if(pto<pfrom||pto>pfrom+count-1)
        while(count--)
            *pto++ = *pfrom++;
    else
    {
        pto = pto+count-1;
        pfrom = pfrom+count-1;
        while(count--)
            *pto-- = *pfrom--;
    }
    return result;
}


int strcmp(const char *s, const char *t)
{
    assert(s!=NULL&&t!=NULL);
    while(*s&&*t&&*s==*t)
    {
        ++s;++t;
    }
    return (*s-*t);
}


int stricmp(const char *dst, const char *src)
{
    assert(dst!=NULL&&src!=NULL);
    int ch1, ch2;
    while (*dst&&*src)
    {
        if((ch1=(int)*dst)>='A'&&(ch1<='Z'))
            ch1+=0x20;
        if((ch2=(int)*src)>='A'&&(ch2<='Z'))
            ch2+=0x20;
        if(ch1==ch2)
        {
            ++dst;++src;
        }
        else break;
    }
    return (ch1-ch2);
}


int strncmp(const char *s, const char *t, unsigned int count)
{
    assert((s!=NULL)&&(t!=NULL));
    while (*s&&*t&&*s==*t&&count--)
    {
        ++s;++t;
    }
    return (*s-*t);
}